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Alexandre Yersin
sadfasHome > Da Lat City > Nature
Nature 

1. Geographic Location:

Da Lat is located in Langbiang highland with the average height above sea level of about 1,500m

Coordinates:

108019’23’’ E to 108036’27’’ E

11048’36’’ N to 12001’07’’ N

Da Lat is on the Northeast side of Lam Dong Province, without common administrative borders with the contiguous provinces. It is bound by Lac Duong District to the north, Don Duong District to the east and southeast, Lam Ha District to the west and Duc Trong District to the southwest.

Most of the city’s administrative borders run along the typographic divides passing high terrain and along streams.

Natural area: 391.06km2

According to the national classification, Da Lat is a type-2 urban area. Da Lat serves as the cultural, economic and political centre of Lam Dong Province.

2. Geology

2.1 Tectonic Geology

2.1.1 Tectonic Activity

Based on the research results relating to Da Lat’s tectonic zone, the city’s land formation process has three stages:

- ocean Stage

- Continental stage

- Terrace stage

ocean Stage

In the Acheozoic Era (about 3,000 million years ago), the continental crust of Kontum zone was born. At that time, to the south of Kontum massif was ocean. In the area of Da Lat, the ocean stage existed through the Proterozoic Era (about 2,000 million years ago) and Paleozoic Era (about 600 million years ago).

Continental Period

This stage started from late Jurassic Period, through the Kreta Period until the end of the Mesozoic Era (about 100 million years ago). During this stage a lot of fierce magmatic activities led to intrusive formations and eruptive formations, which covered and caused deformation, metamorphosis to previous deposit. Eruptive formations such as dacite and andecite built huge mountain ranges and sharp terrain. Intrusive formations built granite mountain ranges at the fringe of the land mass such as Southwest Du Sinh, bound by Nam Ban, Ta Nung, etc. Together with ore stone, eruptive rock (rhyolite, rhyodacite, volcanic tuff, etc) were found in Northeast and Southeast Da Lat (from Datanla to Fimnom). These activities, lasting for 30 million years of the Cretaceous Period, were defined into different phases by geologists. The land mass’ direct foundation was Middle-Late Jurassic derivative rocks, which were exposed in few places, while the rest was cut through or overlaid by Cretaceous intrusions. Cretaceous formations include continental deposit and eruptive rocks, typically distributed around Da Lat.

After the continental formation, Da Lat area went through a quiet phase of tectonics under the impact of external forces. Weathering, erosion, overwashing, and accumulation processes almost flattened the ancient terrain. Tectonic activities were weak and occurred in a narrow area, on a small scale. Therefore, they did not made considerable change to the existing continental crust. This phase lasted the whole Paleogene Period of the Cenozoic Era, which is about 30 million years. By the end of the Paleogene Period, the whole Tay Nguyen area, the Southern Centre, and the Southeast had been covered by a big even denudation-caused plain with wavy gradual residual mountains of 300-400m height.

Terrace Plateau Stage

The formation of Tay Nguyen plateau had not occurred until about 25 million years ago, during the neotectonics from the Neogene Period to the Quaternary Period of the Cezonoic Era. This formation process had 3 phases:

Phase 1: In Middle Miocene (some believe even earlier, in Late Oligocene) strong tectonic activation and intrusion raised the terrain with the maximum throw of about 500-700m. Because of these orogenic movements, the whole Lam Dong area was raised and Da Lat was the highest. The movements also raised the existing low granite intrusive mountain blocks and formed the plateau Da Lat.

After that, upraise movements slowed down and was taken place by the destructive process of external forces, which resulted in wide valley-like flat surface between mountains of above 1,000m. Those were the residual mountains or the peaks of exposed intrusive granite blocks.

Phase 2: In the Middle Pliocene intense magmatic activities (mostly basalt) plus intrusive activities once again raised the land. New fault systems development had opened the way for strong basaltic eruption throughout Tay Nguyen during the Pliocene. Not only the raised parts but other calmer parts were also influenced by the strong tectonic process with clear differentiation. Those tectonic occurrences plus external process caused deformation to the existing flat surface and built low mountainous terrain. Troughs were filled with eruptive materials and the terrain features became less contradictory. Volcanic landscape and new terrain were the favorable conditions to built layers of silt deposit, bentonite and ceyssatite.

In this period due to fierce basaltic eruption the whole area from Bao Loc - Di Linh to Duc Trong - Lam Ha was blanketed by a thick basalt layer of Neogene age. However, in Dalat, because of intrusive activities with almost no eruption, the existing formations were folded and mountain ranges in the west of north of Da Lat were upraised. By Late Pliocene, Lam Dong terrain had gotten the obvious bench shape. After that raising and eruptive activities slowed down and gave place to external flattening process.

Phase 3: In early Quaternary Period (about 3 million years ago), tectonic activities exploded again and raised Lam Dong terrain to a throw of 400-500m. Basalt magma flowing from cracks covered accumulative plains, the lowest surface at that time. Eruptive and raising activities continued along the folding and fault lines. Da Lat land mass was raised even more and the terrain contradiction became clearer. Basaltic eruption also caused deflection and rejuvenation to rivers and streams. Faulting shift turned Neogene and basaltic layers upside down and led to the area’s blocky structure.

These basaltic eruptions were in both forms of effusion and explosion. As a result, basalt rocks were formed in honeycomb structure, which was favorable condition for later weathering to form good soil profile and structure. Basaltic eruption was weak and sparse in Datanla, Cam Ly, Ta Nung, Xuan Truong, Xuan Tho. After this cycle, Da Lat became higher and had more folding. Most geologists believe eruption in this phase was the last in Da Lat area.

In the second half of the Quaternary Pediod, Da Lat terrain was still under the force of weak pulsation-like tectonic activities but hardly of any eruptive activities.

After the 3 phases mentioned, Da Lat has the terrain of terrace plateau. Since then, external forces has polished the area to the current conditions.

2.1.2 Strata and Tectonic Fault

Strata

Da Lat city is found in the “Da Lat hollow”which is a unit of the South-Southeast structure and located in the south of Kon Tum land mass and in the southeast of Cuu Long.

This is an active magmatic tectonic area with strong intensity. Main structural units include:

- Architectural structure before Late Mesozoic

- Active magmatic tectonic formation of late Mesozoic - Cenozoic age. This formation consists of 3 subformations:

* Lower subformation (T2), filled with lagoon phase materials. This was the relative synclinal stage, in blocky structure, without reverse tectonics, and folding.

* Middle subformation (J3-K), filled with volcanic intrusive materials from multiphase and multi-component.

* Upper subformation (N-Q), filled with continental volcanic materials.

Strata in Da Lat terrain includes Mesozoic erathem and Cenozoic erathem.

Mesozoic Erathem

* Late Jurassic Formation (J2)

Jurassic sediment has folding and metamorphic facies. Its profile starts with a thick 350-400m set of agglomerate, polymere gritstone, alternating with marl and aleurolite containing animal fossils. The middle set includes aleurolite, shale, argillaceous limestone and flour sandstone with mussel fossils and early Jurassic sediment. The upper set has clay shale, aleurolite and coarse sandstone with sea fossils belonging to Mid-Upper Jurassic. The whole suite is more than 2,000m.

This Jurassic sediment is usually folded. Many places are cut through by Cretacious intrusion and diagenesis is observed in a large area. Moreover, a few areas are unconformably covered by Cretaceous, Neogene and Quaternary formations.

* Cretaceous Formation

Don Duong formation of Cretaceous age consists of continental sediment and eruptive rocks, distributed around Dalat. This formation is about 1000-2000m.

The lowest set has agglomerate, tuff agglomerate alternating with thin layers of felsites, porphyritic rhyolite, sandstone, brown-red aleurolite. This set is about 200m and lies along Jurassic sediment and granitoid blocks of Cretaceous age.

The next set includes porphyritic rhyolite, porphyritic dacite, porphyritic andesitic dacite with few thin layers of sandstone, tuff aleurolite, and gritstone. The set’s thickniss is about 350-450m.

Next is the set of alternating sandstone, tuff aleurolite and shale of about 150m high.

The upper set is about 150m and consists of porphyritic dacite, tuff dacite, dacite rhyolite, and porphyritic rhyolite. In this Don Duong formation, only Lygodium sp., Picea sp. and Oedrus sp. are found. Those are common pollens in Early Cretaceous. There are Late-Cretaceous granite intrusions cutting through the sets.

Cenozoic Erathem

Dalat’s basalt is classified as Middle-Late Pleistoncene basalt, which is not common in Dalat.

It is alkali-clivine and dolerite-clivine basalt, about 90-120m thick. In this area of basalt, there are often explosive eruption formation, different ultramafic such as alkali ultramafic, meymechite, ankaramite, inclusion and minerals such as olivine, olgite, albite, zircon, magnetic, ilmenite, crystalline chist, granite, etc. From one to four layers of this basalt, separated by the weathering surfaces of laterized red earth, sometimes alternate with few layers of calc-placer and alluvial materials (2-15cm) that cover ancient river terrace.

* Quaternary Formation

In Tay Nguyen as a whole and Da Lat in particular, Quaternary alluvial sediment hardly developed. Recent alluvial has been developed in many places and is about 4-5cm thick. Its variable profile consists of pepple, fine pepple, sand, silt, clay and even peat as in Mang Lin. The weathering crust has been well developed and change with geologic and terrain formation. Weathering thickness could be 0 or tens of meters. Kaolinite weathering is even found on some acid rocks.

Intrusive Magma

In Da Lat terrain, intrusive magma formations are common and diverse and its exposed areas are relatively big.

Late Mesozoic - Cenozoic intrusive formations are widely distributed in South Tay Nguyen. Many formations, medium size to big size, sparsely penetrate older formations and cause metamorphosis to the surrounding rocks. Some places are overlaid by later formations whose two kinds are granodiorite-granite intrusion and granite-granosyneit intrusion.

* Granodiorite and granite intrusions usually form in medium to big size. They have unequal exposed areas which may sometimes spread in the northeast-southwest direction. They cause metamorphosis to the surrounding rocks and form chert such as cordierite biotite, cordierite chiastolite inside Jurassic sediment rock. The petrographic composition of granodiorite and granite at the contact zone is mainly syenite diorite, and pyrosene.

Ore stone is generally uncommon. The found ones are limited to porphyritic granodiorite, and aplite. Ore stone is usually distributed within block range or in littoral zones.

Mineral composition includes plagioclase, orthoclase, quartz, hornblende, biotite, and pyroxene.

Granodiorite and granite are quite common in Prenn, Datanla, Ta Nung of Dalat. Granite usually forms independent intrusions. It is sometimes albitized or turn to quartz with typical mineralization.

Being of Late Jurassic - Cretaceous age, they penetrate Jurassic sediment and have absolute age of about 100 million years.

* Granite - granosyenit intrusions, distributed in the North and West of Dalat, Cam Ly - Ta Nung, Lap - Be Nam, Trai Mat, Trai Ham, usually form intrusions of the size of medium to big.

Tectonic Fault

Along Dalat’s tectonic history, the impact of regional oscillations and magma blocks’ contraction have deformed intrusions and shifted, folded the previous rock layers. The whole process has altered the area’s tectonic plan.

Faulting activities in Da Lat are quite diverse with different directions, among which the northeast-southwest direction is most common (clearly shown in the region of Cam Ly fall and Datanla fall).

Faults appeared late Cretaceous Period, in Paleogene Era. They developed mostly to the northeast-southwest direction, stretched hundreds of kilometres penetrating the North of the land mass through Di Linh plateau down to Dong Nai, and formed cataclasic zones. They can be observed clearly in the areas formed by Cretaceous granite intrusions and eruption that are overlapped by basalt.

Contraction in the opposite direction which was weaker and appeared later formed the reverse fault system. Along with the fault system were feather joints and secondary faults that would become rivers and streams. Cam Ly stream is a fault whose feather joint system Da Lat market and Phan Dinh Phung river belong to.

