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Date
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Author
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Revision
Description
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27/11/2012
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G. Podger
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Review
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and rationale
Source models the use of water by a combination of supply point and water user nodes. The water user node provides a range of demand models that can be configured to represent irrigation demand. Three different models of irrigation demand have been incorporated into Source to represent the different approaches used in Australia. These three approaches include:
1) Regression models, used by the Murray Darling Basin Authority (MDBA)
2) PRIDE Demand model - SRG, used in Victoria
3) IQQM Crop Model SRG, used in NSW and QLD.
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Figure 1 — Schematic of Irrigator Demand Model
Processing Logic
There are three key processing steps in the Irrigator model.
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Figure 3. During Flow Phase- Ordered Water Supplied
During flow phase – Ordered Water Supplied
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p is the average fraction of Total Available Soil Water (TAW) that can be depleted from the rootzone before moisture stress (reduction in ET) occurs [0-1].
For Dr > RAW, Ks is given by:
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Figure 5. Relationship between water stress coefficient and soil water depletion.
Crop Evapotranspiration
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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Figure 6. Escape loss and return flows.
Escapes apply at both the district and crop level (Figure 6). At a district level they could represent processes such as channel seepage, channel escapes and meter errors. The user can add as many escapes as required at the district level. For each irrigation district, there is one default escape factor for the net return flow to the water user. This is used to scale the total return flow from the irrigator demand model. The escape volume from this is considered a ‘loss’ and the user is not able to define a proportion of this escape factor that is returned.
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If the Refill Trigger is activated, and a soil depletion value greater than the Regulated Target defined, orders will not occur until the forecasted soil depletion reaches the specified trigger. Orders are then placed at the current timestep, and if necessary at subsequent timesteps, until the forecasted soil moisture depletion is less than or equal to the Regulated Target. This allows for the possibility that restrictions such as pump capacity mean that it is not possible to meet the target in one timestep.
Forecasting soil depletion
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The regulated and opportunistic requirement are calculated between now and the maximum travel for each crop as:
| Equation 12 |
| Equation 13 |
where:
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ne is total number of district escapes
- The volume of water applied is then distributed between regulated requirements and opportunistic requirements.
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- The volume applied to each crop is in proportion to the regulated and opportunistic irrigation requirements.
| Equation 24 |
- Soil depletion is updated for the applied irrigation water (Equation 1)
- Escape volume (Equation 9) and return volume (Equation 11) for each crop resulting from irrigation deep percolation and runoff are evaluated.
- The deep percolation from irrigation and rainfall are totaled for each crop.
- The runoff from irrigation and rainfall are totaled for each crop. The total returned crop runoff is calculated.
- The crop return flow is added to district return flow.
- The final return efficiency is applied to the return flow to provide a final flux of water that is returned to the water user (Equation 11).
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RemainingGrowingDays= number of growing days left in the irrigation season for this crop.
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IrrigatorRemainingUsage = Estimate of the total volume of irrigation required to complete crops in the current irrigation season
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| Parameter | Description | Unit | Default | Typical Range | |
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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 |
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Plant Date | Date that the planting decision is made and the crop is planted | Calendar date | 31 May | 1/1-31/12 | |
Harvest Date | Date crop is harvested. By default this is disabled | Calendar date | n/a | 1/1-31/12 | |
Under Irrigation Factor | Reduces irrigation application by adjusting the target depletion level. | % | 0 | 0-50 | |
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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| Parameter | Description | Unit | Default | Typical Range | |||
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Runoff - Supply Escape Efficiency | Percentage of applied irrigation water that becomes runoff | % | 0 | 0-20 | |||
Runoff - Return Efficiency | The percentage of runoff that is able to be harvested and returned |
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Crop Deep Percolation
| Parameter | Description | Unit | Default | Typical Range |
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Deep percolation - Supply Escape Efficiency | Percentage of applied irrigation and rainfall that becomes deep percolation and drains below the rootzone. | % | 0 | 0-20 |
Deep percolation – Return efficiency | Disabled as it is assumed that no deep percolation can be harvested and returned to water user | % | 0 | 0 |
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| Parameter | Description | Unit | Default | Typical Range | ||
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Yield | Water stress coefficient (Ky) describes the effect of water stress on crop transpiration | % | 0 | 0-100 | ||
Expected Usage | Expected total irrigation requirement for crop for the growing season. This is used to evaluate expected surplus/shortfall of water availability | mm | 0 | 5-15 | ||
Productivity | Multiplier to convert yield into user specified output units | none |
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References
DIPNR (2004), IQQM Reference manual, Version 1.2, NSW Department of Infrastructure Planning and Natural Resources, NSW.
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