Modelling nitrate from agriculture into public water supplies

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Abstract
A wide range of models and techniques are briefly reviewed within the context of the Thames nitrates study in which models were developed to assess impacts of agricultural practices on nitrate levels in the river system. Here a semi-distributed approach was adopted in which a series of component models was developed to simulate hydrological and chemical behaviour of the Thames River basin. These components included: ( a ) a daily hydrological model for the Thames basin, which included 17 tributary sub-catchments and several major aquifer systems. The model provided input flows such as tributaries, groundwater, surface runoff, effluent returns as well as abstraction flows; ( b ) a soil zone and aquifer model for calculating the nitrate concentrations of surface runoff and groundwater given a particular land-use and fertilizer application rate; ( c ) An integrated model of flow and water quality for the main river, which provided a mass balance along 22 reaches of the main river, allowed for denitrification processes and incorporated all inputs from the non-point sources derived by ( a ) and ( b ) above. Model results will be presented together with an assessment of the major problems of nitrate modelling and predictions, which occur within the hydrologically active soil zone.