A SIMPLE-MODEL OF STREAM NITRATE CONCENTRATIONS IN FORESTED AND DEFORESTED CATCHMENTS IN MID-WALES

A model is developed specifically for simulating the nitrate concentra tions in a stream at approximately weekly intervals in an upland fores ted and a moorland catchment. It is constructed on the basis of observ ed nitrate concentrations in rain water and in two streams, the Hafren and the Hore, at Plynlimon, Mid-Wales, where long data records exist. The Hore catchment has recently undergone extensive deforestation. A simple regression model relating temperature to stream nitrate concent rations was capable of simulating the seasonal fluctuations in nitrate concentration observed in the Hafren. However, this ignores the influ x of nitrate through wet deposition and is incapable of simulating lan d-use change. The regression model was developed into a simple dynamic model which includes a deposition term and a biomass indicator. The e xtended Kalman filter algorithm was used to estimate the optimum value s of the parameters and to assess the model structure. The model was a pplied to both catchments, and the fit between observed and simulated nitrate concentrations at the Hafren was good. At the Hore, the model was able to capture the major changes in nitrate concentration through the deforestation and replanting phases although detailed short-term dynamics were not well represented. Finally, the model is related, spe culatively, to processes which are known to occur in most catchments. This simple model is intended as a step towards the development of sim ilar but more comprehensive catchment models of stream nitrogen dynami cs.