Date | Author | Revision | Description of Change |
| Matthew Bethune | Creation of material | ||
|---|---|---|---|
27/11/2012 | G. Podger |
| Review |
Description and rationale
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The soil water balance for each crop is modelled using a single layer moisture store (Equation 1), as outlined in 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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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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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 j of the crop
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Effective rain considers that a proportion of rainfall will be interception or lost and not infiltrate. 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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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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Forecast values of P and ETo are required to estimate soil depletion into the future. Two options for forecasting ETo and rainfall. The user can specify an average daily pattern, which represents long term average ETo and rainfall on each day of the year. Alternatively, you can specify the number of previous time-steps, and the model calculates the forecast ETo and rainfall 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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The under irrigation factor is used to adjust 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 and not maximise production ie. stressing the crop and getting a reduced yield..
| Equation 16 |
where:
UFactor is user specified under irrigation factor
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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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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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- 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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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 |
|---|---|---|---|---|
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 |
|---|---|---|---|---|
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 |
|---|---|---|---|---|
Decision Type | Allows user to select different methods for specifying the crop area on the planting date. Either a fixed area (which supports expressions and data sources) or a lookup table that relates available water on the planting date to the planted area. |
| Fixed Area |
|
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 an expression 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 |
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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 modifed | 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 (Ks) 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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