Abstract: Report on biophysical catchment-scale modelling in the River Dee catchment, Scotland
Here we report on the use of hydrochemical models to investigate the likely changes in hydrology and water quality under scenarios of climate and land use change in the River Dee demonstration catchment, in northeast Scotland. Specific novel objectives were to: (1) predict the effect of any changes in water quality on freshwater biota, by linking catchment-scale, process-based hydrochemical models to measures of ecological impact; and (2) to integrate modelling outcomes with cost-effectiveness analysis carried out in work package 6, to determine the most cost-effective way of reducing eutrophication, both now and under future scenarios of environmental change.
The Dee catchment is a large, relatively unspoilt area, famed for its salmon fishing, shooting and hill walking. It has been designated at European level for the species it supports, in particular Atlantic salmon (Salmo salar), freshwater pearl mussel (Margaritifera margaritifera) and otter (Lutra lutra). The catchment is subject to significant pressures, including morphological alterations and nutrient inputs from sewage and agriculture, and the area remains of top conservation priority.
Three ecological indicators were identified: freshwater pearl mussel (M. margaritifera), macroinvertebrates (ASPT score) and macrophytes (MTR score). To be able to link hydrochemical model output to ecological impact, relationships were developed between nutrients and these ecological indicators. These were based on literature values (freshwater pearl mussel) or on empirical relationships. A nitrogen leaching model was then used to explore possible changes in nitrate concentration throughout the Dee catchment, and associated changes in ecological status.
Two sub-catchments of the Dee, subject to different pressures, were selected for more in-depth modelling applications: (1) the Tarland Burn sub-catchment, in the middle reaches of the Dee catchment, where stream flow, nitrogen (N) and phosphorus (P) were simulated using the STREAM-N and INCA-P models; and (2) the Loch of Skene sub-catchment further downstream, where work focused on P and suspended sediment concentrations in the Corskie Burn, the main tributary to the loch. Another study in the upland Gairn sub-catchment looks at stream temperature and salmonids and will be published elsewhere, as will a study examining total phosphorus in the Loch of Skene.
Within the Tarland and Skene sub-catchments, a suite of model runs were carried out using the climate and land use scenarios in isolation, in combination, and with a series of cost-effective mitigation measures identified previously. Key findings are that:
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