Date | Author | Revision | Description of Change |
| Matthew Bethune | Creation of material | ||
|---|---|---|---|
27/11/2012 | G. Podger |
| Review |
Description 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.
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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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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Figure 4. Returned Water to Water User
Forecasting Requirements
Soil water balance
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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 equals 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.
Readily available water is defined by:
| Equation 4 |
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| Info |
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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:
| Equation 4 |
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].
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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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The target depletion is a user specified input and can be specified as either a pattern or as an expressiona function. The targets are defined in terms of soil water depletion. They are positive numbers for non ponded crops where the soil is typically in deficit. For ponded crops, the soil water depletion is negative. Thus targets for ponded crops are entered as negatives.
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- Regulated Target. This is the target at which Irrigators water and attempts to maintain soil depletion; and
- An Opportunistic Target. This target is used to generate opportunistic requests. For ponded crops, it also represents the maximum pond level prior to runoff. By default the opportunistic target is disabled and no opportunistic requirement is generated.
- A Refill Trigger - May allow better representation of irrigation scheduling for individual properties as irrigation does not commence until the forecasted soil depletion reaches the specified refill trigger
The behaviour of the first two target levels If you have an order debit system you can use an opportunistic target to generate requests which will extract water opportunistically, decreasing the depletion and thereby reducing the regulated water required to meet the regulated target depletion which is higher than the opportunistic target depletion. For ponded crops, opportunistic targets also reduce the amount of rainwater rejection because runoff will occur above the regulated target pond level.
The behaviour of the first two target levels is illustrated in the following two Figures for a ponded and non ponded crop.
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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
The Irrigator model calculates a list of regulated and opportunistic irrigation requirements between now and the maximum travel time. This requires that soil depletion, as defined by the soil water balance (Equation 1), is forecasted from the current time-step to the maximum travel time. This requires forecasts of evapotranspiration, rainfall, deep percolation, runoff and water that has been previously ordered, runoff and water that will be ordered. If an opportunistic target is not configured, the forecast soil depletion is supplemented by the forecast water ordered to meet the regulated target. If an opportunistic target is configured, the crop assumed it is going to receive the opportunistic volume ordered, so the forecast soil depletion is supplemented by the forecast water ordered to meet the opportunistic target - noting this is still capped by water available.
Forecast values of P and ETo are required to estimate soil depletion into the future. Two There are two options available for forecasting P and ETo and rainfall. The user can specify an average daily pattern, which represents long term average ETo and rainfall on P and ETo on each day of the year. Alternatively, you can specify the number of previous time-steps, and the model calculates the forecast P and ETo and rainfall by by averaging the previous specified number time-steps.
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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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This function defines the planted area for a specific crop on the specified date, taking into consideration available resources. If the user defines the planted area using the expression function editor, then other factors such as economics can be considered.
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:
- A lookup table between available water and planted area. The available water is specified by the Water User. The planted area is reassessed each time the planting decision is triggered (annually);
- Fixed Area – using a Data Source representing a time series of areas. The planted area is a reassessed each time the planting decision is triggered (annually); or
- Fixed Area – Defined using the expression function editor. The planted area is a reassessed each time the planting decision is triggered (annually). This option allows the planted area to be exposed to an economic model or other drivers.
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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 potentialcompared to potential. 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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- A lookup table between available water and planted area. The available water is specified by the water user for the current time-step;
- Fixed Area – using a Data Source of representing a time series of areas; or
- Fixed Area – Defined using the expression function editor.
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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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 |
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- 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.
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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| 1 | Decision Type | Allows user to select different methods for specifying the crop area on the planting date. Either a fixed area (which supports expressions 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 | 1/1-31/12 |
| 4 | Under Irrigation 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 an expression 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 |
|---|---|---|---|---|
Start Date | The first day where target levels are modified | Calendar date | 15 March | 1/1-31/12 |
End Date | The last day where target levels are modifedmodified | Calendar date | 30 April | 1/1-31/12 |
Modifier | The amount that crop water use is to be reduced during modifier period. | % | 0 | 0-50 |
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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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