Water Budget means the balance between the available water in the country and the water under use. There is a great variation in the distribution of water resources in space and time. Water is available in sufficient quantity during rainy season.
As the dry season sets in, there is a shortage of water. The reserves of our surface and underground water are about 23840 billion cubic metres. Out of this only 10860 billion cubic metre water is required for use.
The unit of measurement of amount of water is cubic metre or hectare metre. If water standing one metre deep on a perfectly level area of one square metre, then the total volume of whole of that water would be one cubic metre. In the same way, if water standing one metre deep on a perfectly level area of one hectare then the total volume of water would be one hectare metre.
In India, 90 percent rainfall take place during the short period of three months from June to August. There is a great variation in the number of rainy days in India. Average number of rainy days on the western coast is 137. In Rajasthan average number of rainy days is reduced to less than 10.
There is a variation in the nature of rainfall also. The rainfall may be heavy and continuous in the areas of more rainfall where as the rainfall may be low and intermittent in the areas of less rainfall. Hence, there is a great variation in the regional distribution of rainfall.
About 8 percent areas of the country receive more than 200 cm rainfall, 20 percent areas receive rainfall between 125 to 200 cm. and remaining 30 percent areas, receive less than 75 cm, rainfall. Uneven distribution of rainfall is responsible for the uneven distribution of surface and underground water.