Intensity of irrigation

Once the amount of water to be taken as seasonally available for design purposes is determined, the key question is then the area to be supplied. This involves consideration of cropping pattern, water requirements of individual crops, land availability, and the socioeconomic question of intensity of irrigation. The latter is the contentious item. Should the project be confined to an area all of which can be fully irrigated with the available water (intensive irrigation)? Or should the benefits of irrigation be spread more widely, supplying less than the full irrigation requirements to a larger area (extensive irrigation)? In the second case each cultivator can irrigate only part of his holding, or optionally he can supply all of it with less than the "optimum" quantity of water. The alternatives are described by the irrigated crop intensity (irrigation intensity). This is the percentage of the holding which is to be supplied with irrigation in a particular season, or annually if all seasons are totalled. The question of whether the figure is based upon application of the full "optimum" amount of water, or less than that (a common practice), is usually left unanswered. In some respects a more useful index of intensity of irrigation is simply the depth of water to be supplied, seasonally or annually, calculated as if applied uniformly over the whole area of the holding. Use of this index avoids the question of what water requirements to assume in calculating irrigation intensities.

The relative merits of intensive vs. extensive irrigation system design are much debated. The intensive approach leads to a smaller area to be served by canals (the "command") and lower canal cost, also lower total cost of land development. The extensive approach is often imposed by social pressures. In fact some states decree an upper limit on the design irrigation intensity, on the grounds that any higher intensity would unfairly benefit those within the command at the expense of those excluded from it. The pressure to expand the area served may continue through the life of the project, with petitions to extend the canal system to peripheral areas, or to introduce or permit pumping from canals to higher areas not served by the original system. Extensive irrigation has certain advantages. By limiting the supply of water to less than apparent need, it imposes an incentive for prudent use of water. It may also permit on-farm rotation of irrigated crop benefiting productivity in light soils. Of particular importance, it encourages development of supplemental groundwater, where wells are technically possible. This in turn may benefit watertable control.

Extensive irrigation may well increase productivity per unit of water supplied. However, it may introduce operational problems, particularly in large projects. In a small system that is village owned and operated, decisions on water-management, including the use of stored water, are likely to be made by consensus of the cultivators. In a large public system the cultivator is aware only of the canal which serves him. He is not aware of project-wide supply problems, the "grand design" of the system. If he receives less water than his apparent needs, he may endeavor to take it by whatever means are available. The subject of operation of supply systems in situations of water deficiency is discussed later. For present purposes, it is sufficient to underline the fact that supply of sufficient water to irrigate the whole command, in at least one season, is not automatically a design feature. It is a question to be decided in each case.