Critical Source Times of Nutrient Loss from Agricultural Land to Water.

This study investigated nutrient concentrations, stream discharge and weather data in five intensively monitored agricultural catchments over four years (2010-2014), with a view to investigating links between slurry spreading restrictions during ‘closed’ (and adjacent) periods and water quality. Nutrient losses were disproportionately high during the closed period, accounting for on average, 43% of annual total phosphorus (TP) losses and 45% of annual total oxidised nitrogen (TON) losses. For TP, the beginning of the closed-period was reflective of incidental and residual transfers with high storm flow-weighted-mean (FWM) concentrations and some catchments showing elevated storm TP/SS ratios (> 2%). This changed to a more residual transfer pattern at the end of the closed period in all catchments, with lower storm FWM P concentrations and TP/SS ratios. Total oxidised N behaved similar to TP in the poorly drained catchments and revealed a long lag time in other catchments.

There was no significant difference between storm FWM TP concentrations in the four weeks after the closed period compared with the four weeks at the end, despite the continued vulnerability of low soil moisture deficits (estimated from weather data). The continued low TP/SS during the weeks following the closed period suggests that nutrients either weren’t applied during this time (best times chosen) or that they were applied to less risky areas (best places chosen). For other periods such as late autumn and during wet summers, where storm FWM TP concentrations and TP/SS ratios were high, an augmentation of farmer knowledge of soil drainage characteristics with local and detailed current and forecast soil moisture conditions will help to strengthen existing regulatory frameworks to avoid storm driven incidental nutrient transfers.