The use of indices of flow variability in assessing the hydrological and instream habitat impacts of upland afforestation and drainage

Although the impact of plantation forestry and ground-preparation drainage on headwater runoff response has been widely studied, there are remaining uncertainties concerning the time scale of changes, scale effects of catchment size and impacts on flow variability. Flow variability, along with changes in sediment loads and water quality, is likely to be a defining element of the overall instream habitat quality of headwater catchments. In this paper a method is described for the characterisation of flow variability using 15-min data on the 1.5 km2 Coalburn catchment, from 1967 to 1998, over a period of change from natural moorland to closed canopy coniferous forest. The method is based on annual number, and average and total duration of pulses above selected threshold flows but decouples the effects of variable annual rainfall. The number of pulses increased from pre- to post-drainage but pulse number has declined steadily and pulse duration increased with forest growth—the catchment has become more, then less ‘flashy’. The method provides a comprehensive, continuous and quantitative picture of changes in hydrological regime that is relevant to current assessments of instream physical habitat and ‘environmentally acceptable flows’.