Nutrient loads from agricultural catchments in response to extreme wet or dry conditions : how will N and P loads change with climate change?

Sammanfattning

Climate change is predicted to lead to more frequent and intense extreme weather events, which poses a risk of increased nutrient leaching from agricultural land and as a consequence, worsened eutrophication in aquatic ecosystems. The aim of this thesis was to investigate how long-term trends in nitrogen and phosphorus export are influenced by extreme wet or dry conditions. The research question was: How do extreme wet and extreme dry periods affect total nitrogen (TN) and total phosphorus (TP) loads in agricultural catchments in southern Sweden? To answer this, 30 years of environmental monitoring data from three small agricultural catchments (I28, M42, and N34) were analysed. The Standardised Precipitation-Evapotranspiration Index (SPEI) was applied at multiple timescales to identify periods of hydrological stress, which was combined with Generalised Additive Models (GAM) for trend analysis and event-based high-flow separation. The results showed that wet agrohydrological years were associated with substantially higher nutrient export and more frequent high-flow events than dry years. In catchment N34, approximately 19% of the days during 1998–2002 had SPEI-3 values > 1.5, yet these periods accounted for around 70% of annual TN export. Similar patterns were observed for TP export during prolonged wet periods. TN export was primarily associated with sustained hydrological transport and subsurface flow pathways, whereas TP export showed a more episodic response linked to runoff events and first-flush processes. The study further demonstrated that SPEI provided important hydrological context beyond runoff-based event identification alone. SPEI-1 showed the most consistent and statistically significant relationships with nutrient export across all three catchments and both nutrients, indicating the importance of antecedent moisture conditions and short-term hydrological variability. During dry periods, reduced hydrological connectivity generally lowered annual nutrient export, although prolonged drought conditions also appeared to increase the risk of elevated nutrient mobilisation during subsequent rewetting events. In conclusion, current field-scale mitigation measures may be insufficient during extreme weather events, as they are not designed for the massive water volumes generated. To mitigate extreme nutrient export in a future climate, mitigation strategies must increasingly shift towards landscape-level measures focused on water retention.