Water is an essential ingredient for food production. Initially, natural rains provided water supply to agriculture in forest areas. There was no conscious effort to tap water resources. As the population increased, civilizations came up on the banks of rivers. Rain water is available only on the day of rain, but river water is available for a longer duration. Hence dependability increases with river water. In the case of perennial rivers, one can say that production is assured. Further increase in population led to the growth of communities away from riverside also. Then we have the situation of one set of communities depending on rain water only for its agriculture and another on rivers. The first one had an unpredictable situation - if rains were delayed or rainfall was poor in a particular year, the resultant droughts caused major short falls in production and occasional famines.

It is not always possible to use river water effectively. When water is needed - during non monsoon season -, it may not be available in a river in requisite quantities. When river water is not required for agriculture - during rainy seasons -, rivers may overflow causing floods and most of the flood water will flow into the seas. In order to even out the demand-supply function, minor irrigation through tanks was conceived. Many tanks and reservoirs were built to harvest rainwater. It was also possible to have a sequence of tanks connected by canals and waters going to rivers were diverted to these tanks.

Tank based irrigation brought in stability in agricultural production. Tank water was used for other purposes as well - bathing, washing etc. Even now, tanks play a significant role in irrigation.

Technology brought in construction of large dams impounding large waters in a reservoir. Krishnarajasagar on Cauvery is an early example. Our five year plans emphasized this mode of irrigation so as to exploit the maximum amount of water in a river system. A major irrigation project, today, consists of a large/very large dam normally constructed in hilly areas so as to take into account volume efficiency available in valleys, submerging thousands of hectares of mostly forest lands and taking the reservoir water to fields over hundreds of miles by means of canals. Since the activity can be coordinated in a project mode, this is becoming the main mechanism of irrigation in our country.