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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 2 Demand 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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The Irrigator demand model is based on the existing demand models, including the NOW IQQM Crop Model 2, Victorian Pride Model and the FAO crop water use guidelines (FAO-56FAO56). The strengths from each of these demand models have been extracted and included into the Irrigator demand model. In addition, new functional requirements identified by NSW NOW reviews and Victorian studies have been included.
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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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The soil water balance for each crop is modelled using a single layer moisture store (Equation 1), as outlined in FAO 56 FAO56 – Chapter 8. Equation 85 from FAO56 represents the water balance of the rootzone.
| Equation 1 |
where:
Dr,i root zone depletion at the end of day i [m],
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The user specifies the soil moisture capacity at the district level and this is assumed constant for all crops in the district (Equation 2). The soil moisture capacity is given by the difference in soil moisture content between field capacity and permanent wilting point. The total available water (TAW) for each crop is a product of the depth of the rootzone (when fully grown) and q Cap (Equation 3). The depth of the rootzone is specified for each crop. The depth of rootzone is also specified for fallow. While the fallow has no rootzone, this is treated as the depth of soil from which evaporation can occur.
| Equation 2 |
| Equation 3 |
where:
TAW the total available soil water in the root zone [m],
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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:
| 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:
| Equation 5 |
where:
Ks is a dimensionless transpiration reduction factor dependent on available soil water [0 - 1],
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Figure 5. Relationship between water stress coefficient and soil water depletion.
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Crop Evapotranspiration
Crop evapotranspiration is calculated using the single crop coefficient approach described in FAO-56 FAO56 (Equation 6). The effects of soil water stress on crop ET are calculated by multiplying the crop coefficient by the 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.
The effects of soil water stress on crop ET are calculated by multiplying the crop coefficient by the water stress coefficient (Ks). The Ks value is evaluated based on soil water depletion at the start of the time-step.
| Equation 6 |
where:
Kc = Single crop coefficient on growth day i j of the crop
Ks = water stress coefficient describes the effect of water stress on crop transpiration
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Effective rain considers that a proportion of rainfall will be interception intercepted or lost and not infiltrate. FAO-56 FAO56 proposes an interception loss of 0.2 * ETo. This interception loss is included into Irrigation to define effective rain as described in (Equation 7).
| Equation 7 |
Rainfall Runoff
Rainfall runoff occurs when rainfall results in soil moisture exceeding saturation or a maximum target pond level for ponded crops.
| Equation 8 |
where:
RainfallRunoff = the amount of rainfall runoff depth for cropping area (m)
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DeepPercolation = amount of deep percolation (m)
Runoff occurs from both cropped and fallow areas. In addition, runoff occurs from an Undeveloped Area if configured.
Deep Percolation, Irrigation runoff, district losses and escapes
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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 processing of escapes at both a district and crop level is the same. For each escape, the escape volume is firstly calculated (Equation 9), the escape volume is removed from the volume of water supplied (Equation 10) and then the return volume is evaluated (Equation 11). Where there are multiple escapes, the escape factors are effectively multiplicative.
| Equation 9 |
| Equation 10 |
| Equation 11 |
The volume supplied is reduced by the escape volume
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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.
Two Three target depletions can be specified
- 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.
The behaviour of the target levels is illustrated in the following two Figures for a ponded and non ponded crop.
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- 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
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.
Figure 7 is a regulated –non ponded crop. On day 5, when the crop is planted, a Regulated Target of 25 mm is established. Water is ordered to bring the crop soil water depletion up to this target level. From this day on, water is ordered to maintain soil depletion at the regulated target level. On day 10 there is a large rainfall event. This results in runoff and the soil water depletions falls to less than the regulated target. No further water is ordered until the soil dries out to the regulated target level.
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Figure 8. Ponded crop pond level maintenance
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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.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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Regulated and opportunistic requirements
The irrigation regulated requirement is defined as the amount of water required to maintain soil depletion at a target level. The opportunistic requirement is defined as the amount of water that could be utilised if it is available. Both irrigation regulated and opportunist requirements can be limited to within an irrigation season. If user has specified irrigation season, orders and requests are only generated within the season.
The irrigation regulated and opportunistic requirement are calculated between now and the maximum travel for each crop as:
| Equation 12 |
| Equation 13 |
where:
Rcrop is crop regulated requirement (m3)
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At district level, the regulated and opportunistic requirements are summed and then scaled for any district escape factors.
| Equation 14 |
| Equation 15 |
where:
t is model time-step (day)
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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 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))
| Info | ||||||||||
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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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The planting window allows the planted area, as defined by the planting decision trigger, to be established over a period of time, rather than the whole crop area being established on the plant date. The user specifies the planting window, which corresponds to the number of days the crop is planted over. The area planted each day is defined by:
| Equation 17 |
The soil water depletion of the area planted today is initialised from the fallow soil water depletion
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This allows the initial soil depletion of the new crop to be defined by
| Equation 18 |
where:
DNewCrop is soil water depletion of new crop (m)
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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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The simple, linear crop water production function provided in FAO Irrigation and Drainage Paper 33 is used to predict the reduction in crop yield resulting from water stress. This is a very simplistic approach.
| Equation 19 |
where:
Yr is relative yield.
