Determining water requirements

7.1.The irrigation system may not be designed to supply the total amount of moisture required for crop growth. In some cases, precipitation or upward flow from a water table may contribute substantially towards fulfilling crop water requirements. It is also unrealistic to expect that irrigation can be practiced without losses due to deep percolation, or tailwater runoff. The fraction of the water that is used should be maximized, but this fraction cannot be 100 percent without other serious problems developing such as a salt build-up in the crop root zone.

The dependency on irrigation in an area requires some analyses of the water balance. Water balance may have three perspectives. The first is the balance of agricultural demands within a watershed as depicted in Figure 1. The outcome of such an analysis establishes the safe yield of water from various sources and thereby indicates the area of a project, the priorities among projects, and the configuration of the large systemic components of the project. An evaluation at the field level presumes that this information is available, and it should be generally understood in as much as the limits of on-farm irrigation may be dictated by the magnitude and distribution of the total water supply.

Figure1. The perspective of water balance at the river basin level

The second water balance perspective, illustrated in Figure 2, is the water balance within the farm or command area. An individual field is generally irrigated in concert with others in the command or farm through sharing the water delivered through a canal turnout or a well. Fields also typically share drainage channels. Water balance at the farm or command area level is established on a field's access to water, its priority, timing and duration. Again, a field evaluation presumes that these factors have been formulated and can be determined. Figure 3 illustrates the perspective of water balance at the field level.

Figure 2