Evaluation of land management impacts on low flows in northern England

Abstract

Low flows are becoming an increasing issue in the UK. The effect of an increasing population on water supply demand is bringing awareness of the issue of extreme low flows risk to the attention of water and environmental managers across the country. Summer droughts in the Lake District in 2010 which followed winter flooding have raised the question of whether land management can be applied to reduce low flows risk in the area. This is the issue considered in this project. This master’s thesis, funded by the Adaptive Land-use for Flood Alleviation (ALFA) project of the EU set out to discover whether land management, vegetation change or changes in farming practices, could help reduce the risk of extreme low flows in Cumbria, England.

The hydrological model CRUM3 was applied to simulate the river discharge of the Dacre Beck under different land management change scenarios. Sensitivity analysis and a rigorous Generalised Likelihood Uncertainty Estimation experiment proved the model’s efficiency at predicting low flows discharges as well as flood peaks. Results of vegetation change scenarios demonstrated that a cover of natural grassland provided the best water supply to the river during low flows. Increases in cover of the land by each 1% of the catchment area in natural grassland resulted in a 1% increase in stream discharge during extreme low flows periods. The location of the land assigned to vegetation change was shown to be insignificant. Scenarios of improved agricultural practice were modelled to simulate the reduction of compaction in the catchment by soil aeration. This revealed more impressive increases in river discharge during extreme low flows than the vegetation change. Though the compaction scenarios were theoretical, feasible increases in low flows discharge could reach 100%.

Since flooding has also been a proven issue in this region, the scenarios were also assessed for their impacts on high flows. The most beneficial vegetation type at reducing high flows was deciduous woodland, though this had been seen to have a negative effect on low flows. Natural grassland had negligible effect on catchment high flows. Compaction reduction was however discovered to be a potential simultaneous management solution to both high and low flows, as whilst potentially increasing low flows by up to 100%, it could also decrease high flows by up to 8%. Further research would be required to make accurate estimates of the potential improvements to high and low flows, but this project has demonstrated that reducing compaction is definitely beneficial to the catchment hydrology.