Catchment-scale simulation of stream water chemistry by spatial mixing: theory and application

A modelling framework is presented which gives catchment-wide estimates of long-term stream water chemistry using complete spatial coverage of catchment characteristics combined with measurements of stream water chemistry at a limited number of sites. It has been developed for application to catchments having acid headwaters which are diluted downstream by less acid runoff, usually from improved agricultural land. The catchment is subdivided into landscape types based on land use, soils and other characteristics defined by existing national databases. Each landscape type is assumed to generate stream water having a characteristic annual mean concentration of chemical constituents. This concentration is estimated by regression analysis using measured stream water chemistry data from a number of subcatchments, knowing the proportion of each landscape type in each subcatchment. Catchment-wide stream water chemistry is estimated for conservative ions by mixing contributing area estimated loads. Concentrations of non-conservative ions are estimated using hydrochemical equilibrium equations constrained to give the acid neutralising capacity estimated from the conservative ions. The model is applied to the river Tywi, Wales and its performance investigated.