2.2 Engineering Geology

Can geological partitioning of Da Lat city into three groups:

2.2.1 Average-height Mountain Erosion-Tectonic Region

This group is distributed in the average-height mountain areas in the North, the South slope of Dalat, and the Prenn mountain pass. The region’s geological architecture includes crystalline metamorphic rock, intrusive rock, acid eruptive rock, little Cenozoic eruptive rock and Paleozoic and Mesozoic terrigneous eruptive rock.

The terrain has an absolute height of 1,200-1,500m, a profile of 300-500m, and cross section of 0.8-1km/km2. River valleys are usually narrow, steep with many falls and rapids. The cross sections along rivers are usually very steep (10 - 15O and above). Continuous water horizons are rare, and underground water is found only in monsoon seasons.

Neotectonic movements are quite strong. External activities are mostly weathering, stream and overflow water erosion, stone collapsing, and shear slide.

Soil is composed of hard rock, loose soil with breccias. At riverbed there are pebble, boulder but seldom sand. Pebble and boulder are often used as high-quality building materials.

This region is not suitable for residential, industrial buildings, power lines, and pipe lines. But it can bear transportation, irrigation systems of medium to small scale, as well as defence buildings.

2.2.2 Low Mountain and Hill Erosion Region

Next to the above region is the terrain of low mountain and hill erosion region, located in Xuan Truong, Xuan Tho, and West Dalat. The geologic architecture has Proterozoic metamorphoic rock, intrusive Paleozoic, terrigneous formations, Paleozoic and Mesozoic eruptive rock, Late-Mesozoic terrigneous sediment, and little Cenozoic eruptive rock.

Mountains and hills are about 1,200-1,300m high. There cross section is 0.5-0.7km/km2 on average. Gradual slopes and ephemeral streams are widely developed.

This region is suitable for construction because of hard bearing rock, and convenient water supply due to continuous water horizons. Building materials are quite diverse.

2.2.3 Plain and Plateau Erosion Regions

Da Lat plain has the absolute height of 1,400-1,500m. The terrain is composed of Proterozoic metamorphoic rock, Mesozoic eruptive terrigneous and intrusive rock, Paleozoic terrigneous rock, Middle-Mesozoic rock, and little Cezonoic basaltic eruptive rock. As a result of the plain formation process, the plateau terrain is rather even. There are few residual hills that are about 50-200m higher than the surroundings. These hills have higher peaks, gradual slopes and profiles of 50-100m. There are continuous underground water horizons, but the diversity is poor and water horizons at 5-10m deep have week corrosive effect on concrete.

Neotectonic movements are weak. Feralite weathering, ephemeral streams, and surface erosion are common.

This region’s terrain is totally suitable for construction, especially industrial, residential, power lines, pipe lines, and transportation. Irrigation systems can only be at small to medium scale. Soil bearing capacity is up to 3-5kg/cm2. Materials from soil, rock, sand, pebble and gravel are quite diverse.

3. Minerals

3.1. Non-Metallic Minerals

Non-metallic minerals consist of kaolin and building materials.

3.1.1 Kaolin

Kaolin is formed from the weathering of feldspar, known as kaolinization process.

In the composition of magmatic rock that is commonly seen in Da Lat (granite, dacite, basalt), there are mineral spirits such as quartz, feldspar, mica, amphibole, peridotite, pyroxene, etc. Weather rock releases mineral spirits like feldspar.

Feldspar is composed of white or pink crystals. It can easily turn to clay, which is silicate aluminium with Ca or Na.

Kaolin in Da Lat mainly comes from weathered granite mingled with Ca Na complex and feldspar, and Don Duong formation rhyolite.

The quality of kaolin depends on the quantity of color minerals (biotite, hornblende) in granite and whether geomorphic conditions are suitable for exploiting.

Based on the weathering origin, kaolin in Da Lat can be classified into:

* Kaolin from weathered biotite Ca Na complex granite

This kind is well distributed in the weathering zone of Ca Na complex phase 2 granite, which has been explored in Trai Mat, West Dalat, Lap-be Nam, etc. There are two subsets:

- Kaolinite - gypsite: Kaolin mine Trai Mat

This mine has rather good quality due to low content of color minerals. Kaolin has high fineness: sieve size of 0.1mm - 47.4%

Kaolin of 1.1-0.05mm diameter - 58.55%, of 0.05-0.01mm diameter - 18.43%, of 0.01-0.005mm diameter - 12.37%

Trai Mat kaolin is from thoroughly weathered feldspar. Therefore, raw mineral here is brighter than in other places.

Mineral and chemical composition of kaolin depends on weathering level, weak or average. Alumni kaolin and alkali kaolin have quite high content of Al2O3 (20-45%) and low content of Fe2O3 (0.42 - 1.9%).

With high heat resistance (1680-1760OC), alumni kaolin could be used to produce high-grade porcelain, insulated porcelain, thermally stable porcelain. Kaolin with high alkali content could be used in producing ceramics, filling materials in paper industry, and aluminium sulphate.

Elasticity (<0.1mm): 16.35 - 23.17

pH range: 4.8 - 6.4, average 5.61

Reserves: about 55,910,000 tons

- Kaolinite - gypsite - goethite, distributed in Cam Ly mine, in West Dalat

This kind is composed of SiO2 47.3- 64.9%, Al2O3 21.7 - 32.2%, Fe2O3 3.4 - 3.6%, TiO2 0.25 - 0.28%. Heat resistance is 1670-1770OC. Due to high iron content, it is not used in producing porcelain. It can be used in producing thermally stable bricks or aluminium sulphate. Preliminary estimation of reserves is 38 million tons.

* Kaolin from weathered Don Duong formation rhyolite and felsites

This type of kaolin is typically distributed in Prenn and Dalanta mines. Kaolin from felsites has better quality than kaolin from rhylite because of lower content of Fe2O3 and brown minerals.

Prenn mine has semi-weathering kaolin, mixed with feldspar. The mine runs in northeast-southwest direction with the length of 4km and the width of 1.3km. Kaolin here has high fineness and does not need to be filtered and ground before going to production. Its Al2O3 content is not high, so its heat of solidification is lower than the one of Trai Mat kaolin. With microlithic quartz and high Fe2O3 content, it is a material in civil porcelain industry.

Its reserves is 73,144,421 tons, in which 23,727,841 tons is powder kaolin and 49,416,580 tons is kaolin in piece.

3.1.2 Minerals for Building Materials

Minerals in Da Lat has really big potential as building materials. Since 1996, the Centre of Tectonics, Catastrophe and Environment of Geological Association 6 has been gathering materials, doing surveys, collecting samples and preliminarily evaluate Lam Dong province’s potential in terms of building materials. According to these data, building materials in Da Lat consist of building stone, brick argilla, etc.

Building Stone

Dalat’s building stone consists of the following types:

- Ca Na complex granite

- Xuan Loc formation basalt

- Don Duong formation rhyolite-dacite

* Ca Na Complex Granite

Research on radiation parameter shows that its equivalent dose always exceeds permitted rate. Especially in building using granite and built on granite foundation, the radiation level in closed room is always much higher than in open spaces.

Regarding petrographic, chemical and physic-mechanical composition, Ca Na complex granite has construction quality. With high compressive strength, it is suitable for pavement gravel base, producing concrete, rubble stone and dressed stone for buildings on ground and in water. Regarding scale, reserves of this type of granite is really big. Each single region may have up to tens of million cubic meters. However, radiation level at mines of construction projects using this material is higher than permitted limits.

Until now, there have been 10 registered mines on the mineral maps (Cam Ly, Ta Nung, Trai Mat, etc.). These mines are all near roads and their distribution areas are from a few to tens of km2.

* Xuan Loc Formation Granite

Xuan Loc formation granite is distributed in Mang Lin, Cam Ly airport, Prenn-Dalanta area, Xuan Truong, Xuan Tho, etc. Their distribution areas are from a few to tens of km2. Its estimated reserve is about 1 million m3.

This granite has high compressive strength, suitable qualities to be used in building, road pavement, producing concrete, rubble stone and dressed stone for buildings on ground and in water. With high lime absorption, it could be used to produce cement admixture, and adobe. Low radiation level makes it suitable for building with poor airflow. However, most mines are distributed far from roads. Thick weathering layer of 5-10m adds into the exploiting and transporting disadvantages. The potential mine is the Datanla building stone mine with the length of 250m, width of 200m, and depth of 20m. Its reverse is about 1 million m3. Prenn-Dinh An mine is 400m long, 200m wide, 10m deep, and has a reserve of about 0.8 million m3.

* Don Duong Formation Rhyolite - Dacite

Its exposed area is 300km2 with the width of 5-15km, length of 35m from Lam Ha through Prenn-Dalanta, Xuan Tho to upstream of Da Nhim. This type of material is also distributed in Ta Nung. The exposed area is near highway 20 and other roads, so exploitation and transportation is convenient.

Don Duong formation rhyolite-dacite has high compressive strength, therefore could be used in paving roads, and building construction. With low radiation intensity, it is suitable for building with poor airflow.

This type of rock usually contains ore veins such as tin ore, gold ore or arsenic ore.

Brick Argilla

Brick argilla has two origins: weathering and depositing. Due to terrain characteristics, there is no deposit type in Dalat.

Da Lat Brick argilla is from weathered argillite and aleurolite of La Nga formation. It was discovered in Northeast Mang Lin village with a distribution area of 30km2 from Lac Duong district to North Dalat. The weathering layer could be up to 10m.

Brick argilla may be yellow, brown-yellow, or brown-red. There could be some residual thin veins of white quartz of 1cm thickness.

Compared to deposit argilla, weathering argilla is generally less fine. However, weathering argilla of this area is fine enough to make brick. With low Al2O3 and SiO2 content, mineral components of brick argilla are mainly kaolinite and hydromica, and no monmorylonite. Therefore, when baking, argilla with high content of SiO2 needs to be added in order to lower baking temperature and enhance brick quality. Besides, this argilla has high lime absorption, so it could be used to produce cement admixture, and adobe.

3.2 Metallic Minerals

Metallic minerals consist of tin and gold.

3.2.1 Tin

Tin ore in Lam Dong generally and in Da Lat particularly belongs to the cassiterite type, which is found in Dalat-Bao Loc metallogenetic zonule.

Results from the activation method shows that a ground sample consists of tin - 64%, stibinite - 0.7%, arsenic - 0.008%, copper - 0.04%, volfram - 0.09%, scandium - 0.003%, sodium - 0.5%, aluminium - 2.5%, manganese - 0.0013%, magnesium - 0.6%, vanadium - 0.003%.

There are two types of tin, original tin and alluvial tin.

* Original Tin has hydrothermal and megmatitic origin. Tin ore is related to Ankroet complex granite which is concentrated in berezitized and greisenized rock with high hydrothermal origin.

Until now, 3 mines in Cao mountain, Thai Phien, Da Thien, and 3 ore emplacements in Thai Phien, Da Thien, and Khon mountain have been discovered. The total reserve is about 10,000-12,000 tons.

Alluvial tin is widely distributed in concentrations of alluvium, diluvium, and proluvium in the region from North Da Lat to Da Chais. Reserves have not been thoroughly evaluated but are estimated around 1,600-1,800 tons SnO2.

3.2.2 Gold

Gold has two types: original gold and bullion.

* Original Gold with the following formations:

- Quartz - yellow sulfur formation has developed widely and had big proportion. Veins usually have size of few centimeters to 1 meter, and forms of single veins or vein zones. Its reserve is 0.8-5.5 g/ton. In Dalat, ore emplacements in Da Thien, Trai Ham have huge potential but they are only being explored so their reserves are still unknown.

- Quartz - yellow arsenic formation has high hydrothermal origin. This type of formation is not common, and only found in Trai Ham, Mang Lin, and Sao Nam. Gold is distributed in quartz veins whose length is from 1-2cm to 50cm. Gold content is 2.55g/ton and silver content is 4.8g/ton.

- Yellow sulphur - silver is quite commonly developed in Don Duong formation eruptive rock with low content (0.1-1.7g/ton). Its distribution is around Datanla.

* Bullion, in flake or particle forms, is found in valley flat near rivers and streams with small reserves and low content.

