Load estimation methodologies for British rivers and their relevance to the LOIS RACS(R) programme

Data collected from rivers within the LOIS study area have been analysed to test the reliability of commonly-used interpolation and extrapolation methods for estimating loads from the concentration and flow records that are typically available for rivers in Eastern England. Detailed time series of suspended sediment concentration and discharge, which were collected by means of turbidity monitoring and ultrasonic flow gauging on the River Wharfe at Tadcaster over a 6-month period, have been used to compute a reference load. These data were then artificially decimated to evaluate different procedures for calculating suspended sediment loads. None of the methods investigated produced very reliable load estimates when weekly suspended sediment concentration data were used, and replicate estimates ranged from 24.4 to 550.1% of the reference load. Extrapolation by means of a simple rating relationship was found to produce estimates of suspended sediment load with the highest level of accuracy and precision, but loads calculated by this procedure still varied from −57.2% to +29.1% of the true value at the 95% level of confidence. Reduction in sampling frequency from weekly to fortnightly or monthly intervals generally caused a greater underestimation and increased imprecision in the suspended sediment load estimates. The effect of applying different calculation procedures to estimate chemical fluxes was evaluated using information from the Harmonized Monitoring database for the River Derwent at Church Wilne. For 20 out of the 36 determinands investigated, the difference between the minimum and maximum load estimate, expressed as a percentage of the minimum value, exceeded 50%, and for five determinands, including arsenic in both dissolved and particulate form, the difference was 100% or more. The results of this preliminary analysis suggest that considerable caution should be exercised in calculating river fluxes to the LOIS study coastline, and underscore the need for new and improved procedures to be developed.