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3) IQQM Crop Model SRG, used in NSW and QLD.
In addition, Melbourne University as part of the eWater CRC, undertook a number of years of research looking at how to improve demand modelling. A prototype irrigation demand model (NGenIrr) was developed as part of this research. The Irrigator demand model was developed by combining the best functionality from the existing models into a common demand model. A key focus was on keeping the model as simple and parsimonious as possible, while not compromising the key functional requirements.
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Crop water use occurs at potential rates until soil depletion equals the readily available water (RAW) (Figure 2). RAW is defined as in FAO56 (Equation 4) .The water stress coefficient (Ks) defines crop water use response relative to soil water depletion (Equation 5). Ks decreases linearly from 1 when soil depletion exceeds RAW to 0 at soil depletions levels greater than TAW (Figure 2). When Ks falls to 0.05, the crop is assumed to die. When the crop dies, the area is returned to fallow and no more irrigation requirements are generated for this crop. The crop can only be re-established if triggered by a new planting decision.
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The crop water stress (Ks) and available water asymptote to 0, meaning the actual crop water use progressively reduces as the crop gets more stressed. It therefore takes a long time for the crop water stress coefficient to reduce to 0.05 and cause crop death. |
Readily available water is defined by:
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Crop evapotranspiration is calculated using the single crop coefficient approach described in FAO56 (Equation 6). Crop coefficients at various growth stages can be modelled in Source as described in the Irrigation Demand Model Crop Factors SRG entry. Alternatively, Source also offers sufficient flexibility to apply daily crop factors if they are known for a particular crop based on a different source or method.
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For each crop, the user configures a planting decision trigger. The planting decision trigger includes a decision type, plant date, optional harvest date, planted area definition and an under irrigation underirrigation factor.
The decision type defines the method for calculating the planted area for a crop. The planted area can be specified using one of three methods:
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The Plant Date defines when the crop is established.
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Underirrigation Factor
The under irrigation underirrigation factor is used to adjust change the target soil depletion to achieve a reduction in water use compared to potential. This option may be used where you are trying to keep a crop alive The factor refers to the percentage of the tension water (RAW - TAW) when it is larger than zero. The underirrigation is only assessed on planting decision day and remains the same throughout the planting period. This option may be used where you are trying to keep a crop alive and not maximise production ie. stressing the crop and getting a reduced yield..
| Equation 16 |
where:
UFactor is user specified under irrigation underirrigation factor
TargetDepletionRegulated is target soil water depletion (mm)
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The model will ignore the functions of
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Planting Decision Reassessment Trigger
The planting decision reassessment trigger allows the user to reassess the planted area of a crop. The user can configure as many reassessment triggers per crop as desired. Each reassessment trigger includes a decision type, reassessment date, an under irrigation underirrigation factor and an area relationship.
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If the calculated planted area has reduced, then the crop area will be reduce to the new calculate value. The difference in planted area is returned to fallow. The planted area cannot increase through a reassessment trigger. If the under irrigation underirrigation factor is specified, then a new target depletion is evaluated for the remainder of the crop period.
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Zfallow is depth of the fallow (m)
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Target Modifiers
Irrigator behaviour can change during the year. For example, in grazing industries, less water may be applied coming into winter to reduce the risk of water logging. Another example is the horticultural industry when a fruit crop has been harvested, irrigation intensity can be reduced to a maintenance level without impact on yield.
Irrigation Target modifiers are included into Source to allow a simple mechanism to reduce irrigation intensity for recurring periods of time. Basically, this allows a simple way of modifying the target level. This is achieved by the user defining a reduction in irrigation application over a date range. The % reduction in irrigation is used to modified modify the target depletion level during the selected date range.
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| Parameter | Description | Unit | Default | Typical Range | |
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| 1 | Decision Type | Allows user to select different methods for specifying the crop area on the planting date. Either a fixed area (which supports functions and data sources) or a lookup table that relates available water on the planting date to the planted area. | Fixed Area | ||
| 2 | Plant Date | Date that the planting decision is made and the crop is planted | Calendar date | 31 May | 1/1-31/12 |
| 3 | Harvest Date | Date crop is harvested. By default this is disabled | Calendar date | n/a | 1/1-31/12Under Irrigation |
| 4 | Underirrigation Factor | Reduces irrigation application by adjusting changing the target depletion level. When it is zero or negative it is inactive and when it is >0 the functions of Soil Depletion Target and Target Modifier will be ignored. | % | 0 | 0-50 |
| 5 | Area relationship | Defines the crop area as a function of either available water using a lookup table or from a function or data source | ML vs ha | Fixed area | 3-14 ML/ha |
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