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The average annual precipitation received in India is 4,000 km3, out of which 700 km3 is immediately lost to the atmosphere, 2,150 km3 soaks into the ground and 1,150 km3 flows as surface runoff. The total water resources in the country have been estimated as 1,953 km3. Nearly 62% or 1,202 km3 of the total water resources is available in the Ganga-Brahmaputra-Meghna basin. The remaining 23 basins have 751 km3 of the total water resources.
The annual water availability in terms of utilizable water resources in India is 1,122 km3. Besides this, the quantity of 123 km3 to 169 km3 additional return flow will also be available from increased use from irrigation, domestic and industrial purposes by the year 2050. The per capita availability of utilizable water, which was about 3,000 m3 in the year 1951, has been reduced to 1,100 m3 in 1998 and is expected to be 687 m3 by the year 2050.
Per Capita Availability of Water
Atmospheric Water Balance
The atmospheric water balance equation for India can be written by equating inflow with outflow plus the change in storage:
VI + ET + VAI = P + V0 + VAE + e
where VI represents the inflow of water vapor to the Indian atmosphere from land routes and sea routes, ET is the total evapotranspiration, VAI is the initial water vapour present in the atmosphere, P is the total precipitation, V0 is the outgoing water vapour, VAE is the water vapour present at the end of period under consideration.
Atmospheric Water Balance of India
(All components in million ha-m)
Hydrologic Water Balance
The equation for hydrologic water balance of the country for average annual conditions can be written as:
where P is the total precipitation, ET is total evapotranspiration, I is the total inflow as surface water (Is) and ground water (Ig), Qs is the outflow as surface water to oceans and other countries, (Qg) is the ground outflow, and DS represents the change in soil moisture storage. A diagrammatic representation of the hydrologic water balance for India is shown in Figure 2.2.
Hydrologic Water Balance of India
(All components in million ha-m per year)