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:  

                V_I + E_T + V_AI = P + V₀ + V_AE + e                                                                                                                                                                   

where V_I represents the inflow of water vapor to the Indian atmosphere from land routes and sea routes, E_T is the total evapotranspiration, V_AI is the initial water vapour present in the atmosphere, P is the total precipitation, V₀ is the outgoing water vapour, V_AE 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, E_T is total evapotranspiration, I is the total inflow as surface water (I_s) and ground water (I_g), Q_s is the outflow as surface water to oceans and other countries, (Q_g) 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)