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second letter of the threeletter code that indicates rainy seasonality , is not used because rainfall seasonality is superseded by irrigation scheduling .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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