Irrigator operates on a daily basis generating demands and extracting water to meet these demands via the water user and supply nodes. Irrigator maintains a daily water balance for each cropping area during its planting season, to calculate the daily soil water deficit and an irrigation requirement. The irrigation requirements are used by the Water User to generate orders and opportunistic requests and to subsequently place orders and requests and to extract water from a water source.The model can be applied in both regulated and unregulated systems.
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Figure 1. Irrigator demand model
Target Modifier
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- Evapotranspiration - defined in FAO56 as the amount of transpiration that would occur from a reference crop. Procedures for calculating evapotranspiration are documented in FAO56. In addition, the BOM and SILO climate products produce daily estimates for FAO56. Alternatively, pan evapotranspiration and pan factors can be used to define evapotranspiration;
- Average evapotranspiration (Figure 3) - used when forecasting orders. Two These options are provided for specifying the average evapotranspiration. You can enter a daily pattern of average evapotranspiration, specify an expression, or you can select to calculate an average evapotranspiration runtime, where the average is calculated as a rolling average of the number of previous model time-steps specified. A value of 14 days would be a good first estimate; and
- Rainfall and average rainfall are used to specify the actual forecast rainfall. The former can be specified as a constant value, a time series or a function (as shown in Figure 4). The latter is similar to Figure 3.
Figure 2. Irrigator demand model (Climatic parameters)
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Figure 3. Irrigator demand model (Average evapotranspiration)
Figure 4. Irrigator demand model (Rainfall)
Fallow Crop
Areas that are not being cropped are referred to as fallow. Every irrigator demand model has, by default, one fallow area. Fallow area occurs between cropping seasons or during the irrigation season if the maximum developed area is not planted. The fallow cropping area does not order or receive irrigation water. However, a soil water balance of the fallow crop is maintained, and impacts on rainfall runoff from the fallow area and the initial souild moisture assigned to new crops area. The depth of rootzone for fallow represents the depth of soil from which evaporation can occur.
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This is used to define parameters specifically related to the crop to be planted. To add a new crop, right click Irrigator and choose Add Crop Type. Figure 4 5 shows an example of a crop, named Wheat, that has been added to the model. Consult the Source Scientific Reference Guide for greater detail regarding parameters for the following sections.
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5. Irrigator demand model (General and Soil Configurations)
Soil Configuration
Crop specific soil factors can be defined on the same page as General Configuration - see Figure 45. Soil moisture capacity is specified at the district level and this is assumed constant for all crops in the district. The Depth of the Root Zone, Depletion Factor and Initial Depletion are specified for each crop.
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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 expression editorFunction 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. |
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6. Irrigator demand model (Planting Decision, 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
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, 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.
Two target depletions can be specified (as shown in Figure 46):
- A Regulated Target - the target at which Irrigators water and attempts to maintain soil depletion; or
- An Opportunistic Target - used to generate opportunistic requests.
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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:
- Yield factor - represents the Yield response factor, as expressed in equation 90 of FAO56. This parameter defines how yield is reduced as a result of water stress. It is used in the calculation of the relative yield daily time series (one of the outputs of the demand model). Note that in Irrigation, the yield response factor is entered as a percentage rather than as a fraction;
- Expected Usage - this is a depth measured in mm (note that 100mm is equivalent to 1ML/ha); and
- Productivity - allows you to multiply the relative yield by a productivity term (eg. $/ha or tonnes/ha) to calculate socio-economic outputs.
For more information consult the /wiki/spaces/SD35/pages/57872388Irrigator 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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