Soil Water Balance
The best estimates of crop water use result from direct measurements, but this is not always feasible on large scale. The next most accurate approach would be one which integrates our understanding of the soil-plant-atmosphere continuum as mechanistically as possible. Taking the supply of water from the soil-root system, and the demand from the canopy-atmosphere system into account is essential to properly describe crop water use. The FAO Penman-Monteith reference crop evaporation (Smith et al., 1996) together with a mechanistic crop growth model, which uses soil water and grows a realistic canopy and root system provides the best possible estimate of the soil water balance. This approach has been out of reach of irrigators due to the specialist knowledge required to run the models. This high management cost can be drastically reduced by packaging the model in an extremely user-friendly format, eliminating the need for a detailed understanding of the intricacies of the soil-plant-atmosphere continuum. The benefits are increased too, because of the accuracy of the mechanistic, and therefore universally valid, estimation procedure.
THE SWB (SOIL WATER BALANCE) IRRIGATION SCHEDULING MODEL
Soil Water Balance (SWB) is a user-friendly irrigation scheduling model. It is based on the improved generic crop version of the soil water balance described by Campbell and Diaz (1988). A brief description follows with more detail given in Chapter 4 (Technical manual).
SWB is a mechanistic, real time, generic crop, soil water balance, irrigation scheduling model. It gives a detailed description of the soil-plant-atmosphere continuum, making use of a weather, soil and crop databases.
SWB performs the calculation of the water balance and crop growth using three units, namely weather, soil and crop.
The weather unit of SWB calculates the Penman-Monteith grass reference daily evapotranspiration according to the recommendations of the Food and Agriculture Organization (FAO) of the United Nations (Smith et al., 1996; Smith, 1992a).
In the soil unit of SWB, potential evapotranspiration is divided into potential evaporation and potential transpiration by calculating canopy radiant interception from simulated leaf area (Ritchie, 1972). This represents the upper limits of evaporation and transpiration and these processes will only proceed at these rates if atmospheric demand is limiting. Supply of water to the soil surface or plant root system may, however, be limiting. This is simulated in the case of soil water evaporation, by relating evaporation rate to the water content of the surface soil layer. In the case of transpiration, a dimensionless solution to the water potential based water uptake equation is used. This procedure comes up with a root density weighted average soil water potential which characterizes the water supply capabilities of the soil-root system. This solution has been shown to work extremely well by Annandale et al. (1996). If actual transpiration is less than potential transpiration the crop has undergone stress and leaf area development will be reduced. The multi-layer soil component of the model ensures a realistic simulation of the infiltration and crop water uptake processes. A cascading soil water balance is used once canopy interception and surface runoff have been accounted for.
In the Crop unit, SWB calculates crop dry matter accumulation in direct proportion to transpiration corrected for vapour pressure deficit (Tanner and Sinclair, 1983). It also calculates radiation limited growth (Monteith, 1977) and takes the lower of the two. This dry matter is partitioned to roots, stems, leaves and grain or fruits. Partitioning depends on phenology calculated with thermal time and modified by water stress.
SWB also includes a model based on the FAO crop factor approach (Smith, 1992b). This model can be used to calculate the soil water balance.
A mechanistic, and therefore universally valid, approach to estimating crop water use, like that described here, has several advantages over the more empirical methods often used. Using thermal time to describe crop development removes the need to use different crop factors for different planting dates and regions. Splitting evaporation and transpiration solves the problem of taking irrigation frequency into account. Deficit irrigation strategies, where water use is supply limited, can also be more accurately described.
Extensive use is made of graphics with the soil water balance presented at the end of the simulation. Valuable information on the components of the soil water balance with the deficit to field capacity and recommendations for the next irrigation are also given.
SWB uses a crop parameter, weather, field, water and soil database which negates the need to make several ASCII files in a text editor to handle each simulation. This, together with the fact that several fields can be simulated simultaneously, makes it an ideal tool for the large farmer or irrigation consultant.
User-friendly models can make accurate, high technology approaches to irrigation scheduling feasible on farm. This approach can both reduce the costs and increase the benefits of irrigation scheduling. The grower, however, needs to be convinced that it is important to manage irrigations.
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