Text 7. 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 a perspective of the on-farm water balance

Figure3 The perspective of water balance at the field level

nty commonly used methods for calculating evapotranspiration, ranging in complexity from the Blaney-Criddle Method using primarily mean monthly temperature to more complete equations such as the Penman Method requiring radiation, temperature, wind velocity, humidity and other factors comprising the net energy balance at the crop canopy.

The actual crop water demand depends on its stage of development and variety. Generally it is estimated by multiplying E_tp by a crop growth stage coefficient, k_CO. Values of k_CO have been published by Jensen , Kincaid and Heermann and Doorenbos and Pruitt for a wide range of crops grown worldwide.

Some irrisome irrigated areas maintain a salt balance in the root zone with excess leaching during only years of plentiful water supplies, which may occur as infrequently as every three important soil characteristics in irrigated agriculture include the water-holding .

7.2.The water balance within the confines of a field is a useful concept for characterizing, evaluating or monitoring any surface irrigation system. In using this aspect of water balance, an important consideration is the time frame in which the computations are made, i.e. whether the balance will use annual data, seasonal data, or data describing a single irrigation event. If a mean annual water balance is computed, then it becomes reasonable that the change in root zone soil moisture storage could be assumed as zero. In some irrigated areas, precipitation events are so light that the net rainfall can be reasonably assumed to equal the measured precipitation. Under other circumstances, various other terms can be neglected. In fact, the time base and field conditions are often selected to eliminate as many of the parameters as possible in order to study the behaviour of single parameters.

One of the more important is crop evapotranspiration. The upward movement of groundwater to the root zone can usually be ignored if the water table is at least a metre below the root zone. Then if the soil moisture is measured before and after a period when there is no precipitation or irrigation, the depletion from the root zone is a viable estimate of crop water use.

There are two particularly important components in the field water balance which impact design and evaluation. The first is the irrigation requirement of the crop, or its evapotranspiration and leaching needs. This is a design parameter and will be briefly described here, but a detailed treatment is left to the FAO Irrigation and Drainage Paper 24, Crop Water Requirements, by Doorenbos and Pruitt (FAO, 1977). The second important component deals with field evaluation and concerns the nature of moisture content changes in the soil profile.

73. Evapotranspiration, ET, is dependent upon climatic conditions, crop variety and stage of growth, soil moisture depletion, and various physical and chemical properties of the soil. A two step procedure is generally followed in estimating ET: (1) the seasonal distribution of reference crop "potential evapotranspiration", E_tp, which can be computed with standard formulae; and (2) the E_tp is adjusted for crop variety and stage of growth. Other factors like moisture stress can be ignored for the purposes of design computations.

There are perhaps twegation water should be applied in excess of the storage capacity of the soil to leach salts from the rooting region, although this does not have to be achieved during each irrigation event. It can usually be applied on an annual basis. As a matter of practicality, the normally occurring deep percolation under most surface irrigation systems exceeds the leaching fraction necessary for salt balance, particularly for the first and second irrigations each season when deep percolation losses are typically greatest. In addition, precipitation helps leach salts throughout the year. Nevertheless or storage capacity of the soil; (2) the permeability of the soil to the flow of water and air; (3) the physical features of the soil like the organic matter content, depth, texture and structure; and (4) the soil's chemical properties such as the concentration of soluble salts, nutrients and trace elements.

The volume of soil actually monitored in readings by the neutron probe depends on the moisture content of the soil, increasing as the soil moisture decreases. The accuracy of soil moisture determinations near the ground surface is affected by a loss of neutrons into the atmosphere thereby influencing measurements prior to an irrigation more than afterwards. As a consequence, soil moisture measurements with a neutron probe are usually unreliable within 10-30 cm of the ground surface.

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Field capacity

The most common method of determining field capacity in the laboratory uses a pressure plate to apply a suction of -1/3 atmosphere to a saturated soil sample. When water is no longer leaving the soil sample, the soil moisture in the sample is determined gravimetrically and equated to field capacity.

A field technique for finding field capacity involves irrigating a test plot until the soil profile is saturated to a depth of about one metre. Then the plot is covered to prevent evaporation. The soil moisture is measured each 24 hours until the changes are very small, at which point the soil moisture content is the estimate of field capacity.

Permanent wilting point

Generally, at the permanent wilting point the soil moisture coefficient is defined as the moisture content corresponding to a pressure of -15 atmospheres from a pressure plate test. Although actual wilting points can be somewhere between -10 and -20 atm, the soil moisture content varies little in this range. Thus, the -15 atm moisture content provides a reasonable estimate of the wilting point.