ESTIMATING PEAK NITRATE CONCENTRATIONS FROM ANNUAL NITRATE LOADS

The feasibility of adopting an empirical approach to predicting peak n itrate concentrations in water draining agricultural land is examined in the light of the relationships derived from leaching experiments on a range of soils, and from river data for England and Wales for the p eriod 1974-1986. Well-fitted linear regressions were obtained of peak nitrate concentration in leachate on the annual load of leachable nitr ate in the soil. The slopes to the regressions ranged in value from gr eater than 1.3 for measurements in shallow soils over limestone in a l ow rainfall area, to 0.25 for a well-structured clay soil with mole dr ainage in a high rainfall area. (A slope of 1.0 means that a load of 1 00 kg N ha(-1) gives rise to a peak concentration of 100 mg N l(-1)). This range mainly reflected the ranges in soil water capacity and degr ee of preferential flow occurring during drainage. The slopes to the r egressions for river waters were consistently smaller than those for c lay soils in the same areas. The slopes for rivers draining mainly low -lying clay or chalk catchments in drier areas (Thames, Anglian and So uthern Water Authority areas) were generally greater than those for ri vers draining mainly wetter and more upland areas (South West and Wels h Water Authority areas). The differences between the slopes to the re gressions for rivers could be partly accounted for by differences in a verage rainfall, suggesting possible effects from systematic differenc es in land use, soil structure and date of rewetting of soils in diffe rent climatic areas. The regressions indicated that peak concentration s in excess of the European Community limit (11.3 mg l(-1) nitrate-N) might be associated with total nitrate loads in rivers draining clay c atchments of 30 kg N ha(-1) in central and eastern England, or greater than 70 kg N ha(-1) in Wales. The results indicate that the regressio ns for soils might form the basis of a catchment-scale model of nitrat e leaching, provided that it is linked with a hydrogeological sub-mode l that accounts for the contribution to river discharge from upland an d groundwater sources, and for the effects of different land uses on t he timing of the peak nitrate concentrations.