A new watershed eco-zoning scheme for evaluate agricultural nonpoint source pollution at national scale

The efforts to accurately assess the impacts of agricultural nonpoint source pollution on surface water quality remains an ongoing challenge at national scales. Dividing the country into several zones based on different types of nonpoint source pollutant transportations and integrating hydrological modeling is generally considered as an effective method to achieve this target. However, many studies about eco-regions have specifically focused on a single environmental factor, making it difficult to meet the actual demand of developing nonpoint source pollution modeling at national scales. To fill this gap, the present study developed a new collaborative eco-zoning technology system and used it to identify a set of eco-zones that can reflect the transportation characteristics of nonpoint source pollutant at the national scale based on geostatistical analysis, hydrological analysis, and cluster analysis. The main results show: (1) the first grade eco-zoning scheme includes 10 regions in China and served as basic measurement unit to identify the nonpoint pollutant load units (NPLUs); (2) the second grade eco-zoning scheme includes 43 sub-regions, of which the Yangtze River contains the largest number of sub-regions, followed by the Yellow River and the Pearl River; however, the number of sub-regions in the Haihe River was lowest; (3) the third grade eco-zoning scheme includes 7,775 NPLUs with the average area of 1,212 km² which serve as the best spatial scale for nonpoint source pollution modeling; (4) the most important finding is that nonpoint source pollutant transport should be divided into six sub-processes, which can be present as precipitation index, terrain index, runoff index, subsurface runoff index, erosion index, and buffer strips retention index in each NPLUs. The NPLUs contained in the same sub-regions with similar processes determine nonpoint source pollutant transport to receiving target water bodies. The major contribution of this study is the presentation of a systematic watershed eco-zoning scheme that considers the transportation of nonpoint source pollutants at national scale. It will be producing more useful for nonpoint source pollution management in the future environmental management of China.