Diffuse Surface Water Pollution: Driving Factors for Different Geoclimatic Regions

The quantification of point and non-point losses of Nitrogen (N) and Phosphorus (P) to surface waters is currently a major issue for the implementation of Environmental Directives, such as the Water Framework Directive in Europe. However, the drivers behind nutrient pollution are location specific and are affected by regional hydroclimatic and geomorphological characteristics. In this study the river basin model SWAT was used in order to improve the process-based understanding of mechanisms behind nutrient transport from land to water recipients in two European catchments with significantly different meteorological conditions: the Greek catchment of Ali Efenti, representative of the Mediterranean climate, and the Norwegian catchment of Vansjo-Hobolv, representative of the cold climate typical in Scandinavia. The models were firstly calibrated according to measured river flows and nutrient loads, and then four Best Management Practices (BMPs), related to nutrient application and soil management were applied in order to examine their effectiveness under the different geoclimatic conditions of the two catchments. The results of the baseline indicated that diffuse agricultural sources were the largest contributor of N losses in both catchments and of P losses in the Greek catchment, while point sources were significant contributors to P levels in Norwegian rivers. Nutrient losses to surface waters in Ali Efenti exhibited high seasonal variation, attributed to the extremeness of precipitation events that is typical in the Mediterranean, as well as to the temporal distribution of sediment losses to waters. On the other hand, in Scandinavia, the losses of N and P occurred with less deviation throughout the year and independently of the freezing of soils. The values of the calibrated parameters that mainly governed the hydrological and erosion processes in the catchments demonstrated the natural driving forces of nutrient losses to waters and their temporal distribution indicating that these forces are also crucial in determining the appropriate implementation of agricultural management practices in various geoclimatic regions.