Tracking the sub-catchment of nutrient contributors: The case of a typical mixed hilly-plain watershed in China

As the inlet river of Poyang Lake, it is important to master the source of nitrogen and phosphorus concentration in water body of Le 'an River for the water environment safety management in Poyang Lake. Therefore, this study aimed to quantify the weight of nutrient loss contribution from each sub-catchment to the inlet estuary cross-section. A SWAT model was constructed to simulate total nitrogen (TN) and total phosphorus (TP) loss of Le 'an River Watershed in 1995-2020. A key parameter Ci was proposed to quantify the contribution values and weights of the nutrient load from the sub-catchment to the inlet estuary cross-section and identify the crucial contribution areas. The results showed that: (1) The emission intensity of the TN and TP at the confluence section the Le 'an River Watershed in the past 30 years is 16.54 Gg/yr and 3.69 Gg/yr. (2) Nutrient losses in the watershed exhibited clear spatial variability. The average intensity of TN and TP loss in the middle and downstream plains was 1.38 and 1.12 times higher than in the upstream hilly areas. (3) The weight of nutrient contribution in sub-catchments varied significantly in space, and the TN and TP from the highest contributive sub-catchment was 11.52 and 25.91 times of the lowest, respectively. Through the analysis of contribution values, sub-catchments of No. 15, 20, and 22 were identified as the nutrient crucial areas.