3.3 Gemstones

Da Lat gemstones have their origins related to eruptive basaltic and eruptive dacite rock. 3 types discovered are opal-caxedoan, sapphire, and tektite.

3.3.1 Opal - caxedoan

Opal - caxedoan exists in the lower part of Pleistocene eruptive basalt’s profile. The thickness of this gemstone layer is 10-30cm. The stone’s plane of fracture is glossy with vitreous luster. It could be used for fine arts jewelry.

3.3.2 Sapphire

Sapphire’s origin is alkali basaltic rock. It is found in alluvium and diluvium overbank deposit. An ore emplacement in Xuan Truong has been discovered but not yet studied and evaluated.

3.3.3 Tektite

Tektite is found in many places on Pleistocene erosion surfaces. In some places, tektite is formed in deposit of Pleistocene to Holocene terrain and valley flat. Tektite is the formation of basaltic explosive eruption in Pleistocene Era. Being explosively erupted, burning rock was flying in the sky, so some people believe that tektite is from outer space. Tektite stone is a kind of pellet glass stone which has elongated shape, or shapes or drop, peach seed. Its color is master black, glittering and glossy. Tektite is used in jewelry and handicrafts.

3.4 Active Group

Basaltic and andesitic rock that is greisenized usually has radiation intensity of 50-70 μRh.

At anomalous points, radiation intensity can be up to 70-170 μRh.

Within Dalat, there is one anomalous point: Prenn.

Prenn lies in Don Duong formation fensitic rock. Stone veins are 1-2m wide and run in northeast-southwest direction, penetrating through Don Duong formation tuff eruptive rock. Observed exposed area is 100-150m long and 50-70m wide. Radiation level is high, up to 1700μRh. Radiation intensity of the rock surrounding porphyritic felsites veins is 26 - 28μRh. Composition content is U:52.9g/ton, Th:18.0g/ton, Ce:55.7g/ton, La:38.6g/ton.

4. Terrain

Da Lat plain is a part of Da Lat zone. Since long ago people have called this plain Lang Biang or Lam Vien plateau. Among the plateaus and plains of Tay Nguyen, Da Lat is the oldest, formed from late Paleogene to Miocene. Other plateaus are actually not typical.

Da Lat plain has an average height of 1500m, lowering from 1600m in the North to 1400m in the South. The West and Northwest borders are marked by bow-shaped mountain ranges of almost 2000m high. There are a few residual mountains of 200-400m high. Da Lat plain is of ancient valley type, with the best-preserved ancient flat surface in Vietnam. This surface is composed of shale, aleurolite and eruptive sediment that have been strongly dissected with amplitude of 100m to form quite steep hills. These hills have weathering crust up to and even exceeding 30m thick. Outwash process plays the main role in this area. However, the residual mountain slopes are still developing by gravity erosion.

Da Lat plain has two types of plain, mountain and plain in mountain.

4.1 Mountainous Terrain

Mountainous terrain is distributed around the highland centre area.

From Dalat, looking to the north is the majesty LangBiang mountain running in the northeast-southwest direction from Da Sar fall to Dan Kia Lake. This mountain range has a few peaks among which the highest ones are 2167m and 2064m high.

Along the border between Da Lat city and Lam Ha district, from North Mang Lin to Tuyen Lam Lake are mountain ranges of above 1600m height. These ranges have bow- shape and obstruct the South of the city. A few of the highest ones are You Lou Rouet (1.632m), Nao K’lan (1.763m), B’Nam (1.710m), and B’Nam Qua (1.666m).

The mountain blocks, running in northeast-southwest direction, is mainly composed of granite rocks. Their profiles and cross sections are very long. This is the mountainous area belonging to the transitional margin to Duc Trong-Lam Ha highland, so the Southwest slope is steeped.

To the East, mountains are arranged in bow-shape running north-south and embracing the centre trough. Those mountains are North Lap-be (1.733,7m), South Lap-be (1.709m), Gio Hu summit (1.644m), Bnom R’Me (1.570m), and Tan Ho (1.633m).

North Lap-be and South Lap-be are composed of shale and aleurolite. Gio Hu summit is composed of granite. These are residual mountains with round and quite even top, and relative height of about 150-200m.

From the bow of North Lap-be - South Lap-be - Gio Hu summit to Dalat-Don Duong border, there are hills and low mountains of 50-100m relative height, alternating with feather valleys.

Composed of granite, dacite, some small-scale terrains have Miocene to Peistocene continental eruptive basalt that forms quite even surface (Cau Dat tea factory area).

In the South, mountainous terrain transits to lower terrain. The typical area is the Prenn mountain pass with deep valleys. Mountains here have deep profile, big cross section, relative height of 200-300m, and steep slopes. Geological structure is complicated with mainly acid rock and shale.

4.2 Plain-in-Mountain Terrain

This type of terrain is distributed in the city centre. It is surrounded by hill ranges with round top and gradual slope running downwards Xuan Huong Lake centre of 1477m high. Most of them are bald hills with relatively equal height. The farther from the centre, the higher the surrounding mountains, which makes the city centre an oval cauldron with the 18km long axis in north-south direction and the 12km short axis in east-west direction. The surface is even. River valleys’ profiles do not exceed 50-70m, and cross sections do not exceed 0.3-0.7km/km2. A few residual hills formed by erosion and pedimentation have round tops and very gradual slopes running in north-south direction. Some of such areas are Cu Garden, Lam Dong General Hospital, Hoa Binh area, Da Lat Univeristy. Geological architecture consists of continental sediment formations and acid magmatic rocks (mainly distributed in South Cam Ly river) and shale.

5. Geological Structure

5.1 Parent Rock

In Da Lat area, many types of rock were formed in different tectonic geological conditions and periods. Those are magmatic, sedimentary, and metamorphic rocks.

5.1.1 Magmatic Rock

In intrusive or eruptive form, magmatic rocks mainly have mineral crystals such as quartz, feldspar, mica, amphibole, peridote, and pyroxene.

Common magmatic rocks in Da Lat are granite, dacite, and basalt.

Granite is very hard acid magmatic rock erupted from volcano. The rocks that have yet to disintegrate and are located at about 1 - 10 meters deep remain gray, white, or as black as their elements like quartz (SiO2), feldspar (feldspar) and mica.

Granite is very common and distributed in Southeast Da Lat (Trai Ham, Trai Mat), Sam Son, and around the Ta Nung area... It has large exposed area and many mines that have been exploited.

Dacite stone is also a type of granite but is composed of very small particles. It is distributed in Prenn Pass, Ngo Quyen, Da Cat and the area of North Dalat.

Basaltic Rock

Basalt rocks are basite magmatic rock, which are black, heavy and also erupted from volcano. These rocks are located 3 to 10 meters deep or above. Rocks that have yet to disintegrate have the black and olive gray color of pyroxene (containing Mg, Fe), tin magnet (rich in Fe) and micro-feldspar.

Basalt is distributed in small areas of Van Thanh, Cam Ly airport, Ta Nung center, Cau Dat.

In acidic magmatic rocks, quartz particles are hard and unlikely to disintergrate, but on the contrary, feldspar crystals can easily transform into clay.

In magmatic rocks, potassium feldspar (ortoze) is more difficult to decompose than calcium and sodium feldspar while - iron and manganese silicate (as pyrotcene, olivine) are very susceptible to damage.

Acidic feldspar that is rich in silicon oxide SiO2 hardly disintegrates. The alkali feldspar containing less than 52% of SiO2 easily decomposes. Therefore basic magmatic rocks decompose faster than acidic magma rocks. On these rocks, weathering layers are usually thicker.

Basic magmatic rocks decompose faster, and the products contain less quartz and clay. In addition, rocks containing iron and manganese when decomposed will produce iron oxide. This oxide turns soil into red, red brown or yellow red depending on the oxide’s hydrate level.

5.1.2 Sedimentary rock

The area of Da Lat has shale, aleurolite, and sandstone. The stones are usually composed of many quartz particles held together cement by a kind of silicone cement. Terigneous Mesozoic sediment rocks are distributed in low-lying areas, the North, the Northwest and the East of the city centre.

When cement links are destroyed, quartz particles are released and therefore the soil formation is very sandy.

5.1.3 Metaphoric Rock

Metamorphic rock resembles magmatic rock in the crystals and resembles sedimentary rock in the parallel layer arrangement. Sedimentary rock was deeply buried, melted and re-crystallized into metamorphic rock.

Metamorphic rock is distributed in small areas of Phuoc Thanh stream, and South of Ward 11.

Despite its crystal structure, metamorphic rocks are more easily mineralized than magma rock because mica is arranged in parallel thin layers alternating with layers of quartz and light-color feldspar.

5.2 Mineralization

5.2.1 Ferralitization

In places that are well drained and have porous rock, rich in calcium, poor in silicone, silicone will be released completely dissolved and taken away together with calcium and magnesium. Soil will be rich in iron oxide (Fe2O3), aluminum oxide (Al2O3), especially free Al2O3. It is iron-aluminum soil, with SiO2/Al2O3 ratio less than 1.7. In Da Lat this type of soil is common the hillsides with good drainage and deep water supply levels.

In places where silicon vertical movement is incomplete due to high-silicone parent rock or inefficient drainage (low level water supply, hollow or flat terrain), there is formation of kaolin from silica combined with aluminum oxide. This is the soil is rich in kaolin, and poor in free Al2O3. Its SiO2/Al2O3 ratio is less than 1.7. In Da Lat this soil is often observed on acid magmatic rock, in flat areas with shallow water supply level.

In wet areas with poor drainage, silicon oxide (SiO2) as well as the base cannot base on water to infiltrate into lower levels (muddy soil, water supply at ground level). If parent rock release calcium, magnesium (like basalt), then it would create a variety of clay-rich silicon such as montomorillonite. This is often observed in low-lying areas with poor drainage.

Most soil in Da Lat is classified as feralit land which has colors of reddish brown, yellowish red to brown, golden red, reddish yellow, or yellow due to iron oxides present in the soil. As it goes higher, the ferralitization process is weakened, and aluminum accumulation will outweigh iron accumulation; Therefore, SiO2/Al2O3 ratio is higher in rings below 1000m deep.

Some areas have very intense ferralitization and have iron and alumni oxides in the same elevation layer. Those oxides may be brought from upper levels. Or perhaps because of the complete base loss and the lack of silicon, kaolin cannot be formed and the soil is rich in free Al2O3. In this condition, there will appear layers of laterite soil. If this type of soil is fiercely eroded due to lack of vegetation cover, the surface soil will be washed away, exposing laterite layers. Fe++ that is soluble in water will be oxidized into rigid Fe+++ and turn the soil into rocky surface. This phenomenon usually occurs in small scale on bald hill areas of Cam Ly, Phuoc Thanh, Nam Ho, Trai Mat,etc.

5.2.2 Humus Accumulation Process

As temperature decreases in higher elevation, ferralitization process is also weakened and makes room for the humus accumulation to develop. However, Da Lat is not high enough for the ferralitization process to stop completely and give way entirely to the accumulation of humus.

On the higher areas of Dalat, with low temperature, well developed forest flora, rugged terrain, and little exploitation, the rate of organic matter and humus in soil is high. Main vegetation component is pines with high rates of persistent substance. Therefore, the rate of humus in soil is quite high.

5.2.3 Erosion, Washout, and Accumulation

Downslope Movement of Soil and Rock

This process could be divided into three types:

- Rock chute: occurring quite often on high and steep slopes of above 40o, at mountain foots, or in riverbeds to form heaps or streams of stones.

- Rock collapsing: more common in mountain slopes of greater than 30O and composed of weathered magmatic and metamorphic rocks.

- Shear sliding: occurring on steep mountains and hills that have fierce tectonic lifting and erosion activities.

The above types are commonly found on Prenn Pass, and the Da Lat - Ta Nung road.

Erosion and Ephemeral Streams Process

Due to Dalat’s high rainfall intensity in rainy season, this process is very strong, especially on the hillsides that have thin vegetation cover.

Overflow water erodes and washes away colored substances, leaving behind ditches and ephemeral streams of 10-20m to hundreds of meters long, and 0.5 - 5m deep.

