Irrigation methods and their selection

There are three broad classes of irrigation systems: (1) pressurized distribution; (2) gravity flow distribution; and (3) drainage flow distribution. The pressurized systems include sprinkler, trickle, and the array of similar systems in which water is conveyed to and distributed over the farmland through pressurized pipe networks. There are many individual system configurations identified by unique features (centre-pivot sprinkler systems). Gravity flow systems convey and distribute water at the field level by a free surface, overland flow regime. These surface irrigation methods are also subdivided according to configuration and operational characteristics. Irrigation by control of the drainage system, subirrigation, is not common but is interesting conceptually. Relatively large volumes of applied irrigation water percolate through the root zone and become a drainage or groundwater flow. By controlling the flow at critical points, it is possible to raise the level of the groundwater to within reach of the crop roots. These individual irrigation systems have a variety of advantages and particular applications which are beyond the scope of this paper. Suffice it to say that one should be familiar with each in order to satisfy best the needs of irrigation projects likely to be of interest during their formulation.

Irrigation systems are often designed to maximize efficiencies and minimize labour and capital requirements. The most effective management practices are dependent on the type of irrigation system and its design. For example, management can be influenced by the use of automation, the control of or the capture and reuse of runoff, field soil and topographical variations and the existence and location of flow measurement and water control structures. Questions that are common to all irrigation systems are when to irrigate, how much to apply, and can the efficiency be improved. A large number of considerations must be taken into account in the selection of an irrigation system. These will vary from location to location, crop to crop, year to year, and farmer to farmer. In general these considerations will include the compatibility of the system with other farm operations, economic feasibility, topographic and soil properties, crop characteristics, and social constraints (Walker and Skogerboe, 1987).