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- Calculate the escape volume (Equation 9) and return volume (Equation 11) associated with each of the configured district escapes, note by default this is 0. Note that each escape is calculated based on residual volume supplied (Equation 10); and
- Calculate the total district escape and district return volume from escapes
| Equation 20 |
| Equation 21 |
where:
DistrictEscapen is district escape volume associated with district escape n.
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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.
| Equation 22 |
| Equation 23 |
where:
RTotal(t) = total regulated requirement at time-step t
- 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.
IrrigatorRemainingUsage = Estimate of the total volume of irrigation required to complete crops in the current irrigation season
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Table 1. Assumptions and constraints applying to storage flow routing methods
No | Assumption/Constraint |
1 | The crop model has been designed to represent a group of farms and assumes the irrigation behaviour of a group of farms. |
2 | Crops are planted on the same specified date each year of the simulation. |
3 | Crop yield is a linear response to water stress |
| 4 | The areas of crop will be less than or equal to the area of the district |
Data
Refer to the Source User Guide for detailed data requirements and formats.
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Parameters and Settings
District Configuration
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
Maximum Irrigated Area | Nominal maximum irrigated area. Used in defining the area of fallow. Warning generated if area exceeds this fall | ha | 0 | 0-500,000 |
Soil Moisture Capacity | The difference in volumetric soil water content between field capacity and permanent wilting point | % | 0 | 10-30 |
Irrigation season start date | The first day on which water can be delivered to irrigators | Calendar date | Irrigation districts – 15 August Private diverters – 1 July | 1/1-31/12 |
Irrigation season end date | The last day on which water can be delivered to irrigators | Calendar date | Irrigation districts – 31 May Private diverters – 30 June | 1/1-31/12 |
Climate Configuration
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
Evapotranspiration | Reference crop evapotranspiration, as defined by FAO56. Suggest using SILO data | mm/day | 0 | 0-15 |
Average evapo\rationspiration | Average evapotranspiration used for forecasting soil water depletion and demand | mm | 0 | 0-15 |
Rainfall | Rainfall timeseries | mm/day | 0 | 0- |
Average Rainfall | Average rainfall used for forecasting soil water depletion and demand | mm | 0 | 0- |
District Supply Escape
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
Supply Escape Efficiency | The percentage of water supplied to the district that is lost from the supply system | % | 0 | ≥ 0-100 |
Escape Return Efficiency | The percentage of the supply escape that is collected in drainage system and can be recycled | % | 0 | ≥ 0-100 |
District Return Efficiency
Parameter | Description | Unit | Default | Typical Range |
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Supply Escape Efficiency | Disabled. The district return efficiency is only applied to return flows | % | 0 | 0-1000 |
Return Effficiency | The amount of all harvested return flows that is passed back to the water user. | % | 0 | ≥ 0 |
General Crop Configuration
| Parameter | Description | Unit | Default | Typical Range |
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Crop | Crop type | - | - | - |
Crop Type | Type of crop being grown – Either Annual or Perennial | - | - | - |
Planting Window | The number of days over which a crop area is planted. This defines the number of uniform size sub-areas for initial watering of cropped area. | days | 1 | 1-30 |
Planting Margin | Defines the time period following planting that the crop will not die as a result of water stress. This prevents crop death if initial planting is delayed. | days | 1 | ≥ 0 |
Crop Soil Configuration
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
Dr- Depth of rootzone | Depth of soil profile that water is extracted for ET. For fallow, this is the depth of soil that soil water will deplete due to evaporation. | mm | 0 | 30-1000 |
p - Depletion Factor | Percentage of total available water that a crop can extract from the root zone without suffering water stress | % | 0 | 50 |
Initial Depletion | Soil water depletion used to initialise a model run | mm | 0 | 0-100 |
Fallow factor | Limit on evaporation from fallow soils. Equivalent of a crop coefficient, but used for fallow. | - | 0 | 0.4-1 |
Crop Planting Decision
| Parameter | Description | Unit | Default | Typical Range | |
|---|---|---|---|---|---|
| 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 | 1/1-31/12 |
| 4 |
Underirrigation Factor | Reduces irrigation application by |
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 |
Soil Depletion
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
Target Soil depletion | The target soil water depletion that irrigation attempts to maintain. Negative values used for ponded crops | mm | 0 | 150 to 100 |
Opportunistic target soil depletion | Soil water depletion that opportunistic water is used to refill. By default this is disabled. Negative values used for ponded crops | mm | 0 | -150 to 100 |
Crop Runoff
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
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 |
Crop Deep Percolation
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
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 |
Target Modifier
| 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 |
modified | 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 |
Crop Economics - Note: Crop Economics functionality is still in prototype stage
| Parameter | Description | Unit | Default | Typical Range |
|---|---|---|---|---|
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 |
References
DIPNR (2004), IQQM Reference manual, Version 1.2, NSW Department of Infrastructure Planning and Natural Resources, NSW.
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