AN ANALYSIS OF THE EFFECT OF AN UPSTREAM RESERVOIR BY MEANS OF A MATHEMATICAL MODEL OF RESERVOIR HYDRODYNAMICS

The reservoir hydrodynamic model DYRESM is used to investigate the difference in temperature stratification and flows in a solitary reservoir, and the same reservoir while in a cascade. The inflow 10 the reservoir changes in a cascade reservoir from an undammed river to the regulated flow from the hypolimnion of the upperlying reservoir, while the meteorological and other conditions remain unchanged. In agreement with previous findings of the strong effect of theoretical retention time on reservoir stratification, the comparison of a solitary and a cascade reservoir is based on hydrologically similar conditions. It is concluded that the change in the inflow temperatures manifests itself in the change from a lower degree of stratification and, an accumulation of cold water near the bottom in the solitary reservoir to a more pronounced stratification in the cascade reservoir. Comparing years with different RT, in a dry year (longer RT) the stratification is more pronounced both in a solitary and a cascade reservoir, the high flows in a wet year (shorter RT) producing much more rapid heating of the deep layers. The surface temperatures are lower in the cascade reservoir, too, being affected by mixing of the inflowing water with the surface strata. The heat budget of the reservoir is changed, more heat being taken up by the cascade reservoir as a consequence of the greater air-water temperature gradient. More intensive mixing of water masses is produced in the cascade reservoir under conditions of the same retention time, than in the solitary reservoir. The outflow temperature of the cascade reservoir is lower than the temperature of the solitary one. Another effect on the heat budget, not exactly studied in this paper, might be caused by the decreased extinction of light for the water in the cascade reservoir due to sedimenl < '1tion, phosphorus uptake and organic matter decomposition in the upperlying reservoir, causing lower turbidity, less phytoplankton, and less colour.