Modelling the effect of groundwater abstraction on salmonid habitat availability in the river Allen, Dorset, England

The River Allen in Dorset, England is regarded as a classic example of a chalk stream and historically has a reputation as a high quality trout and salmon fishery. Since 1946 the Allen has been subject to groundwater abstraction from chalk boreholes situated close to the stream. Significant reductions in salmonid populations in recent years prompted the National Rivers Authority Wessex (now South Western) Region to initiate detailed hydrological, biological and fisheries studies to investigate the perceived problem. As part of these studies the Institute of Hydrology was commissioned to investigate the effect of the historical groundwater abstraction regime on the availability of physical habitat for life stages of trout and salmon (Johnson et al., 1993). The modelling procedure used the instream flow incremental methodology (IFIM) (Bovee, 1982), which was implemented using the physical habitat simulation (PHABSIM) model (Milhous et al., 1984). Outputs from PHABSIM hydraulic models were combined with habitat suitability data for life stages of trout and salmon to develop habitat (weighted usable area) versus discharge relationships. Habitat suitability indices were developed by the National Rivers Authority Wessex Region by snorkelling and wading in a number of chalk streams similar in character to the Allen. To assess the impact of groundwater abstraction on the temporal variation of habitat availability, weighted usable area versus discharge relationships were combined with a 20 year time series of mean monthly historical and ‘naturalized’ flows. The resulting time series of historical and naturalized weighted usable areas were analysed using a standard duration curve program to demonstrate the effect of the abstraction on the availability of habitat for each species life stage considered.