Environmental flows for English rivers: a focus on modified catchments

Increased human demands upon water resources and growing uncertainty surrounding climate change have focused attention on the need to determine environmental flows (e-flows) to protect and sustain river ecosystems. Internationally there have been several advances but within England an anthropocentric approach to water resources management has led to polices that set minimum flows founded in fear of water shortage. By exploring the history of flow management in England and its influence on current practice, and by introducing the concept of ‘ecological drought’ as a basis for managing future flows, this thesis makes two valuable contributions to the e-flows debate. It also explores the influence of George Baxter who more than 50 years ago proposed that compensation flows below dams could be varied to meet the seasonal ecological requirements without reducing water supplies.

In this thesis, hydrological assessments are made of watercourses spanning the ‘natural – heavily modified’ continuum located across the River Trent and Great Ouse catchments of central England, using a dataset of 48 stations and approximately 1000 station-years. Analyses highlight a variety of ‘ecological drought’ responses, in magnitude, timing and duration, with extreme low flows being rarely observed on all watercourses in the same year. This suggests e-flow determinations at the local, sub-catchment, scale would have benefits for environmental protection and water supply. A variety of potential e-flow metrics are examined and Baxter’s hypothesis tested. It is shown that supporting flows during the key ecological periods of spring and autumn, while sustaining current levels of abstraction, would risk degradation of the rivers through the increased frequency and duration of extreme low flows. Finally, the thesis examines practical issues impacting on any future e-flows policies relating to climate change and hydrometry.