Modeling flow and transport in irrigation catchments 2. Spatial application of subcatchment model

An alternative to spatial distribution is presented for physically based pr ocess modeling of catchment flow and solute transport. The approach is base d on an integral relation for catchment streamflow which describes the accu mulation through the stream network of the hillslope contributions. This re lation is redefined from integration with respect to position along the str eam to an integral that is with respect to the physical properties determin ing the hillslope flow and transport with probability distributions to repr esent the range of parameter values and areas associated with these. Since the integrand is now a relatively smooth function of the variables of integ ration, standard numerical integration can be used to evaluate the catchmen t totals. The assumptions allowing this redefinition are that hillslope hyd rology is primarily determined by driving forces that are local to the hill slope and that catchment flow can be lumped in time so that streamflow rout ing is not required. The interpolation of the integrand involved in the num erical integration results in significant savings over physically based spa tially distributed catchment flow and transport models. The methodology is tested through application to an irrigated catchment where model simulation s of flow and transport were found to be in close agreement with those obse rved.