Influence of nutrient mitigation measures on the fractional export of watershed inputs in an urban watershed

Enhanced nutrient inputs due to human activities have been noted as a significant driving force for riverine nutrient exports which are responsible for the eutrophication issues in freshwaters. Current studies are mostly focused on the relationship between anthropogenic inputs and riverine exports, and little has been done to assess the role of nutrient mitigation measures in the fractional export of watershed nutrient inputs in urban regions. A highly urbanized watershed in Yun-Gui plateau of China, Lake Dianchi basin was studied as a case to assess the impact of nutrient mitigation measures on riverine nutrient exports. Based on net anthropogenic nitrogen and phosphorus inputs (NANI and NAPI, respectively) models, nutrient inputs from human activities in the basin from 1980 to 2015 were calculated, and the impact of nutrient mitigation measures were identified using a statistical model incorporating land use, precipitation, and temperature. Nutrient inputs from human action in the basin has increased rapidly, mainly from fertilizer application and food and feed imports. Enhanced riverine nutrient exports were found at the same time, and significantly correlated to nutrient inputs. The construction of water transfer projects and wastewater treatment plants in the basin has changed the controlling factors and processes of the fractional export of watershed nutrient inputs, which is weak in explanatory ability and eliminated the role of the land use. A modified model was established by incorporating the effect of water transfer projects and wastewater treatment plants, which showed a significant increase in model performance. The results from the modified model reveal that urban land percentage has become a positively driving force for the fractional export of watershed N and P inputs, and temperature a positive driving force for the fractional export of watershed N inputs while precipitation a negative driving force for the fractional export of watershed P inputs.