SAPWAT |
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SAPWAT4 incorporates the internationally recognised Köppen-Geiger climatic system( Figure 5). The system is based on temperature-rainfall combinations so that the climate of the weather station can be classified by using the temperature and rainfall data of a weather station record. One adaptation was made, that is the second letter of the threeletter code that indicates rainy seasonality, is not used because rainfall seasonality is superseded by irrigation scheduling. Selecting a weather station automatically provides the linkage to the climate of that station. The climate map of a weather station is shown in( Figure 6).
Figure 6 A SAPWAT4 climate map
SAPWAT4 makes use of the FAO 56 procedure that separates soil evaporation from plant transpiration and, therefore, conforms to the FAO 56 defaults that determine soil water characteristics and evaporation parameters. Fortunately, FAO 56 specifies soils according to the familiar sand, silt and clay criterion into nine classes. The profile water balance during irrigation is also calculated and tabulated strictly in accordance with FAO 56 methodology.
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The methodology for estimating crop evapotranspiration under standard conditions has been well researched and due allowance can be made for nonstandard conditions, such as water shortages and salinity, arising from unusual circumstances and the realities of practical management. In short, we can be reasonably confident that we can estimate the amount of water being used by the crop and thus the net irrigation requirement.
Water that evaporates in the air or is blown away from sprinkler systems is regarded as a loss, so is water that is applied to uncultivated areas of the field. In SAPWAT4 this is reflected by system efficiency(%). In a study by
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Reinders et al.( 2010), irrigation water conveyance losses were found to vary between 4.3 % and 57 %. Irrigation system efficiencies varied from 38 % to 77 %. Extremely bad cases are isolated, but it is indicative of inefficiency levels that can be expected in worst-case scenarios.
If too much water is applied and penetrates below the roots this is also regarded as a loss, it is normally the result of an uneven distribution of water by the system or by lack of uniformity in the soil itself. In SAPWAT4 this is referred to as standard DU(%). It is very difficult to provide standardised or even defensible defaults for these values. The approach that SAPWAT4 has followed is to provide a preliminary default value for system efficiency and to set standard DU at 100 %. If, through measurement or judgement, the user can come up with real-life values, these should be substituted for the default values( Figure 7).
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Figure 7 The irrigation system set-up screen
The inclusion of an enterprise budget module in SAPWAT3 is now further developed in SAPWAT4( Figure 8).
Figure 8 Enterprise budget screen
This inclusion had been requested because planning irrigation water use without considering the economic impact does not give enough of a picture on which to base future planning for crop production. Provision is made for the introduction of enterprise budgets as part of the irrigation water requirement planning process. Income, expenditure and gross profit margin are reflected in the crop irrigation requirement
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tables. There is a linkage between the economic factors and the crop irrigation requirements so that if there is a variation in crop irrigation requirements with altering strategies, the impact on costs will be reflected and should there be a depression in yield, the impact on income and gross profit margin will also be reflected.
Rainwater harvesting module
SAPWAT4 provides a rainwater harvesting module( Figure 9) aimed at small areas, typically small farms or household gardens, therefore the water harvesting module is only available if the cultivated and irrigated area is less than 1 ha.
Figure 9 The water harvest screen
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The 50-year daily weather records provided by the derived weather stations are particularly useful because a thorough understanding of the rainfall pattern is essential when assessing the viability and developing suitable systems for rainwater harvesting. A water balance is the background to this module. Total of water requirement is the sum of the irrigation and household requirements, while water gain on the irrigated area is the sum of the rain that falls directly on the garden beds and run-off from the roof and surrounding areas that can be augmented by borehole water and grey water from laundry and bathroom waste. Run-off can be harvested from any combinations of roof, hard-packed soil around the homestead or adjoining roadways or from an adjoining area of natural vegetation. The storage to provide water for the dry season can be any combination of totally covered, impervious containers, open impervious containers or open ponds. The module can also be used to estimate the harvest width area of the infield rainwater harvesting techniques where runoff from an area of slow infiltration soil is stored in a shallow basin |
where the water can concentrate and infiltrate into the soil adjacent to the plant row.
Estimating irrigation water requirement
Data input for irrigation water requirement estimates has been redesigned to make these screens less intimidating. Farm-weather station combinations and fieldsoil-irrigation system combinations have been simplified and moved to the first input screen( Figure 10).
Figure 10 Page 1 of the data input screen for the estimation of irrigation water requirements
All information required by higher than farm level administration and planning levels are still available on this page, but detail is not shown unless asked for. First time farm and field input follows a similar approach; the minimum input requirement to run the estimate is asked for( Figure 11).
Figure 11 Page 1 of the data input screen for the estimation of irrigation water requirements
Once calculations have been done, the total farm data can be viewed and changed if needed be( Figure 12).
Figure 12 Farm data edit screen which comes available once first round irrigation requirement estimates have been done
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