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p fraction of TAW that a crop can extract from the root zone without suffering water stress [-].
Figure
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5. Relationship between water stress coefficient and soil water depletion.
Crop Evapotranspiration
Crop evapotranspiration is calculated using the single crop coefficient approach described in FAO-56 (Equation 6). The effects of soil water stress on crop ET are calculated by multiplying the crop coefficient by the water stress coefficient (Ks). The value The Ks value is evaluated based on soil water depletion at the start of the time step.
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Kc = Single crop coefficient on growth day i of the crop
Ks = water stress coefficient describes the effect of water stress on crop transpiration
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Rainfall runoff occurs when rainfall results in soil moisture exceeding saturation or a maximum target pond level for ponded crops.
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RainfallRunoff = the amount of rainfall runoff depth for cropping area (m)
Targetop = the opportunistic target depletion level for a cropping area (m)
Drt-1 = soil water depletion at the beginning of the time step (m)
the amount of effective rainfall (m)
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Escape is used to define a point in an irrigator demand model where a proportion of the supplied water is removed from the delivery system but may return. The volume removed is called the escape volume. The escape volume can either be lost from the system or returned back to the water user as a return flow.
Figure
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6. Escape loss and return flows.
Escapes apply at both the district and crop level (Figure 36). 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 |
Is Equation 10 correct?
| Equation 11 |
The volume supplied is reduced by the escape volume
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The behaviour of the target levels is illustrated in the following two Figures for a ponded and not non ponded crop.
Figure 4 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 under until the soil dries out to the regulated target level.
Figure
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7. Non ponded crop soil moisture target maintenance
Figure 5 8 shows a regulated ponded crop. When the crop is established on day 5, a pond of 100 mm is required. The regulated target depletion is shown as -100mm. Sufficient water is ordered to match the soil water depletion with the target depletion. On day 10, there is a large rainfall event. This causes the soil water depletion to decrease (negative as is depletion) to the opportunistic target level. Any rain in excess results in runoff. No further water is ordered until the soil water depletion increases to the regulated target depletion.
Figure 8. Ponded crop pond level maintenance
Figure 5. Ponded crop pond level maintenance
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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.
Irrigations Irrigation and opportunistic requirements
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Rcrop is crop regulated requirement (m3)
Ocrop is opportunistic requirement for crop (m3)
TargetDepletionRegulated is target soil water depletion that irrigation with regulated water aims to maintain (m)
TargetDepletionOpportunistic is target soil water depletion that irrigation with opportunistic water aims to maintain. (m)
ERunoff is efficiency factor defining the fraction of applied irrigation water that is lost as runoff
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CropArea is current crop area (m2)
At district level, the regulated and opportunistic requirements are summed and then scaled for any district escape factors.
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| Equation 15 |
where
t is model timesteptime step (day)
RTotal(t) is total regulated requirement at timestep time step t
OTotal(t) is total opportunistic requirement at timestep time step t
Escapen is escape factor n, defining the amount of supplied water that becomes an escape.
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This function defines the planted area for a specific crop on the plant specified date, taking into consideration available resources. If the user defines the planted area using the expression editor, then other factors such as economics can be considered.
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1) A lookup table between available water and planted area. The available water is specified by the water userWater User. The planted area is a reassessed each time the planting decision is triggered (annually).
2) Fixed Area – using a Data Source of representing a time series of areas. The planted area is a reassessed each time the planting decision is triggered (annually).
3) Fixed Area – Defined using the expression 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.
The user can have multiple planting decisions for a crop. This would allow the irrigated area of a crop to increase within a season. Every planting decision configured for a crop is modelled as a new crop in Irrigator.
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The Plant Date defines when the planting decision is triggered and when the crop is established
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
Limits to Area Planted
The total area planted within a water year is not limited other than through the planted area definitions. The user is required to configure sensible planting decisions. A warning is recorded during runtime if the planted area exceeds the maximum irrigated area of the district.
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If the rootzone of the new crop is deeper than the fallow depth, then the crop soil moisture store will contain water from both the fallow depth and the subsoil. If the rootzone of the new crop is shallower than the fallow depth, then crop soil moisture store is a function solely of the fallow depletion.
We make assumption It is assumed that the soil below the depth considered by the fallow crop is at field capacity, this equating to a soil depletion of 0 in the subsoil.
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Dfallow is soil water depletion of the fallow (m)
Zfallow is depth of the fallow (m)
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Irrigation modifiers
Irrigator behaviour can changes 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.
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- The soil water balance (Equation 1) is updated using the actual rainfall and potential evapotranspiration for the current timesteptime step.
- Deep percolation of rainfall (Equation 9) is calculated and then rainfall runoff resulting from saturation excess (Equation 8).
- Today’s actual regulated requirement (Equation 12) and opportunistic requirement (Equation 13) is updated for each crop to reflect the impact of todays today's actual rainfall and evapotranspiration.
- Irrigator model updates the actual requirements list for the current time step and informs the Water User (Equation 14) and (Equation 15).
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1) 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 Equation 10).
2) Calculate the total district escape and district return volume from escapes
| Equation 20 |
| Equation 21 |
Renumbered Equations from 23 & 24 to be sequential
DistrictEscapen is district DistrictEscapen is district escape volume associated with district escape n.
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DistrictReturnn is return volume from district esape escape n
DistrictReturnTotal is total return volume
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3) The volume of water applied is then distributed between regulated requirements and opportunistic requirements.
| Equation 22 | |
| Equation 23 |
RTotal(t)= total regulated requirement at time step t
4) The volume applied to each crops crop is in proportion to the regulated and opportunistic irrigation requirements.
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8) The runoff from irrigation and rainfall are totaled for each crop. The total returned crop runoff is calculated.
9) The crop returns return flow is added to district return flowsflow.
10) 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).
Crop productivity
The crop productivity is evaluated for each crop on the harvest date.
Crop Production = CropArea x RelativeYield x Productivity
Crop production is only calculated if the user specifies a harvest date for the crop which is within the crop growing season.
On all other days that the harvest day, crop production is 0.
Where:
Productivity: The maximum amount of production achieved per hectare of the crop. No units for production are specified. This allows user to enter any production factor and the output units will be consistent with the input unit. Eg kg/ha, $/ha, widgets/ha, dry sheep equivelents per hectare.
CropProduction is the production harvested by the crop on the harvest day. Its units are defined by the units of the ProductionFactor
CropArea is the area of the crop on the harvest date.
Expected water use
An estimate is made each time step of the amount of irrigation water that will be required by the crop for the rest of the crops growing season. The concept is that this estimate of required water use can be compared to the irrigators account balance, and a decision could be made on whether to trade water (using an allocation transfer within the resource assessment system). Note: this is a prototype and has not been tested.
An average daily irrigation requirement is calculated by dividing the user specified water use by the number of days in the growing season.
The remaining usage is defined by number of days left in growing season and the average requirement.
RemainingUsageCrop=ExpectedUsage x CropArea x RemainingGrowingDays
RemainingUsageCrop= Expected water use by the crop for the rest of irrigation season
ExpectedUsage= User specified annual irrigation requirement for a crop. Expressed in mm
CropArea = Area of the crop
RemainingGrowingDays= number of growing days left in the irrigation season for this crop.
Equation |
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IrrigatorRemainingUsage = Estimateof the total volume of irrigation required to complete crops in the current irrigation season
Assumptions and constraints
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