This paper presents a newly-derived method for directly determining the amount of transported dissolved phosphorus by water erosion. The results of the method are compared to prediction based on enrichment ratio (as proposed by Sharpley) and average share of dissolved phosphorus (DP) in total transported phosphorus (5%) that is widely used in the Czech Republic. Four study areas (catchments of dozens of sq. kilometer) were chosen for their different characteristics (land use, average slope, average elevation, phosphorus concentration in the soil) which influence their rainfall-runoff behavior. The modeled results are compared with data measured in situ. The two methods provide similar results in intensively agriculturally used regions. Agreement among the methods was observed for three study areas with significant erosion intensity (above 4 t/ha/year). In the catchment with significantly lower erosion intensity (0.5 t/ha/year), the indirect method (Sharpley) underestimates the amount of DP transported in the watercourses. The sum of transports of suspended solids into watercourses and the average available phosphorus content in the soil determined by the Mehlich 3 method (PM3) are the main factors influencing the results provided by the two methods. An analysis of the impact of these factors on the difference between the results of the methods was provided. Transport of suspended solids is related to the method difference (R range from 0.37 to 0.71). However, no significant relationship was found between the difference in the results and the average PM3 content in the soil (R range from 0.15 to 0.36).
Keywords: Dissolved phosphorus; Eutrophication potential; Soil erosion.