Quantification and variation characteristics of baseflow nonpoint source pollution in Yiluo River Basin, China

Baseflow is an important component of runoff that is one of the essential mediums for groundwater pollution to migrate to runoff. Effective water-quality management requires full understanding of the nonpoint source (NPS) pollutants pathways. However, the contributions of baseflow and baseflow NPS pollutants at multiple scales are still challenging. In this study, the baseflow was quantified and compared by the nonlinear reservoir method, the digital filtering method and the Soil and water Assessment Tool (SWAT) in the Yiluo River, and SWAT model was used to simulate and quantify the NPS and baseflow NPS pollution. Then the scaling effect of baseflow NPS pollution was studied. The results indicated that the average annual baseflow of the basin outlet section and baseflow index of the whole basin was 38.07 m3/s and 0.58 during the studied years (2001-2017). The multiyear average baseflow pollution load for TN and TP was 4216.18 t/a and 293.33 t/a. The spatial distribution of the NPS pollution and baseflow NPS pollution showed higher in the northeast and lower in the southwest, and the baseflow NPS pollution contribution rate in most basins was more than 50 %. The spatial scale effects of NPS pollution load became more obvious with the increase of basin area, and the contribution of baseflow NPS pollution increased. The precipitation and temperature played important roles in baseflow pollution, especially baseflow TN load.