Corrosion phenomenon occurs violently on vegetables and flower lands (Tay Ho, Nam Ho, Trai Mat, Tung Lam, Da Thien, Sam Son, etc) and on open land, bald hills around Dalat.

In many places, erosive water washes aways arable layers, exposing parent rocky layer and forming rocky eroded soil. This is commonly observed in the top and hillside areas such as Da Thien, Tung Lam, Xuan Truong, and Xuan Tho.

In the soil profile, washout occurs vertically. Water from upper layers infiltrates to lower layers and carries along soluble material and leaves behind accumulated insoluble substances.

The Erosion Process

The streams in Da Lat are mostly very steep and have yet to reached even elevation. Discharge difference between seasons is very large, so the flood erosion process occurs very violently. In the monsoon season, flood water causes bank to collapse, sweeps away many corrosive products.

Accumulation and Accretion Process

Under the influence of weathering processes and corrosive water, upstream rocks are gradually destroyed, big stones into small stones, small stones into fine mineral. Fine minerals are hydrolyzed, hydrated, oxidized into simple inorganic compounds which are swept away by the streams. As the flow slows down at gradual slopes, the solid materials carried are deposited in certain sections.

6. Climate

With the advantage of elevation and forest vegetation populations, particularly the pine forests, Dalat inherited mild mountainous climate, and it is balmy all year round.

6.1 Solar radiation

6.1.1 The height and duration of solar lighting

In Da Lat in the sun passes the zenith twice a year: the first time is on 22 of April, the second time is on 22 of August. The time between two zenith-passes of the sun is 4 months. April and August is the month with the largest solar height altitude, that in December is the smallest.

Day length in the season does not differ much: the average daily range of 11 to less than 12 hours (in winter) and over 12 hours (in summer).

6.1.2 The level of radiation distribution

Total amount of radiation in an ideal medium in Dalat is 140 kCalo/cm2/year. The ideal distribution of total radiation among months of the year differs: April has the most (over 16 kCalo/cm2), August has the least (only 9.3 kCalo/cm2). This is related to variation in the height of the sun. The greater the amount of solar height is, the more the ideal radiation is.

In the rainy season, the radiation income reduces. September has heavy rain, the overall average clouds is over 8/10 sky, this is the month when radiation income reached the lowest value in the year.

During the dry season, the ideal radiation is larger and it lowers surface temperature at night. It is also one of the causes of the formation of frost in January and February in Dalat.

Ground radiation received during the month are greater than the amount of lost radiation. This result is shown in the balance of radiation. The maximum value of Da Lat radiation scale falls in May and minimum in December. The great average radiation intensity all year round of Da Lat has given abundant energy source. In addition to topographic height, distribution of the radiation level is the decisive factor to the thermal regime, so Dalat temperature is always lower than the surrounding plains.

6.2 Atmospheric circulation

Da Lat has two seasons: dry and rainy seasons.

The dry season coincides with the northeast monsoon season, which lasts from November to April of the next year.

The rainy season coincides with the southwest monsoon season, which starts from May and ends in october.

During the dry season, Da Lat is influenced by air masses of the East Sea. This is the main air mass of winter winds, bringing warm weather, less clouds, no rain, low temperature at night, large temperature range. However, in the months of November, December and January, this air mass occasionally replaced by extreme tropical marine air mass. When the air mass is strongly active in the North, in Dalat it is cloudy, rainy with strong winds, the strongest wind speeds can reach over 20m/s.

During the rainy season, from May, northeast monsoon barely affects Dalat again, instead, equatorial air masses from the south moving northwards, controlled Indochina. Southwest monsoon is created and promote its influence in the region bringing major source of moisture for heavy rain and the long rains. In this season, the weather is bad, cloudy and rainy, average daily temperature and humidity are high. However, tropical air masses of Pacific occasionally controlled Dalat in high pressure mode edge, so in the rainy season here, there are times the weather became clear, not so rainy.

6.3 Heat Mode

6.3.1 The average air temperature

Dalat has the low temperature. The monthly average (MA) temperature ranged from 15.8 to 19.3OC, in the hottest months MA temperature does not exceed 20 degrees Celsius, in the coldest month, the MA temperature is not lower than 14OC.

According to statistics, the average of many years (AMY), from 1964 to 1998, annual average (AA) temperature is 17.9OC, the year with the highest temperature is 1973 (18.5OC), the year with the lowest temperature in 1967 (17.4OC), values of AA temperatures ranged from 17.4 to 18.5OC.

FACTOR OF (AMY) MONTHLY TEMPAERATURE (OC) AT DA LAT STATION (1964-1998)

Factor

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

All year

T.Tb

15.8

16.7

17.8

18.9

19.3

19.0

18.6

18.5

18.4

18.1

17.3

16.2

17.9

T.x

28.5

29.2

29.8

29.2

27.6

26.7

26.4

26.7

26.4

25.9

25.9

26.0

29.8

Tx.tb

22.3

24.0

25.0

25.2

24.5

23,4

22,8

22,5

22,8

22,5

21,7

21,4

20,6

T.n

4.3

5.3

6.2

10.8

11.3

12.3

13.0

12.8

12.3

8.7

8.0

5.4

4.3

Tn.tb

11.3

11.7

12.6

14.4

16.0

16.3

16.0

16.1

15.8

15.1

14.3

12.8

14.3

T.tb : AMY monthly temperature

T.x : monthly highest temperature

T.n : monthly lowest temperature

Tx.tb : highest AMY monthly temperature

Tn.tb : lowest AMY monthly temperature

In Da Lat, January is the coldest month of the year with MA temperature is 15.8OC. From February, MA temperature increases and reaches maximum value in May (MA temperature is 19.3OC), then decreases gradually until the end of the year. According to AMY data, the lowest MA temperature is 14.3OC (in January 1965). In the coldest month, the MA temperature in Da Lat is still higher than 14OC.

Although located in the tropical monsoon climate, but in Da Lat, there is almost no day in which the average temperature is higher than or equal or lower than 10OC -25OC.

6.3.2 The highest and lowest air temperature

The highest MA temperature fluctuations is from 21.4 to 25.2OC, the maximum value of the highest MA temperature is 26.6OC (March 1964) and the minimum value of the highest MA temperature is 19.3 (December 1995 ).

The lowest MA temperature fluctuations is from 12.8 to 16.3OC, the maximum value of the highest MA temperature is 16.9OC (in 1973 and 1987) and the minimum value of the highest MA temperature was 8.9OC (January 1965).

According to monitored data series from 1981 to 1998, the highest monthly temperature in Da Lat ranged from 25.9 to 29.8OC and the highest value of absolute temperature is 29.8OC (March 1998). The lowest monthly temperature in Da Lat ranged from 4.3 to 13OC and the lowest value of the absolute temperature is 4.3OC (January 1996).

6.3.3 The amplitude of temperature

The amplitude of diurnal temperature in Da Lat is very large, the average amplitude in a year is 11oc. In the middle months of the dry season (I - III) the amplitude is very large (the value can reach 13 - 14OC) and in the rainy season diurnal temperature amplitudes are small (only about 6 - 7OC). Compared to some other provinces of the Central Highlands, Dalat is the place which has a large diurnal temperature amplitude. Besides the range of Dalat AMY temperature among months are small, the difference between the warmest and the coldest month is about 3.5OC. From month to month, temperature rarely changes, during the rainy season this change is not significant, the dry season, this disparity has increased a little bit, but also ranges from 1 to 1.5OC.

6.3.4 Temperature Fluctuations

The temperature is always changing from year to year. In the north, this change is quite large, the average temperature of a month in each specific year oscillates around the average value of 3 - 4OC, while in Dalat this value does not exceed 1 oc. During the rainy season, the temperature variation is negligible (fluctuation value is only from 0.1 to 0.3OC), but during the dry season this value is greater, but it is only up to 0.9OC.

6.3.5 The temperature accumulation

In Da Lat, the yearly temperature accumulation in levels of 5, 10, 15 is small, from 6200 to 6.500OC, winter-spring: 2200 - 2.500OC, harvesting season: about 4.000OC. Efficient yearly temperature accumulation t ≥ 5OC is 4.700OC, t ≥ 10 OC is 2.800 OC, t ≥ 15 OC is 1.000 OC. As weather conditions are not the same each year, winter can come early or late, be long or short, so the total yearly temperature accumulation of the years is different.

According to the data observed, the coldest year is 1967:

t ≥ 5 OC : 6.356 OC

t ≥ 10 OC : 6.356 OC

t ≥ 15 OC : 5.92 OC

The hottest year is 1973 :

t ≥ 5oc : 6.718 OC

t ≥ 10 OC : 6.718 OC

t ≥ 15 OC : 6.656 OC

6.3.6 Comparison of the temperature in Da Lat at the beginning and end of the twentieth century

The AA (annual average) temperature between two stages B (1964 - 1980) and C (1981 - 1988) has no big difference. The average temperature between months and the highest temperature, lowest temperature are somehow different, but the difference is in the acceptable level. Thus, the temperature in Da Lat from 1964 to 1998 is very stable.

The AA Temperature in period A (1918 - 1940) is greater than B and C from 0.4 to 0.5 OC, there were months in which the difference was up to 1 oc. The AA Temperature in A is higher than the B and C phases, but it can be said that this discrepancy is not very big.

The highest temperature factors in phase A is clearly higher than that of the next 2 stages, and the lowest temperature factors in phase A is completely lower than that of phase B, C. The difference is very large, and the maximum value, or minimum value almost occurred in period A, it did not occur in two following stages.

There are several causes for this discrepancy, including data measurement position, monitoring methods, and machinery although it plays an important role, it is not very big reason.

Another reason is that human activities change the living environment and lead to changes in climatic factors.

From 1918 to 1940, weather in Da Lat was relatively harsh. Such harshness clearly reflected in the diurnal temperature range (in the dry season, there is the warmest time that it reached 31.5 OC, but at night it is lower than 0 OC). It is clear that the day is hot and it is very cold at night. But from 1964, there is almost no year when the highest temperature is up to 30 OC and the coldest temperature is lower than 0 OC. The AA temperature difference between the beginning and the end of this century is not very big.

TEMPERATURE IN DALAT THROUGH PERIODS

Periods

Factors

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

All year

1918- 1940 (A)

T.tb

16.7

16.9

18.2

19.2

29.8

19.6

19.1

19.0

18.8

18.3

17.7

16.2

18.3

T.x

30.0

31.0

31.5

31.2

30.6

30.0

29.2

29.3

29.7

30.0

29.2

29.4

31.5

T.n

-0.1

-0.6

4.2

4.0

10.0

10.9

10.4

10.6

10.0

8.1

4.4

2.6

-0.6

1964-1980 (B)

T.tb

15.8

17.0

17.9

18.9

19.1

19.0

18.5

18.4

18.3

18.1

17.2

16.5

17.9

T.x

28.7

28.1

28.9

28.9

27.0

26.5

26.3

26.0

26.5

25.5

26.3

26.2

28.9

T.n

4.9

5.0

6.4

8.9

12.9

12.5

12.9

13.0

12.2

10.2

8.0

6.8

4.9

1981-1988 (C)

T.tb

15.7

16.1

17.9

18.8

19.4

19.1

18.6

18.5

18.4

18.0

17.2

16.0

17.8

T.x

28.5

29.2

29.8

29.2

27.6

26.7

26.4

26.7

26.4

25.9

25.9

26.0

29.8

T.n

4.3

5.3

6.2

10.8

11.3

13.4

13.0

12.8

12.4

11.0

8.0

6.4

4.3

6.3.7 Comparison of the temperature in Da Lat with temperature in Ho Chi Minh City

Although Dalat and Ho Chi Minh City are located in the tropical monsoon climate, there is a big difference in the AMY temperature because they are in two different geographical locations and they has large difference in the height from the sea level.

AMY temperature in Da Lat is 9.1 oc lower than AMY temperature in Ho Chi Minh. March has the biggest difference (10 OC), September has the smallest difference (8.3 OC). Temperature difference between the months in Dalat and Ho Chi Minh City is relatively large, ranging from 8.3 to 10 OC.

