Identification of catchment runoff processes as a basis for defining water quality protection zones
Ockenden, Mary Catherine
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`Protection zones' for water quality are often defined as areas within the landscape where potentially polluting activities are excluded or restricted, with the aim of reducing the quantity of pollutants reaching the waterways. Such protection zones are needed in relation to agrochemical pollution from farming. This study uses a combination of hydrological, hydrochemical and modelling approaches to identify the nature and likelihood of generation of different hydrological flow pathways and to recommend hydrology-based protection zones for overland flow within an agricultural catchment. The study is based in the 616 km2 Upper Eden catchment in Cumbria, and one of its subcatchments, Blind Beck (8.8 km2). Transfer function modelling of rainfall-flow is used to investigate how the dominant mode(s) of stream response varies for different scale catchments (1 km2 to 616 km2). This indicates more water taking slower pathways as the proportion of the catchment on permeable sandstone increases. Chemical sampling, high temporal resolution measurements of specific conductivity, stream water temperature and diurnal variations of stream ions show results consistent with the rainfall-flow modelling in identifying a higher proportion of water on slower pathways in a small catchment on sandstone. Spatial distributions of surface soil moisture at 10 plots in Blind Beck investigate how topography, slope and land use affect the local distribution of surface saturation that produces saturation overland flow. At the plot scale, wetness distributions predicted by the Kirkby topographic index show significant discrepancies with the observed soil moisture distributions. A conceptual model for defining hydrology-based protection zones for overland flow combines areas of highest pollutant source risk with areas likely to generate overland flow. Overall, this study shows that while the factors controlling the location of saturated areas are complex, identification of the dominant hydrological pathways is fundamental to the design of water protection zones.