Spatiotemporal Modeling of Watershed Nutrient Transport Dynamics: Implications for Eutrophication Abatement

The main objective of this study was to quantify nutrient transport dynamics of a previously ungauged, temperate watershed (145km2) surrounding a shallow eutrophic lake and discern lake response to external nutrient loading, based on soil water assessment tool (SWAT) and the Organization of Economic Cooperation and Development (OECD) empirical lake models, respectively. A SWAT model was used to simulate baseline nutrient dynamics after its calibration and validation against daily tributary flow, total dissolved phosphorus (TDP), total phosphorus (TP), and nitrate (NO3) loads. On the watershed scale, median annual TDP, TP, and NO3 losses were 0.4, 1.1, and 2.0kgha−1, respectively. The highest median annual TP and NO3 losses were estimated at 3.7 and 7.7kgha−1 for pastureland and 1.7 and 3.8kgha−1 for cropland and mixed forests, respectively. Baseflow was the major nutrient transport pathway over a wide range of precipitation events (450 to 900mmyr−1). Erosion was the predominant surface process exporting P across the watershed. Critical source areas (CSAs) of TP and NO3 comprised 17% and 4% of the watershed, respectively. Annual mean TP, and mean and maximum chlorophyll content indicated a hyper-eutrophication risk for the lake. An external P load reduction by excess of 80% could be necessary to restore mesotrophy in the lake. Our results suggested that subsurface P transport should not be overlooked a priori when groundwater-dependent and extensively farmed watersheds are managed for eutrophication abatement.