6.4 Sunshine volume

Dalat is where the total number of hours of sunshine a year is relatively high: the year that has the highest total number of hours of sunshine is 1987 (2,507 hours) and that with the lowest total hours of sunshine is 1996 (1,883 hours). Total hours of sunshine a year in Da Lat range from 2507 to 1883 hours. On average, a year has about 2,258 hours of sunshine, hours of sunshine of a year are mainly in December, January, February, March and dry season.

The month with highest AMY total sunshine hours is March (263 hours) and the lowest one is September (126 hours). AMY total sunshine hours between months is ranging from 126 to 263 hours.

6.5 The amount of cloud

Overall, AMY amount of cloud in Da Lat ranges from 5 parts of the sky (PS) to 9 (PS) but it seasonally varies. The AMY Cloud amount was 7 PS, the amount of cloud below AMY is 5 PS. The ratio between the amount of below clouds and AMY cloud amount is 74%.

From April, May, cloud amount increased and in September, october, it reaches the maximum value (9 PS). That was the period when the tropical Indian ocean air masses are most active. Average monthly amount of cloud in rainy season is from 6 to 9 PS.

Often at the end of october, early November, when the southwest monsoon completely lost its influence on Lang Bian Plateau, the amount of clouds decreases markedly. Particularly, the period from January to March is when the tropical air masses operate strongly, the cloud amount is small, only about from 2 to 3 PS.

Variation of under-cloud amount is similar to the overall volume of cloud. The greatest amount of cloud is in the months of June, July, August and smallest in the months of January, February, March.

6.6 Relative humidity of the air

MA relative humidity of the air TBT in Dalat reaches the maximum value in August (91%) and minimum value in March (77%), ranging from 77 to 91%.

AMY HUMIDITY OF THE AIR(%)

Factor

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

All year

U.tb

82

78

77

84

87

88

90

91

90

89

85

84

85

U.n

17

13

9

20

43

47

53

48

51

39

27

24

9

Overall, in the rainy season (from May to october), AMY relative humidity of the air in Dalat is high (87-91%), but in the dry season it is only within the range of 77 - 85%.

The year with highest AMY humidity is 1966 (88%), the year with lowest AMY humidity is 1977 (81%), so the amplitude of AMY relative humidity in Dalat is 7%.

The largest value of MA relative humidity of the air is 94% (August 1990) and the smallest value of MA relative humidity of the air is 64% (March 1964).

However, during the rainy season, there are days when the relative humidity values of air reaches 100%, whereas there were days when it was only reaches the value of 9% (March 1998).

6.7 Wind Volume

Although the terrain conditions has strong impact on the wind directions, but wind in Dalat retains two main and typical directions: in dry season (from early September to the end of April of the next year), prevailing wind direction is east north;

- The rainy season (from early May to the end of october), the prevailing wind direction is mainly west.

From october, the northeast monsoon have affected the weather in Da Lat. The wind is active in the months of November, December and January of the next year with the frequency from 45 to 65%. In February, the frequency of northeast wind reduces and reaches only about 26%; in March, April, the east wind has more advantages than the west north wind; however its frequency is hardly over 20%.

From May to September is the period when the west wind is active, interspersed with southwest and northwest winds with frequency range from 10 to 15%. West wind is most active month in August with a frequency range above 60%.

Due to high topography, wind speed is relatively large in Da Lat, the average wind speed reaches approximately 2.2 m/s.

Based on the average wind speed and frequency of calm weather, the wind in Dalat can be divided into three periods:

- Calm period with the average monthly wind speed from 1.2 to 1.3 m/s, the frequency of calm is over 50%, occurs in February, March, April.

- The period of light winds has an average monthly wind speed from 1.5 to 2.1 m/s, the frequency of calm winds is around 30 to 45%, occurs in the months of January, May, September, october

- The period of strong winds has an average monthly wind speeds above 2.5 m/s, the frequency of calm winds is around 15 to 30%, occurs in the months of June, July, August.

Period of strong winds in Dalat is divided into two phases:

The first stage is when the southwest wind is strongly active associated with disturbances such as typhoons, tropical depressions often occurring in the months of June, July, August. Wind speed of this phase has been observed and is over 20m/s.

In the months of November, December and the next January, when the northeast monsoon blows back strongly in the north of our country, in Dalat, the second phase of strong winds appeared. Northeast monsoon usually less affected southern regions, but due to Dalat at altitudes above 1,500 meters above sea level, the impact of this wind is quite clear and strong. In the months above, on the average, wind speed here is up to 3 to 3.5 m/s. Strongest wind speeds can reach over 20 m/s. Strong winds occurred continuously in waves, each wave is lasting several days, sometimes up to 5-6 days.

Because the impact of tropical disturbances landed in some neighboring provinces, in the months of September, october, November, strong winds happen not often every year, but with strong winds of level 6-7, gust of level 8-9 and it blows continuously for several hours, combined with heavy rain, bringing great damage to life and production of the people.

6.8 Precipitation Volume

6.8.1 Starting and ending time of rainy season

The rainy season usually starts in Da Lat last in the end of April, early May and ends in later october, early November. However, the annual rainy season can move, starting and ending time is different, sooner or later, but generally in Dalat rainy season lasts more than 6 months.

6.8.2 Precipitation quantity and quality

Total AMY monthly rainfall is 1,739mm in Dalat. Rainfall volume is concentrated in the months of July, September and october. Those are months of the active southwest monsoon. Total yearly rainfall is greatest in 1973 (2,191mm) and in total yearly rainfall is smallest in 1965 (1,076mm). During the rainy season, the month of highest rainfall is September 1973 (493mm) and the month of smallest rainfall is May 1988 V (41mm).

Total rainfall during the rainy season accounts for about 78.7% of total yearly rainfall (if taking the average from May to october). During the dry season the total rainfall is only about 21.3% of total annual rainfall.

AMY RAINFALL VOLUME (mm)

Factor

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

All year

RRR

11

24

62

170

191

213

229

214

282

239

97

36

1.739

JJ

2

2

5

11

18

20

23

22

23

19

10

5

161

RRR : AMY total rainfall in a month (mm)

JJ : AMY number of days which has rain in a month (days)

At the beginning of the rainy season, there are usually showers and thunderstorms in the afternoon. The rain did not last long but the rain intensity is fast so it can also bring a significant rainfall (May, June). When the month began to step into July, the southwest monsoon is active and thickening, often with moderate rain, heavy rain, continuing several days. Months of September, october are usually two months of most continuous rain, even the heavy rain as it combines effects of the southwest monsoon and several tropical disturbances (storms, tropical depressions, thermal equator,...), largest tropical daily rainfall can be measured is of 140 mm/24 hours. Heavy rains and prolonged rainfall has made a total of this two months rainfall in Dalat often reach the maximum volume during the rainy season.

6.8.3 Number of rainy days

On average, a year has about 161 rainy days. The year with the most total number of rainy days is 1996 (206 days), the year with the least total number of rainy days is 1965 (96 days).

The months which have the most total of number days with rain are July, August, September, october and the months which have the least total number of rainy days are February and March.

6.9 The other extreme weather

6.9.1 Thunderstorm

On average, each month in Dalat has about 5 to 6 days with thunderstorms. Months that have the most number of days with thunderstorms are April and May (13 to 14 thunderstorm days a month). In two months of December and January, there is almost no thunderstorm, these are the two months of the prevailing northeast monsoon.

However, the total number of days with thunderstorms occur in a year more or less well depend on the weather regime of that year. According to AMY statistics chain, the year with the most number of days with thunderstorms occurring is in 1997 (106 days), the year with the least number of days with thunderstorms occurring is 1984 (37 days).

6.9.2 Hail

Hail is a special form of ice rain. Hail usually occurs in the hot period of the year. In Da Lat hail usually occurs in the months of April, May and June, especially the first two months of the rainy season. During this period the buffer zone receive large amounts of radiation, the heat of midday increases and the afternoon is hot, the high streams develop strongly, thermodynamic conditions are favorable for the formation of strong thunderstorms, rainy clouds grow faster, cloud top is very high and hail easily arise.

The size of the hailstones depends on the conditions of its formation. Usually hailstones are about pea-size, others are larger but not much.

Hail rarely happens, but when hail occurs, it damages agricultural production, mainly due to mechanical break that destroy plants, especially trees, vegetables.

6.9.3 Fog

Fog is the phenomenon of water vaporing in the atmosphere condenses into suspended tiny water particles in the air.

In Da Lat is the most common type of fog is radiation fog formed when the buffer (the ground) was colder due to radiation during clear, windless weather. The effect of the rise of air moisture on the mountain wind also made contributions to the formation of fog. When the sun warms the ground up, the fog disappears quickly, in a wet and cold valley, fog may persist longer.

Thick fog is relatively less common. The rainy season usually have this kind of fog over the dry season, especially September and october, on average, there’re about 4 to 5 days with thick fog. The main reason is that during this month of heavy rain, high humidity, low wind, long night and the sun's energy that is absorbed at the surface, the lower temperatures create favorable conditions for the formation and development of fog.

In Da Lat on average, a year has from 80 to 85 days with fog appearing. the year with the most number of foggy days is 1998 (128 days), the year with the least number of foggy days is 1985 (45 days).

Frequency of fog appearing most is in February-May and September-october. March has the most number of foggy days. This is the time when the ground is the most heated period of 6 to 16 hours a day, then showers and thunderstorms usually happen in the evening, to create favorable conditions for the formation of radiation fog.

6.9.4 Hoarfrost

Hoarfrost is formed in the following conditions:

- Temperature base and soil temperature is low;

- Small wind speed or calm weather;

- The sky is often little cloudy or clear.

In Da Lat hoarfrost usually occurs in the month of January and February. In March, the hoarfrost phenomena can occur, but mildly. Hoarfrost causes considerable damage to crops.

6.10 Air pressure

Air pressure in Dalat is lower than that in the plains, the average is about 644mm Hg, making sense of light and refreshing. Dry air contains much more oxygen and less water vapor, that makes breathing easier, faster regeneration of red blood cells, people feel comfortable, the intellectual labor force feel less stress, less work fatigue, better endurance.

7. Hydrologic

7.1 Characteristics of stream water

In Da Lat city, there are more than 20 streams of over 4km in length. They are all headwater streams in the Dong Nai River’s basin. 14 of the 20 streams originate from the altitudes of above 1,500 meters.

As headwater streams, dry ones are over 50%. These streams flow only in rainy season.

The streams are flowing over a mountainous area with high altitude, and deeply dissected terrain. Therefore, the average gradient in some basins is up to 30 to 40%. The flow, driven primarily by topography, usually runs along the valleys between mountains.

With the stream net density ranging from 0.6 to 0.9 km/km2, high mountain areas in the North, East and South of the city have the densest flow net. Because most streams have not reached even elevation, deep erosion is dominant. Riverbeds are deep, narrow, steep, usually do not have terrace. Index of meandering is low.

Because the formation of flow is directly influenced by geographical environment (climate, topography, geology, soil, vegetation), the streams’ annual flow modules fluctuate within 35 to 40 l/s/km2.

Rain in Tay Nguyen in general and in Da Lat particularly is distributed evenly over time. Number of rainy days is large, and rainfall distribution is also quite even. Therefore, surface flow is reduced and underground flow is increased, which keeps the monsoon season not as severe as in other areas.

Topography, geology and soil affect the hydrological regime through water permeability and water retention.

In the high mountains of the North, East, and South of Dalat, and in the residual mountains, the terrain is steep, and its lithological composition is mainly acidic magmatic rocks with low water permeability. In spite of that, the land has a positive effect in retaining water, reducing surface flow and increasing water infiltration thanks to a rather thick weathering crust, and a floral of mostly pine. On the other hand, the streams in this area often have deep bed and can easily get supply from groundwater sources, so the hydrological regime of the streams in the region rather stable. Particularly in open lands, bare hills in the West and Southwest of the city, because there is no vegetation cover to moderate the flow, season differentiation is more severe.

