Evaluation of extreme hydrological events and diffuse pollution losses from agriculture-dominated catchments under Nordic conditions

Excessive phosphorus and nitrogen losses from agricultural areas cause eutrophication, one of the most prevalent global water quality challenges. This study's main goal was to identify and evaluate the occurrence of diffuse pollution attributed to extreme hydrologic events from agriculturally dominated areas. We analyzed the Norwegian Agricultural Environmental Monitoring Program long-term data from four catchments representing different agricultural practices and climate regimes. Extreme flows were set at ≥10% exceedance (Q10) flows and the corresponding nutrient and sediment losses were evaluated. The extreme flow occurrences between the catchments were significantly different, especially in spring and autumn months. In southeastern Norway (cereal crops; distinct winter freeze and thaw), at least 70% of the total suspended sediments (TSS) and total phosphorus (Ptot) losses were attributed to extreme flows. Forty percent of the TSS and nutrient losses were attributed to extreme flows in the grassland-dominated in western Norway (coastal climate, high annual total precipitation). In southern Norway (vegetables, coastal climate), TSS and Ptot losses were largely attributed to extreme events but only 30% of the dissolved phosphorus and 40% of the total nitrogen losses. The occurrence of diffuse pollution due to extreme events varied significantly between agricultural practices and climate regimes. We recommend that the effectiveness and efficiency of conservation measures against extreme hydrologic events in relation to agricultural practices and prevailing climate conditions should be accounted for in planning and implementation.