Irrigation is required when rainfall is insufficient to compensate for the water lost by evapotranspiration. The primary objective of irrigation is to apply water at the right period and in the right amount. By calculating the soil water balance of the root zone on a daily basis, the timing and the depth of future irrigation can be planned. To avoid crop water stress, irrigation should be applied before or at the moment when the readily available soil water is depleted (Dr,i <= RAW).
An irrigation schedule specifies the timing (which day) and amount (depth) of irrigation. Each crop can have many different schedules for non-overlapping periods of the crop season. For example, you may want to irrigate more frequently during the sensitive flowering and yield formation stages. If a branch has more than one crop in rotation, you can specify the irrigation schedules for all crops. Use the Irrigation Scheduling Wizard to help you fill in the parameters for the IrrigationSchedule function.
There are various methods available to determine both the timing and amount of irrigation.
There are four methods for determining on which days irrigation will occur:
Fixed Interval: Irrigate every N days, where N is specified in the IrrigationTriggerValue.
% of RAW: Irrigate when soil moisture depletion is greater than or equal to a specified % of Readily Available Water (RAW). To prevent crop water stress, depletion should never exceed RAW.
% of TAW: Irrigate when soil moisture depletion is greater than or equal to a specified % of Total Available Water (TAW). To prevent crop death (permanent wilt point), depletion should never equal or exceed TAW.
Fixed Depletion: Irrigate when soil moisture depletion is equal to or exceeds a specified depth (in mm).
There are four methods for determining how much water to apply on days when irrigation occurs:
% Depletion: Apply a specified % of the current soil water depletion.
% of RAW: Apply a specified % of the Readily Available Water (RAW) level, regardless of the current soil water depletion.
% of TAW: Apply a specified % of the Total Available Water (TAW) level, regardless of the current soil water depletion.
Fixed Depth: Apply a specified depth of water. Note: the value entered should be the average depth of water applied to the entire area. If the fraction wetted (Fw) < 1, then the amount actually delivered to the crop will be higher: I/Fw . For example, for a drip irrigation system with Fw=0.3, a fixed depth application of 10 mm of water would be mean that 10/0.3 = 33.3 mm of water was applied to that 30% of the land area, and would be available for ET by the crop.
The optimal irrigation schedule and amount would use % of RAW (100% of RAW) as the trigger method, and % of Depletion (100% of Depletion) as the irrigation amount method, which would apply irrigation at the last moment before crop stress would occur, and irrigate just enough to get back up to field capacity. However, in reality, it will be difficult for a farmer to know exactly when depletion reaches the RAW threshold
If there are gaps in the irrigation dates between schedules, there will be no irrigation during those periods. Also, there is no irrigation in the fallow periods (before or after the crop season).
The irrigation amount is the amount available for evapotranspiration. If the irrigation efficiency is less than 100%, then the supply requirement for irrigation will be increased. (Crops that are flooded, such as rice, have their irrigation efficiency set to 1, because all the water added to the surface storage will be available for evapotranspiration.)
Irrigation Supply Requirement = Irrigation Crop Requirement / Irrigation Efficiency
For example, if the irrigation efficiency is 75% (which means that 25% is lost to evaporation, runoff or deep percolation) and the irrigation amount is 100% of depletion, and the depletion is 40mm, then the amount applied will be 40 / 0.75 = 53.3 mm, of which 40 mm will effectively reach the crop and be available for ET, and 13.3 will evaporate, runoff or percolate (as specified by Loss to Groundwater and Loss to Runoff fractions entered as data). Note: The amount of irrigation available for ET is also constrained by the maximum infiltration rate over a 24-hour period.
The Loss to Groundwater and Loss to Runoff fractions determine how much of the irrigation that is not available for evapotranspiration will infiltrate, runoff or evaporate.
Irrigation Loss to Groundwater = Irrigation Supply Requirement * (1 - Irrigation Efficiency ) * Loss to Groundwater%
Irrigation Loss to Runoff = Irrigation Supply Requirement * (1 - Irrigation Efficiency ) * Loss to Runoff%
Irrigation Loss to Evaporation = Irrigation Supply Requirement * (1 - Irrigation Efficiency ) * (1 - Loss to Groundwater% - Loss to Runoff% )
Some fraction of a catchment's runoff can be used for irrigation internally within the catchment (diverted before it reaches surface water inflow of runoff link), via the "Irrigation Use of Runoff" variable. This will allocate some or all of the runoff for use by the catchment for its irrigation, even if there are higher priority demands downstream, and before all other irrigation supplies the catchment would otherwise use. However, runoff from irrigation is NOT available to be used again for irrigation -- this would be circular. Only the runoff from precipitation and from the soil moisture at the beginning of the timestep is available for irrigation within the catchment. Only runoff links to surface water are considered, not infiltration links to groundwater.
TotalRunoff = RunoffNoIrrigation - RunoffUsedForIrrigation + AverageIrrigationRunoffFraction * TotalIrrigation
RunoffUsedForIrrigation = Min(RunoffAvailableForIrrigation, SupplyRequirement) ' do not exceed the total irrigation demand
RunoffAvailableForIrrigation = RunoffNoIrrigation * IrrigationUseOfRunoffFraction
TotalIrrigation = RunoffUsedForIrrigation + IrrigationFromOtherSupplies
where
RunoffNoIrrigation = calculated runoff assuming no irrigation
AverageIrrigationRunoffFraction = calculated average fraction of applied irrigation that will run off
IrrigationUseOfRunoffFraction = data
IrrigationFromOtherSupplies = calculated by WEAP allocation algorithm, based on availability of supplies and other higher priority demands in the system
See also: MABIA Irrigation, Irrigation Scheduling Wizard