The surface of the central area has ancient-valley shape, and is quite flat. The lithological composition is shale and granite. On the bedrock layer is a weathering crust with average permeability, but vegetation cover is very thin (vegetable gardens or residential lands). So monsoon seasons often come and end in similar period with rainy seasons, and season differentiation is more severe. In this area the proportion of shallow one-season stream is more than in the above area. Stream beds are not deep because of small bank gradient. This property is evident for streams originating from the low-lying city centre of Da Lat (Phuoc Thanh stream). It is not evident in the other streams because most of them start from high mountains and flow through the hollow center of the city.

Flow regime in Da Lat depends on climate and is divided into two distinct seasons: flood season and dry season.

Da Lat’s rainy season starts in May. Streams are dry after the previous season, so early-season rain cannot bring surface flow but only infiltrate into the soil. Thanks to high soil permeability, monsoon season comes 1.5 to 2 months later than rainy season.

The period from May to July is the transition to flood season in the streams of Da Lat. During the transition time, the basin is more humid and enters the flood season in July, the same time for heavy and prolonged rains of the wet season to begin.

The rainy season ends in october with a not so abrupt transition. Spring water slowly recedes and during this process there are often big floods caused by late season rainfall into the full basin. During the transition from flood to dry season, underground water has significant role in regulating stream flow and reducing the differentiation in the flow regime. The flood season actually ends in late November.

In flood season, stream discharge is about 70% of the whole year discharge. In october there are biggest floods of the year, and stream discharge in this month accounts for 20% of the entire year. Early-season floods are usually caused by thunderstorms rain. Mid-season floods are affected by storms. End-season floods are associated to storms and heavy rainfall.

As soon as the rainy season ends, water in the streams starts to recede. April has the smallest flow volume, about 2% of the entire year. The streams with small basins (<5km2) in the Southwest, Southeast of the city originate from the altitudes where there are no forest vegetation covers and weathering layers are thin. Therefore they are dried out very soon.

7.2 River and Spring System

7.2.1 Spring System in Northern Dalat

This system of spring is located in the Northern of Da Lat city and flow into Dan Kia Lake. It has two branches: Da Thien spring and Phuoc Thanh spring.

Phuoc Thanh Spring is 4.1 km long, originating from Da Thanh cluster, and has the highest elevation of 1,566m. The stream flows in the south - north direction in the valleys in hilly areas then combines with Do spring and flows into the Dan Kia Lake. Its basin with mostly bare hills, residential land, and arable land to grow vegetables does not have ability to regulate flow. The lowest flow module measured is 7.4 l/s/km2.

Phuoc Thanh spring is the irrigation source for vegetables and flower planting areas in the North of the city (Tung Lam, Phu Da, Phuoc Thanh, Thanh Mau). During the dry season, exploitation at high intensity exhausts the flow water.

Da Shan Spring is 10.5 km long, originating from a mountains of nearly 1550m high, which is close to the administrative boundary between Da Lat city and Lac Duong district. The stream flows from north to south in the valleys in hilly area, then deflects towards the east - west direction and into Phuoc Thanh stream. In 1973, a weir was built upstream to collect water for irrigation of Da Thien vegetables farms. Da Thien Lake is located in a green pine hill which has become the Love Valley tourist area.

7.2.2 Camly Spring System

Cam Ly Spring is 64.1 km length and is the largest spring system in Da Lat. It plays an important role in creating the downtown area landscape. Annual average flow at Xuan Huong Lake is 0.7 m3/s. The yearly average total amount of water is 22.1 million m3.

The upstream portion has two big branches:

Branch 1 is 3.7 km long, originating near Lake Da Thien, and running from north to south. The catchment area is small, and mostly agricultural land. Therefore it has low discharge and is poorly moderated. Two small dams have been built for vegetables irrigation. They are dam I and II Da Thien.

Branch 2 is 5.4 km long, originating near a 1593m mountain in Da Sar commune (Lac Duong district). It flows in the northeast - southwest direction. In 1981, a dam was built and Lake Victory was formed.

Phan Dinh Phung Spring is a tributary stream of Cam Ly with the length of 5.7 km. Originated in the Thanh Mau region with the highest elevation of 1.566m, the stream flows from north to south and is near Xo Viet Nghe Tinh street. After flowing under the bridge on La Son Phu Tu street, the stream flows between Phan Dinh Phung and Hai Ba Trung streets then meets Cam Ly spring down Nguyen Thi Dinh street. Phan Dinh Phung Spring is a natural drainage line along the north - south axis of the ward areas1, 2, 6.

Before, in the area near Tran Hung Dao School (old), the French dammed up the stream, forming the Da Thanh Lake (Van Kiem Lake). The lake was later silted up and became a vegetables garden.

Cam Ly Spring originates from Da-ra-hoa region (Lac Duong District) and receives water from Lap-be Nam and Lap-be Bac mountains. Stream flows from north to south, receiving water from branch 1 and branch 2, then poured into the Xuan Huong Lake.

In 1938, the French put up a small dam on the stream, forming Than Tho lake. A water plant was built next by to supply clean water.

After the completion of the Than Tho Lake dam, a smaller dam downstream was also built, forming the Saint Benoit Lake (Me Linh Lake). Up to now it has been completely silted up.

After passing the dam of the Xuan Huong Lake (Ong Dao bridge), Cam Ly stream flows from east to west near Anh Sang clusters, and Nguyen Van Cu street, Nguyen Thi Dinh street, etc.

2km from the center of the city is a stream running over granite and forming Cam Ly waterfall.

To the area near the Cam Ly airport, Ly Cam stream meanders and flows to the West of the city. At 10 km from the city centre, in the region of Ta Nung commune, Cam Ly Stream deflects and runs from north to south. It flows on basalt bed with many faults and forms Cuu Than waterfall (Liang Pong Yang). Downstream the fall, there is the Cam Ly Lake, which was formed in 1995 due to irrigation dams.

On a tributary stream of Cam Ly, in the town of Ban Nam, there is the majestic Voi waterfall.

Cam Ly Stream continues to flow through Lam Ha district then combines to the Da Dang river in Tan Van commune (upstream the Dong Nai river).

7.2.3 Da Tam Spring System

This is a large stream system distributed in the South of the city. Da Tam Spring has three main branches: Datanla, Prenn, Da R'Cao.

Datanla Spring has many branches, the longest of which originates near Pin Hatt mountain, known as the Tia spring(Da Trea), 6.1 km long. The basin is mountainous terrain with a thick floral of pine. In 1984, the Tia spring was blocked to form the Tuyen Lam Lake, which supplied water for Duc Trong field irrigation. This is a big lake with many small branches, and romantic scenery.

Tuyen Lam lake

Datanla stream flows along the 6.7km valley between mountain.

Approaching the Datanla waterfall, the stream changes to northwest - southeast direction, and runs on very steep terrain along the Prenn Pass valley. Because it flows on the granite bed which has many geological faults, it is deep and has steep banks, and many falls, among which is the Datanla fall tourist site. There are more of small waterfalls downstream, but they have not been exploited due to steep slopes. Gallery forests along the stream have moist tropical forest tree species adding to the wild and majestic features. The confluence of Datanla spring and Prenn spring is near Prenn bridge.

Prenn Spring is 11.7km long and originates near Hung Vuong street. It has moderate flow, running from north to south.Downstream, at the foot of Prenn there is Prenn Pass. This is a beautiful waterfall near roads, so many tourists visit.

Da R'Cao Spring originates in Tuy Son hamlet (Xuan Tho). Its fairly large basin is located in Ward 11, Xuan Tho commune, Xuan Truong district. The stream is 24.2km, 15.6km of which belongs to Da Lat City. Tributaries are small springs. The basin is mostly remote mountains and forests so this spring has little agricultural and tourist value.

7.2.4 Danhim River System

The Da Nhim Lake has two upstream tributaries, Da Nhim and Krong Klet. They originates from the mountainous district of Lac Duong. The Da Nhim River is a natural boundary between Da Lat City and Don Duong District.

The tributaries of the Da Nhim river within the basin in Da Lat City originate from Tho Xuan and Xuan Truong hamlet. They have elevation of 1,450 - 1.550m and flow from the west to the east. Because the terrain is highly dissected, Da Nhim River stream net is quite dense. Most streams flow through the valleys between the mountain and regularly have water thanks to thick forest cover.

Because flowing over the fringe of the terrain, elevation difference is 400 - 500 m. Stream lengths are 4-6 km, so the river is steep, deep and has many rapids.

In the territory of Da Lat, the largest stream is Da Dousoung river (La Ba River). It is 19.2 km long and originates from the base of a 1.604m mountain at Da Sar hamlet (Lac Duong District). Then it runs through Xuan Tho hamlet and finally into Da Nhim river.

La Ba river

8. Plants

8.1 Forest Plant Formation

Da Lat city has 25.646ha forest land. In the opinion of ecosystem Dr. Prof. Thai Van Trung, Da Lat has four main forest types:

- Low mountain moist evergreen subtropical closed forests

- Low mountains moist broadleaf - conifer mixed subtropical closed forests

- Low mountains coniferous subtropical open forests

- Coniferous tropical open forests

Two types of closed forest have better protection effect than two types of open forest, but the former’s area is only 1/7 of the latter’s.

Two types of closed forest have many species of rare plants, but the number of individuals is negligible compared to forests in other districts and provinces in our country.

Due to human impact, these forests change. They might be better developed, or degraded into secondary formation, or even destroyed.

8.1.1 Low Mountain Moist Evergreen Subtropical Closed Forests (Evergreen Broadleaf Closed Forests, Broadlead Forests, Mixed Forests)

The forests have two layers, with no emergent floor. The canopy layer includes trees of Fagaceae family, Re family (Lauraceae), tea family (Theaceae), Mulan family (Magnoliaceae), which clearly dominant. Canapy trees have average height of 15 - 20m, and average diameter of 0.40 - 0.60 m.

Canopy layer is not continuous and has different tree heights. This layer includes individuals of shade-tolerant trees, interspersed with young trees of upper layer.

The low shrub layer has species from the families of Rubiaceae, Theaceae, Myrta-ceae, Euphorbiaceae. In this stage also there is the appearance of the woody fern species (Crylthea podophylla, Cibotium Crylthea spimulosa barometz in the canopy and Crylthea spimulosa at the edge of the forests). In the low shrub layer there are bamboo species of genera Arundinaria and Phyllostachys growing separately, and several species of palm (Palmaceae) as tall as areca-nut tree (Pinanga baviensis, Caryota ocflandra). Forest floor includes ferns of genera: Asplenium, Diplazium, Plagiogryria, Polystichum, Tectaria, Angiopteris. At the edge of a forest there are broadleafed Dieranopteris splendida. Adventitious plants consist of many fern types belonging to the genera: Trichomanes, Hymenophylum, Vandseboschia, Asplenium, Vittaria, Lemmophyllum, and the plants in familia: Ericaceae, Vacciniaceae, Araliaceae, Apocynaceae. There are less vines beloning to familia: Gnetaceae, Connaraceae, Asclepidiaceae, Ampelidaceae. Few trees wheel fenders, even among large-bodied species. This forest type can be found at Prenn Pass. This type of forest can be found in Prenn mountain pass.

8.1.2 Low Mountain Moist Broadleaf - Conifer Mixed Subtropical Closed Forests (Broadlead-conifer mixed forests)

This forest type is intertwined with the type of evergreen broadleaf closed forest, but in higher elevations.

There structures are similar to evergreen broadleaf closed forests, except that there are gymnospermae plants with bigger leaves than conifer such as bamboo cypress (Podocarpaceae), Keteleeria

Valuable spicies that still remain in this forest type:

- Keteleeria davidiana Beissner - Ngo Tung

- Pinus dalatensis Five-needle pine

- Libocedrus macrolepis Benth: blue cypress

- Taxus baccata var. wallichiana (Zuce) Hooks: red pine

- Dacrydium pierrei Hikel

- Podocarpus wallichianus C. Prel

- Podocarpus imbricatus Bulusne: white pine

Right on Ly Tu Trong street, or Phan Boi Chau street, it is very likely to encounter blue cypress, and Ngo Tung

Two types of closed forests accounts for 2,057 ha, which is 7.11% area.It is good for environmental protection.They scatter in the high mountains and rugged slopes: Prenn, Datanla, Tuyen Lam, Cam Ly, Xuan Truong commune, Xuan Tho, Ta Nung.

