An assessment of a simple method for estimating the relative contributions of point and diffuse source phosphorus to in-river phosphorus loads

Implementation of the Urban Wastewater Treatment (UWWT) Directive requires a relatively simple methodology to partition in-river phosphorus (P) loads between diffuse and point sources in a catchment. In this study, standard monitoring data from the River Avon in Warwickshire, U.K. were used to estimate in-river loads. A novel aspect was the application of uncalibrated P run-off coefficients from the literature to areas of different land-use to estimate the diffuse P load. Point source P-loads were estimated either from monitoring data or from published export coefficient values. Analysis of the data showed that the main source of variability in the estimate of in-river loads was from the flow data, which, at many sites, did not cover a sufficient range of natural flow rates to allow reliable estimates of the annual load to be made. Analysis of data from 20 sites where this condition was fulfilled showed that, in the large, urbanised (15% land cover) River Avon catchment the use of uncalibrated run-off coefficients gave reliable estimates of diffuse loads.[PUBLICATION ABSTRACT]