A catchment is a user-defined area within the schematic in which you can specify processes such as precipitation, evapotranspiration, runoff, irrigation and yields on agricultural and non-agricultural land. When you create a catchment in the schematic, a window pops up in which you can select a number of options which will apply for this catchment. In addition to requesting whether the runoff from the catchment will contribute headflow to a river, this box also asks whether irrigation will occur in the catchment (and if so, the demand priority). If irrigation is selected for a catchment, the user will be required to create transmission links from a supply to the catchment for the irrigation water and to input additional variables that parameterize the irrigation activity.
For a catchment, the user can choose one of three different methods to compute water use (both rainfed and irrigated), runoff and infiltration from agricultural and other land cover.
The FAO Crop Requirements (either rainfed or irrigated) method focuses on crop growth, and assumes simplified hydrological and agri-hydrological processes (non-agricultural crops can be included as well). Runoff from these processes can be diverted back to a river or to a groundwater node.
The Soil Moisture method includes a one dimensional, 2-compartment (or "bucket") soil moisture accounting scheme for calculating evapotranspiration, surface runoff, sub-surface runoff (i.e., interflow), and deep percolation for a watershed unit. This method allows for the characterization of land use and/or soil type impacts to these processes. The deep percolation within the watershed unit can be transmitted to a surface water body as baseflow or directly to groundwater storage if the appropriate link is made between the catchment node and a groundwater node. If the latter link is made, the method essentially becomes a 1-compartment model, where the runoff from the upper compartment is divided among runoff to the river and infiltration directly to groundwater.