Long-Term Hydrological Impacts of Land Use Change and Evaluation of Best Management Practices from 2000 to 2020 in the Hulan River Basin, Northeast China

The alterations in runoff resulting from changes in land use and land cover (LULC) were the primary influencing factors contributing to non-point source pollution (NPS). In order to evaluate the long-term hydrological consequences of LULC for the purposes of land use optimization in the Hulan River Basin, Northeast China, the validated Long-term Hydrological Impact Assessment (L-THIA) model was employed to simulate the spatiotemporal distribution of total nitrogen (TN) and total phosphorus (TP) non-point source (NPS) loads from 2000 to 2020. Additionally, the load per unit area index (LPUAI) method was utilized to identify critical source areas. The findings indicated that the regions with elevated pollution levels were predominantly situated in areas designated for agricultural and construction activities. The greatest contributor to nitrogen and phosphorus loads was agricultural land. There were clear increases in both TN and TP during the study period, with increases of 51.73% and 55.56%, respectively. As a consequence of the process of urbanization in the basin, the area of land devoted to construction activities increased, reaching a coverage of 5.02%. Nevertheless, the contribution of construction land to the total basin NPS load exceeded 10% in 2020. This was the primary factor contributing to the observed increase in pollution loads despite a reduction in agricultural land area over the past two decades. TN and TP loads were markedly higher during the flood season than the non-flood season, accounting for over 80% of the NPS load. The sub-watersheds in the southwest and northeast have been identified as significant sources of nitrogen and phosphorus loss, contributing to the overall burden of NPS pollution. Implementing measures such as fertilizer reduction and conversion of farmlands to forests and grasslands can effectively mitigate NPS pollution, particularly TN pollution. This study proposes that the integration of L-THIA with GIS can serve as a valuable tool for local planners to consider potential pollution risks during future planning and development activities.