Depending on the level of degradation, the two forest types are classified into the following states:

Rich forest: small area (44 ha), little explitation, two-layer structure, tree diameter > 35 cm.

Average forest: 1190.83 ha accounting for 4.11% of area, reserves of 179.815 m3, moderately exploited, forest canopy cover of 0.5 to 0.7%, fresh carpet coverage from 30 to 50 %. Species with natural regeneration under the forest canopy include chestnut, brooch, etc. Density is 2000 to 5000 trees/ha. With good preservation, this forest has much potential.

Poor Forest: 161.04 ha, encounting for 0.55% of area. This forest has been violently damaged. Its structure has many layers, mainly small arbor and green floor. Scattered large trees with bad quality include chestnut, teak, Talauma gioi, Combretum quadrangulare... Small tree layer has Spondia pinnata, chestnut, Syzygium cumini... Canopy cover is 2.2 to 0.4%, green floor coverage is from 50 to 70%. Natural regeneration under the canopy include chestnut, Combretum quadrangulare, Xerospermum macrophyllum Talauma gioi, Density is 500 to 1000 trees/ha. With good preservation, this forest has much potential.

Young forest with reserves - 555.38 ha. This is the recovering state after being exhausted from exploitation. Vegetations are still at their young stage and love light. Species structure is complicated and irregular in terms of age. There may be few residual high trees from previous flora but the reserve is not considerable.

Young forest without reserves -106.04 ha. This is restoration state after shifting cultivation. The typical species are the first-shooting class which is lucipetal, grows quickly, evenly-aged, and has one layer.

Broadleaf- bamboo mixed forest - This forest type is characterized by small bamboo species that are usually distributed along streams with a small area (67.37 ha), and by human-caused secondary leaves (due to human destruction) of the two types of evergreen forest

8.1.3 Low Mountain, Half Dry Conferous Subtropical Forests

The dominant plant is three-needle pine trees (Pinus kesiya Royle ex Gordon). They occupy an area of 14.628ha, accounting for 50.6% of forest area and forest land.

Three-needle pine is mainly timber source. Small-diameter trees is used to make pulp; large-diameter trees are used in construction and home appliances.

In the four forest types, three-needle forest has the most significant role. They protect rivers, dams, irrigation lakes, and hydropower lakes, restore landscapes, and serve as tourist area.

Three-needle pine forest can substitute for the other three forest types if they are damaged. Its structure is stable, sustainable, and also likely to become broadleaf-conifer mixed primeval forest.

The current state of the three-needle forests is very diverse, with mixed layer. There can be the main canopy, uneven canopy layers, or natural regeneration. It is also diverse in terms of density classification and formations of grass and shrubs. Altogether, they make abundant, rich and attractive scenery.

The two types of closed forests and this coniferous open forest type are enclosed in the subtropical belt with low mountains, rain, humidity, and the altitude is 1000 - 1.800m.

Da Lat is the distribution centre of three-needle pine forest. However, three-needle pines can extend their natural distribution to the altitude of 2,000 m (range Lang Biang) or down to 600 - 800m (Bao Lam).

Three-needle pine forests have mixed-layer structure. Pipe trees occupy the dominant timber floor, which can reach heights up to 25 - 30m. An area of 1ha without cutting may about 1,000 trees.

Small broadleaf trees occupy the sub-floor, 10 - 12m high. Plant structure is relatively simple. There are over 10 species, mainly belonging to the Fagaceae family. Being cleared, many individuals stay in bushes, and in some areas the whole layer may be gone. Plants at this layer are mostly shade tolerant, resistant to fire, and can strongly sprout. The shoots are dense and grow very fast. This layer of small broadlead trees have the industrial potential that was not noticed before.

Floor grass - shrub layer occupies the space at 3 to 3.6 m and 2 to 0.5 m above the ground. Layer’s species are plentiful. Poaceae family plants are dominant. Perennial grasses hold important positions. Dicot glass appears with many species, but individuals of each species are in small number, scattered with monocot species. Dicranopteris linearis, and Pteridium aquilimum grow in larger masses. Grass is seasonally developed. In rainy season, they grow fast, but in dry season they die and turn into combustible material which is a forest fires cause.

Succession

Three-needle pine forests can easily substitute the two types of evergreen closed forests if they are damaged.

In three-needle pine forests with average density, if there are no clearing and fire over a long period of time, the layer of shrubs and young trees of the broadleaf species (Indigofera, Rhododendron langbianensis, Melastoma villosum, Engenia sp., Pieris ovalifolia, Craibiodendron stellatum, Carpinus polanei, Lithocarpus sp., Quercus sp.,...) will gradually replace the grass and gain dominant position in the small wooden scrub layer below. This will push the tendency of reverting to its primeval forest type, which is the broadleaf-conifer mixed forest.

If pine forests are cut (including selective exploitation) and there are frequent fires, savanna with two dominant species Kerriocholoa siamensis and Eremochloa ciliaris, will appeare. In forests near residential areas, due to grass cutting, grazing, there appear some bunch grasses such as Lantana camara, Saccioleppis indica, Axonopus compressus,... Finally, when the pine layer is lost, savanna appears. There are cases where the grass is very low (5 - 15 cm) and composed of many low grass bushes with such dominant species as Aristida cumingiana, Dimeria sp., Eremochloa ciliaris, E. ciliaris, Fimbustylis diphylia, lanthanum camara, Malastoma villosum,...

Savanna, with two dominant species Aristida cumingiana, Dimeria sp., can be considered as the lowest stage of three-needle pine flora degradation under human impact.

Three-needle pine forests are secondary forests, love light and grow fast. With good preservation, they can revert to their primeval broadleaf forest. Keeping them with no fires in consecutive 5-7 years, they can maintain stable. If cleared and fired often they will degrade into savanna and shrubs.

Growth

Young three-needle pines grow slowly. In the first 20 years they only achieve 30 -40% of their grow-up height, depending on soil quality. In this phase there is lawn intervening. In the dry season, if grass is on fire, all young pines will be burned out.

In their immature stage, tap roots grow first, then fibers follow. Therefore, porous soil is favorable for regeneration. Their average growth in Da Lat at the moment is 8 - 12m3/ha/year.

In the areas of naturally grown pines, after exploiting, the burnt-out plowed land is a good condition for pipe natural regeneration.

Fire is the ecological factor to form a pine forest, and also a factor leading to forest degradation, and destruction.

Pine Forest Age Classes

Natural three-needle pine forests have six age classes:

Young forest: 10-20 years

Pre mid-age forest: 20-30 years

Mid-age forest: 30-50 years

Pre-mature forest: 50-60 years

Mature forest: 60-70 years

Post-mature forest: > 70 years

Young forest area is 323.1 ha, accounting for 1.11% of the area. Diameter (D1, 3) = 10cm.

Nests forest area is 665.15 ha, accounting for 2.3% of the area, D1, 3 = 10 - 19cm.

Young forests and -- forests scatter in clusters, alternating with the pre-mature and mature forests.

Middle-aged forest is 3686 ha, accounting for 12.75% of the area.

Depending on the density, early mid-age and mid-age classes could be divided into smaller classes:

- Small pre mid-age forest: 1000-1500 trees/ha

- Big early mid-age forest: 700-1000 trees/ha

- Small mid-age forest: 500 - 700 trees/ha

- Big mid-age forest: 400-500 trees/ha

Pre-mature forest area is 8712 ha, occupying the largest area (30.66%) and also the largest reserves. Average density is 300-400 trees/ha.

Mature forest has small area (71.1ha). Average density is 200-300 trees/ha.

Scattered forest has the area of 1168 ha, with uneven diameter. Density is 40-50 trees/ha. Age is diverse and average reserve is 40m3/ha.

In general, pine forest inis sparse. Thickness of the middle-aged or older forests reaches only 0.45 to 0.5. Compared with standard density, actual density is of only 40 to 50% of 20m3/ha/year (8 - 12m3/ha/year).

Moreover, small broadleaf tree layer grows beneath the canopy is cut for firewood, and charcoal. That brings favorable lighting condition for floor grass to grow fast. During the dry season die grass leads to the risk of fire.

8.1.4 Semi-dry Conifer Tropical Forests (Two-needle Pine Forests)

The dominant species are two-needle pine (Pinus merkusii Jungh và de Vriese) and Tra Beng oil (Dipterocarpus obtusifolius Teysm.).

Two-needle pine may be the only species of the upper layer, but sometimes there are few Tra Beng oil growing in the same layer.

This forest type is very scarce now. Its area is nearly 20 ha, distributed in five wards adjacent to the Ta Nung hamlet (Rong Troi). This forest type is mainly distributed in Di Linh Plateau. This is the adjacent point of the two plateaus, Di Linh and Lang Biang.

This type of forest is located at lower altitudes compare to the other three forest types (two of evergreen type and one of three-needle type).

From 1980 to 1990, large areas of this type were cleared. Some have been replaced by the three-needle pine species.

In the low shrub layer there are often chevalierii Vaccinium species (nem chua) and Schima crenata (voi thuoc). Common grasses are Kerriechloa siamensis, Dicranofteris linearis (guot), Nephrolefis hirsuta (mong trau), Imperata cylindrica (alang grass), and the species of genera Exotheca, Polytocca, Cymbopogon, etc.

Thanks to symbiotic fungi at roots, both three-needle and two-needle pines can grow on poor soil. If the soil is plowed, plant growth is lush.

At the young stage, three-needle pines grow faster than two-needle pines. Within 4 to 5 years, they can easily outgrow shrubs. Two-needle pines grow slowly, and can only catch up with the three-needle pines from year 10 onwards.

Two-needle pines mainly give sap, starting from 15 to 20 year-old. The average exploiting rate is 5kg/tree/year over 30 years.

8.1.5 Variations of Forest Flora

In Da Lat, the forest flora biodiversity has been reduced.

Although Da Lat has four types of forests, but they are all pine forest. Three-needle pine forests with sustainable mixed layer structure only remain in some areas that are strictly protected. They accounted for less than 5%. The rest of forests have only one-layer structure, and only grass on the floor.

Because the urbanization process, many forests around the residential areas are sinking. Some scenic spots such as Cam Ly waterfall, Than Tho Lake, Valley of Love,... no longer have their original wildness.

8.2 Aquatic Plants

In ponds and lakes in Dalat, remaining aquatic plants only exist in a small number but they still have useful impact in freshwater fish farming.

Floating vegetation includes species like water ferns, lotus, water lilies, Azollaceae family, etc.

Under-water plants include Potamogetonaceae, Ceratophyllaceae, Haloragaceae.

Phytoplankton plants are algae species: silica algae, grey algae, green algae, Dinoflagellata, Euglenophyta.

The development of algal populations in the lake system varies with seasons. In February, March, and April, the dominant species is grey algae (Merismopedia) and the green algae belonging to the Scenedesmus, Tetraedon, Crucigenia, Astinastrum geneva. Silica algae are mainly Melosira and other less developed species.

Phytoplankton is the prey of fish. Fish generally do not eat algae because of the smell but they love green algae.

8.3 Vegetables

In 1898, Agriculture Station was set in Dankia.

In a report dated 15-12-1901, engineer A. D'André - Inspector of Agriculture, Lang Bian Plateau Agricultural Station chief- stated the Station had 16.6706 ha area, and was doing growing trials on temperate vegetables: asparagus, eggplant, lettuce, water crescent, cabbage (red spieces, Cabus, Joanet, Milan, Brussels, Quintal), cauli-flower, kohlrabi, cucumber, onion, radish, beetroot, carrots, green beans, peas, water dropwort, parsley, tomatoes, artichoke, etc.

Since then, these vegetables have been grown in small farms and gardens in Da Lat.

Beside the vegetables varieties bred by farmers, prior to 1960, farmers of the villages of Tan Lac, Ha Dong, Nghe Tinh, Thanh Da, Da, Phu, Anh Sang,... have planted new varieties imported from France.

