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| Note: Irrigator allows more flexible representations of crop based planting decisions than earlier crop demand models, which better align with economic modelling. If the planted area is defined using the Function Editor, then other factors such as economics can be considered. In addition, planting decisions can be reviewed periodically during the season. This allows decisions to be made to cut back the irrigated area, reduce irrigation intensity or potentially trade water if there is insufficient water to finish the crop. |
Figure 6. Irrigator demand model (Planting Decision
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, Fixed Area)
If Lookup Table is choosen as the Decision Type, you must define a configuration table (as shown in Figure 7), which can be entered manually or imported using the Import button. This table is used to determine the Area and Underirrigation Factor for a given value of Available Water. Additionally, for values of Available Water that are not present in the table, linear interpolation is used to determine the other two parameters. For the example shown in Figure 7, if available resources in a RAS are 15000ML, the planting area will be 3000ML based on linear interpolation between (10000, 2000) and (20000, 4000). The available resources from water user (order debit type) is determined from the current account balances plus released water in the river plus on farm storage available water.
Figure 7. Irrigation demand model (Planting Decision, Lookup Table)
Target soil depletion
The amount of water in the rootzone is defined in terms of rootzone depletion, which describes the soil water deficit of the rootzone relative to field capacity. The target depletion can be specified as a time series, a pattern or as an expression.
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You can configure some simple economic considerations in Irrigator, as shown in Figure 8. To enable economic values to be configured for a crop, right click on the crop and choose Enable Economics. A simple, linear crop water production function is used to predict the reduction in crop yield resulting from water stress. Irrigator records the relative yield as a daily time series. The following parameters must be specified as an expression:
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For more information consult the Irrigator Demand Model page of the Source Scientific Reference Guide.
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8 - Irrigation demand model (Crop Economics)
Runoff
The supply escape efficiency defines the amount of applied irrigation water that becomes runoff. A value of 0 results in no irrigation runoff; 10% indicates that 10% of the applied irrigation water becomes runoff. You must also specify a return efficiency, which means that the proportion of runoff that is returned to the water user can be stored in the farm storage or returned to the river. By default, both are set to 0 and do not need to be configured.
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