Temporal dynamics and projected future changes in the nitrate leaching in a small river catchment dominated by under-drained clay soil grasslands: analysis of high-frequency monitoring data

Excessive nutrient levels can have impacts on both human health and freshwater ecology. This research has concentrated on the nitrate response within a small river catchment within the Newby Beck catchment, part of the River Eden Demonstration Test Catchment in Cumbria, UK, that is dominated by grassland underlain by drained clay soil. To understand the catchment hydrology and possible nitrate sources, the types of nitrate response were classified by type of hysteresis loop, obtained when concentration was plotted against discharge for each storm event. To determine the key drivers of nitrate response based on antecedent variables correlation matrices and stepwise regression analysis were used. Potential future river flows were estimated using the HBV-Light hydrological model and the UKCP09 climate projections. High-frequency observations of nitrate concentration and stream discharge provided a database of 235 storm events which could be analysed. The Newby Beck catchment is dominated by source-limited clockwise hysteresis events but anti-clockwise events, when large amounts of nitrate are leached, account for over one third of storm events. Nitrate concentrations are governed by preparatory processes of nitrate production at source in the soil and climate change is projected act to alter these preparatory processes through increasing temperatures and more severe droughts in summer which would result in an increase in high-magnitude nitrate leaching events. Given the model and future weather scenario uncertainty, it was calculated that there is a 71% probability of an increase in the future nitrate flux by the 2050s and 82% by the 2080s. It is suggested that mitigation measures that intercept water that is transporting this nitrate combined with reduction of nitrate supplied by anthropogenic activities are the best choice for reducing the impact of nitrate on freshwater systems.