Relative Importance Analysis of a Refined Multi-parameter Phosphorus Index Employed in a Strongly Agriculturally Influenced Watershed

Eutrophication is a main cause for impairment of freshwater ecosystems, and diffuse phosphorus (P) loss from agricultural land is usually the main cause for freshwater eutrophication. The P index is a simple and practical tool for estimating the potential P loss risk. In a preceding study, a refined P index scheme was developed and validated. In the current study, the relative importance of the 14 input variables used is assessed in order to determine their relative significance to the final P index value. The backpropagation network with Garson’s algorithm was employed in order to capture the significance of interactions among the input variables. The study clearly shows the source factors, especially the degree of P saturation (DPS), along with management practices regarding application of inorganic P fertilizer and livestock manure, are the most important factors governing the P loss in the very high and high risk areas. Conversely, the transportation factors governed P loss risk in the low and very low risk areas. Recommended management strategies for mitigation of P loss from the different risk zones are proposed based on the relative importance analysis and practical constraints. A scenario analysis, based on a gradient reduction of DPS, through decreased application of both inorganic P fertilizer and P emissions factors from livestock manure, gave a reduction of average P index from 7.3 to 57 %. Moreover, the proportion of high- and very-high-risk area may be reduced from 38 to 23 % and 24 to 13 %, respectively.