Estimating peak nitrate concentrations from annual nitrate loads

The feasibility of adopting an empirical approach to predicting peak nitrate 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 period 1974–1986. Well-fitted linear regressions were obtained of peak nitrate concentration in leachate on the annual load of leachable nitrate in the soil. The slopes to the regressions ranged in value from greater than 1.3 for measurements in shallow soils over limestone in a low rainfall area, to 0.25 for a well-structured clay soil with mole drainage in a high rainfall area. (A slope of 1.0 means that a load of 100 kg N ha −1 gives rise to a peak concentration of 100 mg N 1 −1). This range mainly reflected the ranges in soil water capacity and degree of preferential flow occurring during drainage. The slopes to the regressions for river waters were consistently smaller than those for clay soils in the same areas. The slopes for rivers draining mainly low-lying clay or chalk catchments in drier areas (Thames, Anglian and Southern Water Authority areas) were generally greater than those for rivers draining mainly wetter and more upland areas (South West and Welsh Water Authority areas). The differences between the slopes to the regressions for rivers could be partly accounted for by differences in average rainfall, suggesting possible effects from systematic differences in land use, soil structure and date of rewetting of soils in different climatic areas. The regressions indicated that peak concentrations in excess of the European Community limit (11.3 mg 1 −1 nitrate-N) might be associated with total nitrate loads in rivers draining clay catchments of 30 kg N hat in central and eastern England, or greater than 70 kg N ha −1 in Wales. The results indicate that the regressions for soils might form the basis of a catchment-scale model of nitrate leaching, provided that it is linked with a hydrogeological sub-model that accounts for the contribution to river discharge from upland and groundwater sources, and for the effects of different land uses on the timing of the peak nitrate concentrations.