India is basically an agricultural country, and all its resources depend on the agricultural output. Water is evidently the most vital element in the plant life. Water is normally supplied to the plants by nature through rains. However, the total rainfall in a particular area may be either insufficient, or ill-timed. In order to get the maximum yield, it is essential to supply the optimum quantity of water, and to maintain correct timing of water. This is possible only through a systematic irrigation system-by collecting water during the periods of excess rainfall and releasing it to the crop as and when it is needed. Thus, the necessity of irrigation can be summarised in the following four points:

1. Less Rainfall

When the total rainfall is less than needed for the crop, artificial supply is necessary. In such a case, irrigation work may be constructed at a place where more water is available, and then to convey the water to the area where there is deficiency of water. Rajasthan canal is one such example. It conveys water to the arid zones of Rajasthan, where the annual rainfall hardly exceeds 100 to 200 mm.

2. Non-uniform Rainfall

The rainfall in a particular area may not be uniform over the crop period. During the early periods of the crop, rains may be there, but no water may be available at the end, with the result that either the yield may be less, or the crop may die altogether. By the collection of water during the excess-rainfall period, water may be supplied to the crop during the period when there may be no rainfall. Most of the irrigation projects in India are based on this premise. The rainfall during the winter is very scanty, and hence rabi crops need artificial supply of water through the irrigation works.

3. Commercial Crops with Additional water

The rainfall in a particular area may be sufficient to raise the usual crops, but more water may be necessary for raising commercial and cash crops.

4. Controlled Water supply

By the construction of proper distribution system, the yield of the crop may be increased.