Since 1960, farmers have been receiving a number of vegetable varieties from France, America, Taiwan, Japan, etc. through the Da Lat Center for Vegetables Experiments (Center for Agricultural Research in Da Lat). The most important ones are cabbage, potatoes, and onions.

8.4 Flowers

Da Lat is known as the City of Highland, City of Fog, Mount Street, but the most popular name is the City of Flower. Da Lat has a wide range of flower diversity: forest flowers, wild flowers, planted flowers in parks, in gardens, merchandise flowers, tree flower, mountain flowers, plain flowers, oriental flowers, West flowers.

Since 1898, together with the test cultivation of vegetable varieties, Agriculture Station in Dan Kia has also acclimatize the flower species: roses, chrysanthemums, white daisy, purple daisies, arrowroot, balsamine, capucine, dhalia, Antirinum majus L., forget-me-not, carnations, Cam Nhung, opium, pansies, geranium, phlox, Violet, zinnia, etc.

Later, the French brought to Da Lat flower varieties from their homeland and planted them in their villas’ gardens. Vietnamese people coming back from abroad also brought back to their family in Da Lat new flower varieties. In addition, through mailing and importing, new varieties of flowers continue to come and add to the diversity of Da Lat flowers.

The Vietnameses - Kinh people from all over the countries have brought up the plain flowers: marigold, camelia, water coconut, sam, muoi gio, etc.

Since 1993, with the establisment of Da Lat Hasfarm Company, many new flowers and seeds appear on the market: tulipe, chrysanthemum, carnation, rose, lys, etc.

In addition, Da Lat has some flowers that are rarely seen elsewhere countrywide: Mai Anh Dao (Cerasus cerasoides L.), phuong tim (Jacaranda mimosaefolia), mimosa (Mimosa sp.), cajuput willow (Callistemon lanceolatus Sw.), hydrangeas (Hydrangea hortensia DC), pansies (pensee: Viola tricolor L.), Violet (Violette: Viola odorata), forget-me-not (Myosotis sp. L.), etc.

Currently, Da Lat has over 400 species of flowers that have existed locally for very or originated from Asia (China, Japan, India, Thailand,...), Europe (France, Netherlands,...), the Americas, Africa, Australia. They have been either sexually or asexually reproduced. The orchid family (Orchidaceae) and Asteraceae family (Asteraceae) have the most classes and species.

Garden Flowers

Besides the flowers are grown for the purpose of harvesting, flowers in Da Lat are also grown for garden decoration. A few of them are a-ga-pan (Agapanthus africanus (L.) Hoffm.), Zephyranthes rosea (Spreng.) Linds., Iris (Iris halophyla Pall.), Canna indica L., Telosma cordata (Burm.f.) Merr. white daisy (Marguerite: chrysanthemum maximum Ramax. ), Begonia integrifolia, honeysuckle (Lonicera japonica Thunb.) azalea (Azalea japonica (Mollis) Pers.), Lang Bian Plateau rhododendron (Rhododendron langbianensis), poinsettias (Poinsettia pulcherrima (Willd. ex Klotzsch ) Grah.), geranium (Pelargonium in quinans (L.) W. Ait.), Salvia splendens Sello, Althaea rosea (L.) CAV.), Bougainvillea spectabilis Willd. lanterns (Fuchsia L. japonica), Angelonia salixifolia, HymbHippeastrum equestre Herb, Impatiens balsamina L., Verbena peruviana Britt., golden bell (Spathodea campanulala Bean.), two-color Jasmine (Brunfeldsia hopeana Benth.), Antirrhinum majus, etc.

8.5 Bonsai

In recent years, with better-developed economy, people's living standards has been improved. Bonsais are seen everywhere from gardens to house, parks, and even offices. They are also sold to visitors or transportated to Ho Chi Minh city.

Most bonsais are evergreen and shade tolerant, easy to grow roots. They usually originate from forests, and often trimmed, bended and shaped to philosophical and aesthetic ideas.

The common bonsais are azlea, honeysuckle, Bodhi, fir, cypress, etc.

Realizing that temperature in Da Lat is increasing, a number of people tried some bonsais adapted to plains such as Bang Lang, yellow apricot, etc.

8.6 Fruit Trees

Temperate fruit trees need the essential condition of low temperature season for their vernalization. During this time trees rest completely with their buds sleeping, their leaves falling. Only branches remain. Dalat's climate is relatively cold, especially in January and February. The minimum relative coldness in Da Lat can only satisfy a few temperate trees’ demand for vernalization. Thus, since early this century, Da Lat farmers have planted a number of temperate fruit trees such as peach, plum, strawberry,... Most of the fruit trees grown in Da Lat originates from France, America, China and Japan.

8.7 Industrial Trees

8.7.1 Tea

Tee tree (Camellia sinensis (L.) O. Kuntze) is the industrial trees that have been planted in Cau Dat, Xuan Truong hamlet since 1922.

Cau Dat tea has high quality and is produced into black tee, green tea for export.

8.7.2 Coffee

Coffee tree was first planted in Da Lat in 1903 in Xuan Truong, Trai Ham, and then to other places.

Beside the Coffea arabica L.species, Da Lat has Ta Nung hamlet cultivating Coffea robustas species.

8.8 Food Crops

Da Lat farmers have planted rice, corns, sweet potatoes… but with a small area.

8.9 Mushrooms

Since 1990, red Lingzhi mushroom (Ganoderma lucidum (W. Xurt.: Fr.) Karst) has been found and successfully planted in Da Lat

In 1993, Da Lat Agricultural Product Company jointed with three companies from Japan and Taiwan to produce white mushroom (Agaricus bisporus (Lange) Sing.)

Besides, some limited liability companies produce (Auricularia porphyrea Lev.), (Ganoderma lucidum), nấm hầu thủ (Hericium erinaceum), nấm Đông - cô (Donko) (Lentinus edosdes),...

8.10 Medicinal Plants

Da Lat has many medicinal plants, not only tropical but also temperate ones, especially archichoke.

Quinquina (Quinquina: Cinchona ledgeriana (How.) Moens) was planted in Xuan Tho in 1927 by A. Yersin at the Quinquina Experimental Station of 16ha.

Quinquina bark is used to cure fever, malaria, or as tonic.

Since 1980, Center for Medicinal Plants and Pharmaceuticals Research Center have acclimatized many medicinal plants originating from foreign countries (China, Russia, Australia) and from other provinces. Some of them are mint, angelica, white Atractylodes, Stevia rebaudiana, matricaria chamomilla L., Digitalis purpurea L., red multiflorous knotweed, Phellodendron amurense Rupr., Illicium verum, Radix Scrophulariae, Anoechilus roxburglihayata, Geranium nepalense, cinnamon, Ngoc Linh ginseng, solanum, Artemisia annua, Radix Dipsaci, Saussurea lappa Clarke, Ligusticum Wallichii Franch, etc.

9. Animals

9.1 Wildlife

Mountains in the area of Da Lat city was home of tigers, elephants, bears, bison, elk, deer, panolia eldii, tragullidae, wild pigs, chimpanzees, monkeys,... and small forest animals such as rabbits, chickens, forest birds, peacocks, mallards...

In the early twentieth century, the tiger was the horror of those who live in Dalat. Duclaux went to Dalat in 1908 and commented:

“Tiger is an everyday story, not a fabricated fun story but a real concern. Dogs, horses, many servants, many postmen were attacked and many Europeans shared the same fate. “

In 1960, in the old Prenn Road area (Khe Sanh, Mimosa now) people can still also shoot down the tiger.

In 1979, across the street from Chi Phat to the lake Tien (Xuan Truong), a forestry staff also met a tiger.

In 1940, in Chi Phat there was a herd of elephants and on old maps, this region is also recorded famous “Elephant Road”(Piste des Elephants).

Panolia eldii is a 1.15 m high deer, cute, common on the Lam Vien Detours in 1950

Dalat has 45 bird species which attract many ornithologists, including four special species of Vietnam: mi Lang Biang (Crocias langbianis), Garrulax yersini, Sitta solangia, Carduelis monguilloti.

Sparrows and white-chin swiftlet are familiar with the people in city of Dalat. They often appear in the sky and build nest under the roof tiles or in the woods.

In 1921, Le Bosquet conservation area which is 8.000ha wide in the Triangle Dalat - Dran - Fimnom was established and it has contributed to the protection of plants and animals in the region located between Highway 20, 11 and 21.

On 26-1-1951, Bao Dai signed his great order No. 7/QT/TD and and on 12-2-1951 the Decree No. 67 giving rules of the hunting on Reigning Dynasty territory.

Under this legislation, forest Dalat is forbidden forests, shelter for animals, not for hunting, only hunting in the area around Da Lat in the area and hunting in temporary reserve areas are not prohibited.

However, the indiscriminate hunting and the narrowing, degradation of forests in Da Lat has led to the decrease in the number of species and number of individuals of forest species.

Now, there are only some wildlife species distributing mainly in the area around Tuyen Lam lake, Datanla waterfall, Prenn, Elephant Mountain and three communes of Ta Nung, Tho Xuan, Xuan Truong.

The composition of wildlife species distribution in the area of Da Lat includes: leopard, temminck, clouded leopard (Neofelis nebulosa), Asian Golden Cat (Catopuma temmincki), Large-spotted Civet (Viverra megaspila), Small Indian Civet (Viverricula indica), Chrotogale owstoni,Masked Palm Civet or Gem-faced Civet (Paguma larvata),

Melogale moschata, Yellow-throated Marten (Martes flavicula), Asian black bear (Ursus thibetanus), Macaca nemestrina, Herpestes javanicus, Arctonyx collaris, Megamuntiacus vuquangensis, Felis viverrina, Marbled Cat (Pardofelis marmorata), wildcat (Felis silvestris), Porcupines, Otter, Hylopetes spadiceus, chipmunk, dhole (Cuon alpinus), chamois, Mustela kathiah

9.2 Cattle and poultry

Da Lat farmers raise cows, pigs, horses, chickens and rabbits.

On 23-12-1900, engineer A. D' André brought from Phan Rang 10 sheep, in which there was a male sheep. Those sheep are hybrids between Indian and Chinese sheep, so they can produce long but rough wool.

Currently, farmers are no longer raising sheep Dalat.

9.3 Ephemerae

Ephemerae species have food value as milk of animals on the ground, are essential for survival and growth rate of fish.

In lakes, rivers and streams in Dalat there are 20 species of Rotatoria, five species of Cladocera, one species of Daphnia, a species of Moina, a species of Copepods, a species of Cyclops, a species of Asplanchua (rotifers).

The ephemerae species are widely distributed and change in number depending on regions and seasons.

Xuan Huong lake has a lot of Moina, sometimes 8,600 in one liter of water. There are about 100,000 Moina in 1m3 water, from 25,000 to 60,000 Rotatoria, 20,000 Daphnia.

9.4 Animals that live in water

The species that are living naturally in the ponds and streams in Dalat include eels, snails, turtles and fish such as: silver fish head, Aplocheilus panchax, Enophrys, Carassius, Marbled spiny eel, macrropodus, seahorses...

The species of wild fish in Dalat has long-standing, no other fish species from lowland to upland streams because of crossing streams, the species in the streams in the plains can not go up stream.

Later, people set the fish free, drop carp, catfish,... into the lake

In 1950, a delegation of tourism - sports in France brought Black Bass and stocked them into the Great Lakes (Xuan Huong Lake today) for visitors to go boating, throwing bait. This fish had grown so fast.

Dalat - Tuyen Duc Fishery Company released into the lake the carp, tilapia, hypophthalmichthys, to experimental and monitor their growth and reproduction.

In 1966, Fishery Company released in Xuan Huong eggs of a Japanese specie of fish. Fish is very good, has fast growth rate. Unfortunately, Black Bass and that kind of fish no longer exist because of pesticides and lake rinsing.

Now, in ponds, lakes, they also raise the fish like: carp, tilapia, hypophthalmichthys,, snakehead fish, catfish... In addition, the aquarium has several species of fish brought from Ho Chi Minh City.

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