Calculation process and spatial heterogeneity characteristic of non-point source pollution caused by rainfall-runoff in Taizhou City

With improved point source pollution management, the impacts of non-point source pollution on the ecosystem are increasingly evident. However, conventional techniques such as output coefficient approaches fail to adequately capture its geographical and temporal variability. This study proposed a grid-based calculation technique to quantify non-point source pollution from rainfall-runoff in the Jinqing Watershed, Taizhou City, Zhejiang Province, based on the principles of the Soil Conservation Service-Curve Number (SCS-CN) model. The modified SCS-CN model effectively represented both temporal and geographical variations in rainfall-runoff pollution. Precipitation emerged as the primary determinant of non-point source pollutant load over time due to its strong positive association with rainfall. From January to April, there was a constant pollutant load; besides irregular rainfall patterns, land use types also influenced pollutant loads at a geographical scale. Building and cultivated areas exhibited positive correlations with nitrogen (r = 0.44, P = 0.05) and phosphorus (r = 0.54, P = 0.05), while forested and grassland areas showed negative correlations with nitrogen (r = −0.44, P = 0.05) and phosphorus pollutants (r = −044, P = 0.05). Pollution loads were also influenced by landscape patterns; concentrated dominant land use types resulted in higher contamination levels compared with scattered dominant land use types.