The influence of shape on reservoir stratification under changing climates

Reservoir morphology influences a waterbody’s response to temperature and corresponding effects on stratification, water quality, and ecosystem health. Because local reservoirs may respond differently to the same climate forcing, understanding shape-driven influences is critical to optimised reservoir management, particularly under future climate scenarios. For this study, 5 archetypical hypothetical reservoir morphologies (shapes) were created based on constant surface area and maximum depth from a previously modelled polymictic reservoir, Blagdon Lake in southwest England, to establish how bathymetry alters stratification regimes. The shapes were modelled with the Aquatic Ecosystem Model 3D (AEM3D) and then forced with multiple future climate scenarios based on UK Climate Projections (UKCP18). Two stratification predictors, lake geometry ratio (GR) and Osgood index (OI), were used to characterise stratification in the differently shaped reservoirs. Model results show that all reservoirs stratified with similar thermocline depths during simulated summers, with some variations in water-column stability and hypolimnion volume. Morphologies with larger OI values tended to have increased strength, areal extent, and duration of stratification. Results highlight the importance of taking reservoir shape into account when planning management strategies and optimising reservoir design under future climates.