Can large-scale climate patterns predict nitrate export mechanisms from agricultural land?

Nitrate losses to waterbodies is particularly associated with intensive agricultural and rainfall events are important periods of mobilisation, it is therefore important to understand the drivers for these transfers. In western Europe, weather is strongly influenced by large scale atmospheric systems in the North Atlantic. Here, we explore the link between such atmospheric systems and temporal changes in water quality using 12 years of high-frequency nitrate and discharge data. The data was collected from 5 hydrologically-isolated field-scale grassland catchments. Rainfall driven discharge and associated flushing index (FI) for nitrate was calculated. Most events had a negative FI, but the proportion of events that were positive increased over the summer. Values of two large scale climate indices (NAOi and WEPAi) were obtained for each month of the study period and the corresponding monthly mean values for FI and soil moisture were compared considering differences between season and field-scale catchments. The best model for explaining mean monthly FI was as a function of the WEPAi, allowing for differences between catchments and seasons with the differences between winter and summer being significant. For both seasons, a positive WEPAi had a negative response on FI; however, the divergence in slope between the two seasons was most likely due to a potentially greater range of soil moisture conditions in the summer compared to winter. Furthermore, nitrate export from grassland field-scale catchments could be predicted using the WEPAi, but prediction was slightly better with a more local measure of soil moisture.