Establishment and Validation of an Amended Phosphorus Index: Refined Phosphorus Loss Assessment of an Agriculture Watershed in Northern China

Phosphorus (P) loss from non-point sources is a main cause of freshwater eutrophication in agricultural regions. Knowledge-based watershed management plans, aimed at reducing the diffuse flux of phosphorus from specific land-use and site characteristics to freshwater resources, are needed in order to curb eutrophication in agriculture regions. In this context, the use of a phosphorus index provides a simple and practical method for identifying hot-spot source areas and to estimate their potential for contributing a flux of P to the surface waters. However, as a semi-quantitative tool, the P index is usually difficult to validate due to inadequate data representation relative to large spatial and temporal variation in P fluxes. An amended P index scheme is therefore developed and validated, based on comprehensive synoptic soil study and stream water monitoring as well as a previous study that had applied the former P index in the studied watershed in northern China (Zhang et al. 2003). The amendments include the use of data from the individual village units (mean area, ca. 30.6 ha), use of the degree of P saturation (DPS) in the source factor scheme, adoption of flow length factor and modified water course erosion factor into the P transportation scheme, and an adjustment of the organization structure of the P index scheme. The validation of the amended P schemes was performed by comparing the modeled average P index values with the corresponding measured P fluxes for 12 different sub-catchments. The results indicate an improved precision in the simulated potential for P loss using the refined P index scheme. Measured fluxes of total P (r = 0.825), particulate P (r = 0.867), and less-studied yet more relevant dissolved P (r = 0.627) all showed significant correlations with the modeled P index values in the amended P scheme. The primary direct finding of the current research is that the areas with close proximity to rivers and the reservoir, as well agricultural land around villages, are found to be the main hot-spot sources for P loss to the reservoir.