Evaluating the spatial scaling effect of baseflow and baseflow nonpoint source pollution in a nested watershed

Abstract Effective water-quality management requires full understanding of the nonpoint source (NPS) pollutant pathways. However, the spatial scaling effect has become one key barrier for the estimation of baseflow NPS pollutants. In this study, the scaling effect of baseflow NPS pollutants was investigated. The Soil and Water Assessment Tool was used for flow and NPS pollutants simulation at nested river sections, while the baseflow NPS pollutants was then estimated based on 17 levels of nested watersheds in a watershed of China. The results indicated that as the spatial scale increased, the baseflow became an important pathway of NPS pollutants, as its average contribution increased from 0.27 in the small headwater catchment to 0.45 in the nested large watershed. The scaling effects of the baseflow index (BFI) and the contribution of the baseflow NPS pollutants were closely related to the land use distribution among those upstream catchments, whereas for the mainstream, the tributary inflow would become another important reason for the scaling effect. Furthermore, the spatial scaling effect of the BFI was less obvious during the dry period, whereas the spatial scaling of the baseflow NPS pollutants did not show much difference among different hydrological periods.