MANAGEMENT MODEL FOR PREDICTING NITRATE LEACHING FROM GRASSLAND CATCHMENTS IN THE UNITED-KINGDOM .1. MODEL DEVELOPMENT

This paper describes the development of a field-scale model that simul ates the nitrogen (N) cycle in grazed grassland within a catchment-sca le management model which can predict the loading and concentration of nitrate in rivers. The development is comprised of the addition of tw o sub-models of nitrate transport: one relating the amount of soil nit rate to its concentration in drainage water for different types of soi l, and the second accounting for the proportion of permeable rock unde rlying the catchment. The sub-model that calculates the supply and tra nsport of soil nitrate has been made sensitive to annual patterns of w eather according to a classification based on the maximum soil water d eficit. The model predictions were tested against best estimates of an nual load and peak concentration of nitrate in rivers draining 11 smal l, predominantly grassland, catchments in the UK during the period 197 4-1987.