A catchment is a user-defined area within the schematic in which you can specify processes such as precipitation, evapotranspiration, snow and ice accumulation and melt, 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, including 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. You can use WEAP's Automatic Catchment Delineation to help you define catchments and rivers.
For a catchment, the user can choose one of five different methods to compute water use (both rainfed and irrigated), runoff and infiltration from agricultural and other land cover. See Overview of Catchment Calculation Methods for more information.