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Chapter 5

Simple estimation of crop water requirements

Irrigation scheduling is the term used to describe the procedure by which an irrigator determines the timing and quantity of water application. Accordingly, the two classical questions of irrigation scheduling are: when to irrigate? and, how much water to apply?

In conventional low-frequency irrigation by surface flooding or sprinkle methods, the answer to the first question is generally: when the reservoir of available soil moisture in the root zone is nearly depleted. In practice, that implies: when the crop is near the point of experiencing distress. With high-frequency irrigation, in contrast, the farmer need no longer worry about when soil moisture is depleted or when plants are about to suffer thirst. Such situations can be avoided entirely. To the old question, when to irrigate? the irrigator can now answer: as frequently as possible, even daily. To the second question, how much water to apply? the answer is: enough to meet current evaporative demand and to prevent salinization of the root zone.
The evaporative demand is a variable imposed by weather conditions, which
fluctuate over time. It can be determined by monitoring relevant weather variables (e.g. temperature, wind, atmospheric humidity and solar radiation) and then applying any of several functional equations or formulae to calculate the potential evapotranspiration (Figure 31 and 32).


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Weather variables affecting evaporation, transpiration and soil moisture uptake by roots


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Radiation and water balances on a plant under localized irrigation

Alternatively, and more simply, the evaporative demand can be estimated from the evaporation rate measured directly by means of a standard evaporimeter. One of the simplest and most useful of such devices is the evaporation pan. It consists of a shallow water-filled container that is placed on the ground within the irrigated area. The amount evaporated daily can be obtained conveniently by measuring the volume of water per unit area of the pan that has to be added to the pan to bring the water surface back up to a marked level. The pan evaporimeter gives an indication of the integrated effect of radiation, wind, temperature and humidity on evapotrans-piration from an open field (Figure 33).


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The standard Class A pan evaporimeter, developed by the US Weather Bureau

Of the various standardized pans, the one used most widely is the Class A pan, introduced by the United States Weather Bureau. It is a circular container, 121 cm across and 25.5 cm deep, placed on a slatted wooden frame resting over the ground. The pan is filled with water to a height about 5 cm below the rim. This standard design is relatively easy to follow, yet it is not critical to do so precisely. Any configuration that does not differ too radically from the Class A pan will, in the experience of the author, give nearly the same results. However, while inexpensive and easy to install, maintain and monitor, evaporation pans do have several shortcomings.
Although a crop field responds to the same climatic variables as does water in a pan, it does not necessarily respond in the same way. A vegetated surface generally differs from a free water surface in the reflectivity, thermal properties (heat storage), day-night temperature fluctuation, water transmissivity and aerodynamic roughness of the plant canopy. Such factors as the colour of the pan, depth and turbidity of the water and shading from nearby plants can all affect the measurement to some degree.
Pan evaporation depends on the exact placement of the pan relative to wind exposure. Pans surrounded by tall grass may evaporate 20 to 30 percent less than pans placed in a fallow area. Rainfall may occur during the irrigation season and may add water to the pan, or thirsty animals wandering in the area may drink from the pan, thus detracting from its usefulness. To avoid water loss to drinking animals (especially birds), pans are often covered by screens. This may reduce the evaporation rate by some 10 to 20 percent, thus requiring the use of a correction factor.
All these shortcomings notwithstanding, pan evaporimeters, if properly sited and maintained, can be useful inasmuch as they tend to correlate with other measurements of potential evapotranspiration (PET).3 The problem is how to translate pan evaporation into an estimate of the crop's PET, and in turn into actual irrigation requirements.
The first step is to apply a correction factor to account for the fact that free water generally evaporates more than does a crop stand, even if that stand is dense, well endowed with soil moisture and is transpiring at its full potential rate. Many
experiments have shown that the appropriate correction factor can vary from 0.5 to 0.85. In the experience of the author, based on direct measurements as well as a review of the literature, that factor is typically about two-thirds (say, 0.66):

PETfull cover = 0.66 Epan (4)

The second step is to account for the stage of the crop's growth, as indicated by its fractional ground cover. That can be estimated from ground observations of the area shaded by the crop. Since the potential evapotranspiration, while a function of the crop's coverage, is not simply proportional to it, it is proposed here to use the following empirical relationship:

PETpartial cover = 0.33 (1 + C) Epan (5)

where C is the fractional ground cover of the crop, varying from 0 (when the crop is first sown or planted) to 1 (when the crop stand is full). In the latter case, equation (5) becomes equation (4).

The third step is to estimate the irrigation requirements (I), including the actual crop water requirement (W), plus a leaching fraction (L), minus the rainfall that occurred since the previous irrigation (R). Assuming that the actual crop water requirement is about 80 percent of PET and that the desirable leaching fraction is 10 percent of PET (i.e. W = 0.8 PET, L = 0.1 PET), the result is:

I = {0.33 x (W + L)} Epan (1 + C) - R
  = (0.33 x 0.9) Epan (1 + C) - R                     (6)
  = 0.3 Epan (1 + C) - R

These relations should only be regarded as preliminary estimates. Actual field measurements of specific crop responses to varying amounts of irrigation under local conditions should provide more reliable guidance on optimal irrigation amounts. Furthermore, the estimates above refer only to a crop's active growth stages. As a crop reaches maturity and its tissues become senescent, its water requirements naturally diminish. Irrigation is discontinued when its further contribution to crop yield no longer justifies its added cost.

3 Potential evapotranspiration (PET) has been defined as the volume of water per unit area of field evaporated and transpired by a dense stand of actively growing short grass that is well endowed with (never short of) water. Actual evapotrans-piration (AET) is usually less than PET, as a real field may not be uniformly dense and well watered (Penman, 1948; Monteith, 1980).

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