Water is one of the most important inputs essential for the production of crops. Plants need it continuously during their life and in huge quantities. It profoundly influences photosynthesis, respiration, absorption, translocation and utilization of mineral nutrients, and cell division besides some other processes. Both its shortage and excess affects the growth and development of a plant directly and, consequently, its yield and quality. Rainfall is the cheapest source of natural water-supply for crop plants. In India, however, rainfall is notoriously capricious, causing floods and droughts alternately. Its frequency distribution and amount are not in accordance with the needs of the crops. Artificial water-supply through irrigation on one occasion, and removal of excess water through drainage on another occasion, therefore, become imperative, if thecrops are to be raised successfully. Water management in India, thus, comprises irrigation or drainage or both, depending considerably on the environmental conditions, soil, crops, and climate. It is a situation-oriented entity.
Water effects the performance of crops not only directly but also indirectly by influencing the availability of other nutrients, the timing of cultural operations, etc. Water and other production inputs interact with one another. In proper combinations, the crop yields can be boosted manifold under irrigated agriculture.
Water is a costly input when canals supply it. The constructing of dams and reservoirs, the conveying of water from storage points to the fields, the operating and the maintaining of canal systems involve huge expense. The misuse of water leads to the problems of water-logging, salt-imbalance, etc., thus rendering agricultural lands unproductive. Hence a proper appreciation of the relationship among soils, crops, climate and water resources for maximum crop production.
Water resources. Taking the total geographical area of the country at 328 million hectares and the average annual rainfall at about 112 cm, the total annual precipitation in the country is estimated at about 3,700,000 million cubic meters. The south-westerly monsoon contributes over 80 per cent to the total precipitation in the country. The Central Water and Power Commission, New Delhi, has estimated that of the total annual precipitation amounting to 800,000 million cubic meters seeps into the ground, about 1,700,000 million cubic meters flows into the rivers and the remaining amount of about 1,200,000 million cubic meters evaporates back into the atmosphere.
The water, flowing on the surface and that seeping into the ground, forms the two major sources of water for irrigating crops.
Surface-Water Resources. A large number of rivers of various potentials and discharges are spread all over the country. The rivers in the north, which originate from the Himalayas, are snow-fed and, thus, have less seasonal fluctuations in their flow than the rivers in the other parts of the country. The flow in the rivers of the central and southern parts of the country depends entirely on the monsoon. The rivers flow to the full during the rainy season (July to September) and their flow dwindles with the approach of the summer.
The surface-flowing water needs to be trapped in ponds, tanks, lakes or artificial reservoirs when it is available in abundance so that it can be fruitfully used for irrigation during the rainless period to meet the water needs of crops. Of the annual surface flow of 1,700,000 million cubic meters, only about 666,000 million cubic meters can be utilized for the purpose of irrigation owing to the physiographical limitations.
Ground-Water Sources. Substantial supplies are also available from ground-water sources. Of the 800,000 million cubic meters of rain-water that seeps into the ground annually, about 430,000 million cubic meters of it is absorbed by the surface layers of the earth’s crust and, thus, can be utilized directly by the vegetation in the process of evapo-transpiration and growth. The remaining 370,000 million cubic meters of rain-water percolates deep into the porous strata of the earth’s crust, representing the gross annual enrichment of the underground water. This ground-water is tapped by digging or drilling shallow or deep wells and is lifted by using mechanical devices for irrigating the crops.