Stratification dynamics in a shallow reservoir under different hydro- meteorological scenarios and operational strategies

[1] Vertical mixing plays a major role in functioning of seasonally stratified aquatic systems. In this study, we employ a 1-D stratification model and a 9 year forcing data set to simulate the thermal dynamics in a large, but shallow reservoir that regularly displays a polymictic character with complete mixing events during summer. Such mixing dynamics is typical for many water bodies in the temperate zone having an intermediate depth. In many cases summer-mixing events were documented to induce severe water quality deteriorations (e.g., cyanobacterial blooms). We examined and quantified the response of summer-mixing behavior to combinations of hydrological regimes, i.e., water level fluctuations and withdrawal depth, and changes in meteorological variables, i.e., air temperature and wind speed. According to our findings: (i) increasing summer air temperatures considerably increase the resistance of the water column against mixing; (ii) while the combination of maintenance of a high and constant water depth and implementation of epilimnetic discharge results in almost complete resistance to mixing, their individual effects are also substantial, being roughly comparable to the effects of 4–6 K increase in air temperatures; (iii) wind is a critical variable, 30% increase of which can compensate up to 5.5 K increase in air temperatures; and (iv) effects of changes in air temperature, wind speed, and water depth are inter-dependent, as indicated by enhanced importance of wind and temperature in response to decreasing water depth, as well as reduced importance of depth in response to decreasing wind speed and increasing temperature.