GENERAL CHARACTERISTICS

There are eight river basins including Pumqu, Rongxer, Poiqu, Zangbuqin, Jilongcangbu, Majiacangbu, Jiazhagangge, Daoliqu in research area. General descriptions of them are given below: 

Pumqu basin

Pumqu river basin is situated in the southwest of Tibet between the latitude 27º49´N to 29º05´and longitude 85º38´E to 88º57´E. It is bordered by the upriver of the NianChu river and Doqeen Co in the east; and Rongxarzangbo, Bo Qu and Paikuu Co in the west; and Nepal, Sikkim in the south; and YarLungzangbo basin in the north. The total catchments area is 24272 km2. It is about 320 km in length from east to west and 120 km in breadth from north to south. It originates from the YeBokangjiale glacier of XiXiabama Mountain.

There are many tributaries (YairuZangbo, LoloQu and ZhagarQu ) in Pumqu basin. Among these rivers, the catchments area is more than 1000 km2.

Rongxer basin (Tama Koshi)

Rongxer basin (Tama Koshi basin) is situated in the southwest of Tibet between the latitudes 27°56'N to 28°16'and longitudes 86°07'E to 86°32'E. It originates from the DuokaPula Mountain which elevation is about 5611m. There are so many glaciers at the source of the river. The total catchments area is 1439 km2. Among the area, the glacial area is about 324.09 km2. The main river course is about 45 km in length.

Poiqu basin (Bhote-Sun Koshi)

Poiqu basin is situated in the southwest of Tibet between the latitudes 27°55'N to 28°30'and longitudes 85°43'E to 86°18'E. It originates from the Poiqu Co glacier of the south slope of the Himalayan Mountain. The elevation of the river source is about 5320 m and 2000 m at the mouth. It starts from the source of the river flowing from east to west and with the joining of tributaries, it changes the direction near the Nielamu. It enters Nepal and named Bhote-Sun Koshi river near Nielamu about 7km from north to south.

There are five branches on the right of the river and four branches on the left in Poiqu basin. It is about 80 km in length and the total catchments area is 1990 km2. Among the area, the glacial area is about 231.58 km2.

HYDRO-METEOROLOGICAL OBSERVATION

There are only two meteorological stations-Nie Lamu and Tingri (shegar) in the research region. The stations measure daily rainfall, daily maximum and minimum temperatures, relative humidity, wind speed, and evaporation, atmospheric pressure. Precipitation, air temperature, evaporation and relative humidity play an important role in analysis the climate of basin. Characteristics of these parameters are discussed as the follow paragraphs. Because there are few observed data in the research basin, so we can only do a trend analysis with station which in the research area.

Air Temperature

These years, with the global warming, which is caused by greenhouse effect, the atmosphere environment has changed. The increase in temperature can have an impact on the condition of glaciers; higher temperature can cause rapid melting of glacier ice. Now many people pay close attention to the urgent problem that is snowline is rising and water level of the glacial lakes is changing. Here is an analysis for the research basin reveals a clear increase in temperature after 1990s (Figure 3.1). The trends are accelerated after 1998. We can describe the climate characteristics of the research area with the data of Nielamu and Tingri meteorological station. The research area belongs to semi-humid region. The data of the Nielamu meteorological station: The annual mean temperature is 3.5°. The lowest month mean temperature is -3.7° which occurred in January. The highest month mean temperature is 10.4° which occurred in July. The highest temperature is 22.4° and the lowest one is -20.6°. The temperature data of Tingri station show same warm trend (Figure 3.3).

Figure 3.1 : Annual temperature trends for the research regions

Precipitation

The research area is situated at the upstream of the Ganges and the North slope of the Himalayan Mountain. It is also the main vapor passage, which the southwestern monsoon going into the mountainous region of western of Tibet. So the precipitation in this area is dominated by a southwestern monsoon, the rainfall decrease from south to north. The maximum precipitation is about 3000mm in the south and 500mm in the north.

Through analysis for the precipitation data of Nielamu station: annual mean precipitation is 650mm; maximal four months precipitation, which is about 47% of the annual volume occurs from June to September. There is so much snow and rainfall in winter and the rainfall season starts early in this area, so precipitation is uniform distribution in a year and smaller changing every year.

Through analysis for the precipitation data of Ding Ri station which in northwest of the basin: annual mean precipitation is 265mm; precipitation maximum which is about 50% of the annual volume generally occurs in July or August, maximal four months precipitation which is about 94.2% of the annual volume occurs from June to September, precipitation is uneven distribution in a year and smaller changing every year, annual precipitation in the wettest year is 4.5 times that of the driest year.

The Cv value of precipitation in this region is between 0.20-0.30. Consecutive dry years or wet years occurred in the area.


Figure3.2 Annual precipitation trends of Nelamu station

Figure 3.3 Annual precipitation and temperature trends of the Tingri station

Evaporation

Evaporation plays a very important role in water balance. Evaporation is influenced by temperature, humidity, solar radiation, and wind speed. The evaporation increase from south to north: La zi is 2800mm, Tingri is 2550mm, and Nie lamu is 1600mm. There is no more changing for every year; Maximum annual evaporation is less than 1.5 times that of the minimum value.

River discharge

Because much of the river in Himalaya-China region originate from modern glaciers, and many glaciers such as Rong bu glacier distribute around it. Glaciers and snow area is about 25%. The supply runoff belongs to groundwater mix with snowmelt and precipitation. Annual mean runoff is about 50*108 stere.

Here is an analysis for La zi hydrological station: maximum discharge is 1390m3/s (1999), minimum discharge is 25.9m3/s (1992), the timing of discharge coincide closely with seasonal maximum and minimum of precipitation at basin scales, discharge maximum generally occurs in August coinciding with the peak of the monsoon, minimum values occurs during the months of December-January, maximal four months discharge which is about 66.1% of the annual volume that occurs from June to September, discharge is uneven distribution in a year and smaller changing every year, annual maximum in a year is 2.7 times that of the minimum value.
 


Figure 3.4: Hydrograph of discharge and precipitation of La zi station

CONCLUSIONS

The GLOF events have been occurred in Tibet especially the period from 1998 to now. The analysis result of Pumqu river basin show that runoff has a increasing trend from 1990s to now; the temperature are growing up; snowmelt increasing and the trend of precipitation is similar to the temperature. Obviously, there is a very important role played in analysis the flood by snow and ice melt in the basin. We should pay close attention to the effect of global warming on the regional hydrology. Particularly, the basin just like Pumqu and Poiqu that distribute a lot of glaciers, it needed real-time monitoring, which will reduce the hazard to the minimum.