There is a choice among four methods to simulate catchment processes such as evapotranspiration, runoff, infiltration and irrigation demands. These methods include (1) the Rainfall Runoff and (2) Irrigation Demands Only versions of the FAO Crop Requirements Approach, (3) the Soil Moisture Method, and (4) the MABIA Method. You can click on the "Advanced" button at the top of the Data Entry window for a particular catchment to select among these options. Your choice of method should depend on the level of complexity desired for representing the catchment processes and data availability.
Of these four methods, the Irrigation Demands Only method is the simplest. It uses crop coefficients to calculate the potential evapotranspiration in the catchment, then determines any irrigation demand that may be required to fulfill that portion of the evapotranspiration requirement that rainfall cannot meet. It does not simulate runoff or infiltration processes, or track changes in soil moisture.
The Rainfall Runoff method also determines evapotranspiration for irrigated and rainfed crops using crop coefficients, the same as in the Irrigation Demands Only method. The remainder of rainfall not consumed by evapotranspiration is simulated as runoff to a river, or can be proportioned among runoff to a river and flow to groundwater via catchment links.
The Soil Moisture method is the most complex of the four methods, representing the catchment with two soil layers, as well as the potential for snow accumulation. In the upper soil layer, it simulates evapotranspiration considering rainfall and irrigation on agricultural and non-agricultural land, runoff and shallow interflow, and changes in soil moisture. This method allows for the characterization of land use and/or soil type impacts to these processes. Baseflow routing to the river and soil moisture changes are simulated in the lower soil layer. Correspondingly, the Soil Moisture Method requires more extensive soil and climate parameterization to simulate these processes.
Note that the deeper percolation within the catchment can also be transmitted directly to a groundwater node by creating a Runoff/Infiltration Flow Link from the catchment to the groundwater node. The method essentially becomes a 1-layer soil moisture scheme if this is link is made. See Groundwater-Surface Water Interactions for more information.
The MABIA Method is a daily simulation of transpiration, evaporation, irrigation requirements and scheduling, crop growth and yields, and includes modules for estimating reference evapotranspiration and soil water capacity. It was derived from the MABIA suite of software tools, developed at the Institut National Agronomique de Tunisie by Dr. Ali Sahli and Mohamed Jabloun. For more information about MABIA and to download standalone versions of the software, visit http://www.mabia-agrosoftware.net. The algorithms and descriptions contained here are for the combined MABIA-WEAP calculation procedure.
The MABIA Method uses the ‘dual’ Kc method, as described in FAO Irrigation and Drainage Paper No. 56 (Spanish version of FAO 56), whereby the Kc value is divided into a ‘basal’ crop coefficient, Kcb, and a separate component, Ke, representing evaporation from the soil surface. The basal crop coefficient represents actual ET conditions when the soil surface is dry but sufficient root zone moisture is present to support full transpiration. In this way, MABIA is an improvement over CROPWAT, which use a single Kc method, and hence, does not separate evaporation and transpiration.
See also: FAO Method Calculation Algorithms, Soil Moisture Method Calculation Algorithms, MABIA Method Calculation Algorithms.