Total catchment area of Red River Basin is 169,000 km2. The part located in China territory is 81,240 km2 (or 48%). Annual average rainfall of the river basin is about 3,000mm. Total average flow in many years is l0.7 billion m3. The basin is consisted of 5 sub-basins.
Total water volume of Thao sub river basin is 26 billion m3, of which 18 billion m3 is from China territory. The rainfall flow is rather abundant, i.e. 7.9 billion m3, with 70-85% as flood flow. Upstream of Thao river has enough humidity and flow storage to meet water demands of agriculture and other economic sectors.
Groundwater potential for exploitation is rather plentiful. The aquifer structure of the upstream reach can provide 4 billion m3 per annum whereas downstream reach has a discharge of 450 m3 per day per km2 which can meet domestic demands, irrigate cash trees and supply water to industrial activities as processing and mineral exploitation.
DA RIVER BASIN
The catchment area of Da Sub River Basin in Vietnamese territory is 25.200 km2. Total water volume outside the river basin is 26 million m3 per annum. Annual rainfall in the whole river basin is 50 million m3 per annum; total evaporation volume in the basin 24 million m3 per annum; total flow arisen in place and from outside the basin 52 million m3 per annum. It can be said that water resources of Da river basin is quite abundant. Branches of Da river have high slopes which mean greatest potential for hydropower development among Vietnamese rivers. Cascades in Da river produce up to 35% of total hydropower potential of the country.
LO – GAM RIVER BASIN
Sub river basin of Lo – Gam in Vietnamese territory has a catchment of 22,570 km2. Total water volume outside the basin is 13 million m3 per annum. Annual rainfall in the whole river basin is 49 million m3 per annum; total evaporation volume in the basin 26 million m3 per annum; total flow arisen in place 21million m3 per annum; total flow arisen from outside the basin 34 million m3 per annum. . It can be said that water resources of Lo – Gam sub river basin is quite abundant. Upstream reach of Lo sub basin see a water volume of 14.000-16,000 m3 per ha. The upstream basin then is considered to be rich in water. Branches of Da river have high slopes which means greatest potential for small hydropower development i.e. 8,535 kw.
UPSTREAM THAI BINH RIVER BASIN (ALSO CAU-THUONG RIVER BASIN)
Like the whole northern region, upstream of Thai Binh river is totally located in the tropical region of the north hemisphere. Under the impacts of monsoon winds, climate here is characterized by two distinct seasons: dry season lasts from November to next April and is under effects of north east wind. Dry season is cold and sees less rainfall. Rainy season lasts from May to October and suffers south west wind and climatic turbulences as storms, air convergence, fractures etc… which result in hot and humid weather and a lot of rains. Typical climate of the region has following features:
Average air temperature: 22.6oC
Highest air temperature: 40.5oC
Lowest air temperature: -1.8oC
Average air humidity: 83%
Average evaporation: 786 mm
Total average sun shining hours: 1,540 hours
Largest wind velocity: 36 m/s
Average rainfall in the basin: 1,669 mm
The main topographical feature of the region is high mountains with great slopes. That is why most of rivers, especially tertiary rivers have great gradient.
Total water volume of the whole region is 11 billion m3 which is contributed by 3 rivers of Cau, Thuong and Luc Nam with respective volumes of 4.5 billion m3, 4.2 billion and 2.4 billion m3. Modest rainfall results in normal flow with average module of 22 l/s per km2 which is far less than northern east region of Quang Ninh and some regions in Red River Basin. The distribution of annual flow in the whole region depends on rainfall regime and rainfall regime is also distinguished into 2 seasons, say flood season from June to September and low flow season from October to May.
RED RIVER DELTA
Red river delta is totally located in the tropical region of the north hemisphere. Annual climate here is characteristic of two distinct seasons: winter lasts from November to April and is under effects of north east wind and monsoon wind bringing in cold weather and less rain; summer lasts from May to October and is under effect of south west wind and climatic turbulences as storms, air convergence, fractures etc… which result in hot and humid weather and a lot of rains. This kind of weather also affects livelihood of local people in forms of flooding. Typical climate of the region has following features:
Average air temperature: 23.3 oC
Highest air temperature: 42.8 oC
Lowest temperature: 2.7 oC (in Bac Giang, dated January 12, 1955)
Average evaporation: 878 mm per annum
Highest evaporation: 1,012 mm (in Bac Giang)
Lowest evaporation: 698 mm per annum (Phu Lien)
Average air humidity: 84%
Average wind velocity: 2.0 m/s-2.5 m/s
Total sun shining hours: 1,500-1600 hours
Average rainfall in the delta: 1500-1800 mm per annum
Red river delta receives water from large rivers upstream as Red River system including main branches of Da, Red, Lo, and Thao. These tributaries meet each other in Viet Tri city. Thai Binh river system has main tributaries of Cau, Thuong, and Luc Nam which have confluence at Pha Lai.
Red and Thai binh rivers have total annual water volume of 137 billion m3 per annum. Red river up to Son Tay has a water volume of 117 billion m3 per annum. Water volume in Thai Binh river up to Pha Lai is 11 billion m3 per annum. If the whole water resources of Red and Thai Binh rivers are only made available to the delta area, each hectare will receive 1 million m3. This means the area is quite abundant of water resources. These plentiful sources of water are not evenly distributed in terms of time and space. Uneven distribution causes serious effects on livelihoods of local people.