The Plant Growth Method (PGM) is a daily simulation of plant growth, transpiration, evaporation, irrigation requirements and scheduling, and yields. Portions of its plant growth routines are based on the SWAT and APEX models. Evapotranspiration is simulated using the algorithms found in the EWRI Standard Reference Crop specifications. Soil water movement is simulated using a 13 layer model of the plant root zone. The model allows for calculation of the effects of changes in atmospheric CO2 concentration and temperature on plant water use and growth. As such, it can be used in climate change studies to explore possible implications of global climate change on crop productivity and water use. This method can be used to model both agricultural crops as wells as non-agricultural land classes, such as forests and grasslands. The required parameters can be found in the included database or in other models using similar algorithms including SWAT and APEX.
Although the timestep for Plant Growth Method is daily, the timestep for the rest of your WEAP analysis does not need to be daily (although it can be daily). For each WEAP timestep (e.g., monthly), the Plant Growth Method would run for every day in that timestep and aggregate its results (evaporation, transpiration, irrigation requirements, runoff, and infiltration) to that timestep. For example, in January, the Plant Growth Method would run from January 1 to 31, and sum up its results as January totals, including most importantly, the supply requirement for irrigation. WEAP would then solve its supply allocations, using this monthly irrigation requirement from the Plant Growth Method catchments. In the case where the supply delivered to the catchments was less than the requirement, the Plant Growth Method would rerun its daily simulation, this time using only the reduced amount of irrigation to determine actual evaporation, transpiration, irrigation requirements, runoff, and infiltration.
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
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
For more information about the Plant Growth Method calculations, please refer to Plant Growth Model.pdf in the WEAP Program folder.