Modeling the oxygen regime of stratified reservoirs

Abstract

The existence of stable thermal stratification in a reservoir during the summer alters appreciably the conditions under which its oxygen regime is formed, since the thermocline prevents diffusion of oxygen supplied to the body from the atmosphere and during photosynthesis of algae, and from the epilimnion to the hypolimnion. As a result, water supersaturated with oxygen may be observed in the epilimnion, and, conversely, an oxygen deficit in the hypolimnion. In the near-bottom layers, its content may be diminished virtually to zero with fatal consequences for near-bottom and bottom fauna.The thermal regime of a stratified reservoir can be schematized by a two-layer model consisting of an epilimnion and hypolimnion, which separates the plane with a pronounced change (jump) in water temperature, being conditionally adopted as the thermocline. Here, the temperature of the upper boundary of the thermocline can be assumed equal to or less than the temperature of the water surface, and the temperature of the lower boundary equal to or greater than the temperature of the water at the bottom, depending on specific conditions. In the latter case, the temperature variation of the water throughout the depth of the epilimnion and hypolimnion is assumed to be linear. A similar assumption is also made for the variation in the oxygen content over the depth of the hypolimnion, which, as a rule, decreases with depth in mesotrophic and eutrophic bodies of water. As for the epilimnion in which, in contrast to the hypolimnion, the oxygen content is largely determined not by diffusion, but by adequate transfer, i.e., by the mixing of water (runoff, wind-generated, and convective flows), its oxygen content may be assumed constant over the depth. A mathematical model [1] on which the hypothesis of the ideal mixing of water in a reservoir on the whole, or taken layer at a time, is based, can then be used to calculate the oxygen regime of a stratified reservoir. Using this model, the oxygen regime is calculated successively in the epilimnion at first, and then in the hypolimnion. In that case, the thermal regime of some layers is assumed to be known, and the average velocities of the flows in these layers are assumed to be directly proportional to their average water temperature [2]. Proceeding from this condition, which is valid for poor circulating and deep reservoirs, irrespective of the degree of circulation in the latter (for which temperature stratification in the summer is also characteristic) and the draw down of water into the tailrace is known, the flow of water is determined separately for the epilimnion and hypolimnion, which will also determine the water volume in these layers, and, accordingly, their oxygen regime.The fact that the oxygen-saturation concentration of water O~ in both layers is determined by the surface temperature of the water is common to calculation of the regime in the epilimnion and hypolimnion. This temperature controls the equilibrium concentration of O 3 in the epilimnion. Maximum oxygen concentration...

Tailoring dam structures to water quality predictions in new reservoir projects: Assisting decision-making using numerical modeling

“…conditions, the entire volume of the reservoir was considered as the hypolimnion, and exchanges with the atmosphere were allowed. Oxygen exchanges between epilimnion and the atmosphere were estimated following Sokolov and Dmitrieva (1998). See the Supplementary File for further details.…”