Search

In order to effectively manage nutrient river load reductions and target remediation strategies, it is important to determine the relative contributions of diffuse and point sources across the river catchment. This study used a geographical information system (GIS) to apply phosphorus (P) export coefficients (obtained from the literature) to 58 water quality monitoring sites across a large, urbanised, mixed land use catchment, typical of southern lowland England (the River Avon, Warwickshire, UK). These coefficients were used to estimate the annual P load at each monitoring site, and also the relative contribution of point source (from sewage treatment works (STW)) and diffuse input (from both livestock and agricultural land use). The estimated annual P loads showed very close agreement (r2=0.98) with the measured total phosphorus (TP) loads. Sites with the highest proportion of P derived from STW had the highest TP concentrations and loads, and also had greater variations between seasons, with elevated P concentrations occurring during the summer months. The GIS model was re-run to determine the effect of an 80% reduction in P output from STW serving over 10,000 people, thereby assessing the effect of implementing the European Union's Urban Waste Water Treatment Directive (UWWTD). The exported TP load was reduced by 52%, but the sites with the highest TP concentrations were still those with the highest proportion of P derived from STW. The GIS model was re-run to estimate the impact of 80% P reductions at a further 11 STW of varying sizes. This reduced the total TP load by only 29 tonnes year-1, but greatly reduced the P concentrations in many highly nutrient contaminated tributaries. The number of sites with P concentrations greater than 1 mg l-1 was cut from 15 (before UWWTD implementation) to 2. These findings suggest that after UWWTD implementation, resources should focus on introducing tertiary sewage treatment at the remaining large STW, before targeting diffuse inputs. This conclusion is also likely to apply to other lowland river catchments in southern England, most of which have similar population densities to the River Avon.

Drainage network analysis includes several operations that quantify the topological organization of stream networks. Network analysis operations are frequently performed on streams that are derived from digital elevation models (DEMs). While these methods are suited to application with fine-resolution DEM data, this is not the case for coarse DEMs or low-relief landscapes. In these cases, network analysis that is based on mapped vector streams is an alternative. This study presents a novel vector drainage network analysis technique for performing stream ordering, basin tagging, the identification of main stems and tributaries, and the calculation of total upstream channel length and distance to outlet. The algorithm uses a method for automatically identifying outlet nodes and for determining the upstream-downstream connections among links within vector stream networks while using the priority-flood method. The new algorithm was applied to test stream datasets in two Canadian study areas. The tests demonstrated that the new algorithm could efficiently process large hydrographic layers containing a variety of topological errors. The approach handled topological errors in the hydrography data that have challenged previous methods, including disjoint links, conjoined channels, and heterogeneity in the digitized direction of links. The method can provide a suitable alternative to DEM-based approaches to drainage network analysis, particularly in applications where stream burning would otherwise be necessary. [ABSTRACT FROM AUTHOR]