Nepal is among the richest in terms of water resource availability and it is one of the most important natural resource of the country. Water resources are abundant throughout the country in the form of snow covers, rivers, springs, lakes, and groundwater. The total renewable water resource of the country is estimated to be 237 km³/year (225 km³/year for surface sources and 12 km³/year for groundwater sources) and per capita water availability for 2001 was 9600 m3/capita/year.

Snow cover

Snow cover in Himalayas provides huge natural storage of freshwater. Glaciers, permafrost, and glacial lakes are main forms of water storage. Snow-melt discharges from Himalayas maintain the water levels in downstream rivers and wetlands and thereby provide vital ecosystem services and support dependent livelihood. There are about 3,252 glaciers with total coverage of 5,323 km² in Nepal. Similarly, there are about 2323 glacial lakes located in this region with total coverage area of 75.70 km². Due to impacts of global warming and climate change phenomenon glaciers are retreating at alarming rate and glacial lakes are expanding rapidly. Glacial Lake Outburst Flood (GLOF) disaster poses imminent risk to downstream infrastructure, households and livelihood. In Nepal at least 12 GLOF events have been reported to date and about 20 glacial lakes are identified as potentially dangerous from GLOF.

River Basins

Whole Nepal is a part of the Ganga Basin and it is estimated that approximately 70% of dry season flow and 40% of annual flow of the Ganga River comes through Nepal. It is estimated that there are altogether 6,000 rivers (including rivulets and tributaries) in Nepal and drainage density is about 0.3 km/km2. The cumulative length of rivers is 45,000 km. There are 1000 rivers longer than 10 km and about 24 of them are more than 100 km. Rivers in Nepal can be classified into three broad groups on the basis of their origin. The first group of rivers is snow fed-types such as the major rivers systems: the Koshi, Gandaki, Karnali, and Mahakali. They originate from snow and glaciated regions in Himalayas and their flow regimes are mostly governed by the melting of snows and glaciers. As a result, flow in these rivers is perennial and sustain flow during the dry season. These rivers are reliable source of water and also provide potential opportunities for hydro-power generation and irrigation in the downstream. The second group of rivers originates in the middle mountains and hilly regions. Their flow regimes are affected by both monsoon precipitation and groundwater. Contribution from groundwater yield maintains the minimum flow level and prevents from drying during non-monsoon periods. The Bagmati, Kamala, Rapti, Mechi, Kankai, and Babai rivers fall into this group. The third group of rivers originates in Siwalik zone. Tinau, Banganga, Tilawe, Sirsia, Manusmara, Hardinath, Sunsari and other smaller rivers are examples of rivers falling in this group. The flow in these rivers is mostly dependent on monsoon precipitation and their flow level could deplete significantly low during the non-monsoon period. Summer monsoon is important period when about 60-85% of annual runoff of all river systems in Nepal occurs during July to September.

River water discharge flowing through Nepal

Enclosed water

There are numerous enclosed water bodies all over Nepal and those includes lakes, ponds, dams, and other small wetlands. Recent report of the National Lakes Conservation Development Committee has identified total 5,358 lakes in Nepal (including 2323 glacial lakes). Lakes are spread at different elevation as well as along entire east-west longitudinal range. There are nine wetland identified as Ramsar sites in Nepal.

Other important lakes include Phewa, Rupa, and Begnas in Pokhara (Kaski district). These enclose water bodies are important source of water for irrigation, recreation, fishing and other domestic uses. In addition these are habitats for different flora and fauna. Wetlands are sanctuary for migratory birds and other fauna. The wetlands of the country's lowlands alone support 32 species of mammals, 461 species of birds (among which 15 species are rare), 9 species of turtle, 20 species of snake and 28 species of fish.

Groundwater

Nepal also has abundant groundwater resources. The estimated renewable groundwater potential of the country is estimated to be 12 km3. They are major source of domestic uses and irrigated agriculture in Terai regions. Apart from Terai region, cities such as Kathmandu are highly dependent on groundwater resources to fulfil daily water needs. Groundwater discharges are vital for maintaining water levels in rivers originating from Middle Mountains."Groundwater Depletion in Kathmandu Valley: Need for Management"