Complex summer water temperature behaviour below a UK regulating reservoir

Detailed monitoring over a 15-year period has revealed considerable complexity in summer water temperature behaviour below Wimbleball Lake, a large regulating reservoir in south-west England. Continuous records of water temperature in the regulated River Haddeo and the neighbouring unregulated River Pulham, coupled with information on flow in the tailbay of the dam and on the thermal structure of the impounded water body, reveal that the nature and extent of the thermal modification below the reservoir varies from day to day, month to month and year to year. This complexity largely reflects changes in the balance between a springflow and a direct reservoir release component of the compensation runoff, which arise in response to variations within and between summers in hydrometeorological conditions and in the way the reservoir is operated to regulate downstream flows. Temperature behaviour in the regulated river is made more complicated by progressive warming of the springflow during the summer months, the thermal inertia of the stored water, the development of thermal stratification within the reservoir and the alteration of release depth, although the detailed effect of some of these factors depends on weather conditions, the use of destratification equipment and the rapidity of reservoir drawdown.