Correction factors for estimating suspended sediment export from loess catchments

This study focuses on problems related to estimating the exact exported sus pended sediment quantity and quality from flow-proportional samples taken a t a fixed height at the outlet of a small, agricultural catchment in centra l Belgium. First, a correction factor is introduced to estimate the real su spended sediment concentration. This factor depends on the stream depth and the grain size distribution of the sediment and is necessary because of th e presence of a concentration gradient in the stream. Secondly, water depth is corrected for the sediment volume, and thirdly, the grain size distribu tion of the exported sediment is corrected. The application of these correction factors leads to a reduction of the est imated total sediment export by 18 per cent (from 4810 Mg over a period of three years to 3930 Mg). Furthermore, the importance of small grain size fr actions increases, leading to higher enrichment ratios for the clay fractio n and lower ratios for the sand fraction. Suspended sediment export is thus more selective than was previously thought. Due to the overestimation of t he sediment export, total phosphorus losses from the catchment are also ove restimated by 17 per cent. This paper therefore emphasizes that point sampl ing of suspended sediment is biased and that a method should be incorporate d to correct the sediment export in terms of quantity and quality. Copyrigh t (C) 2001 John Wiley & Sons, Ltd.