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 dif ference in temperature stratification and flows in a solitary reservoi r, and the same reservoir while in a cascade. The inflow to the reserv oir changes in a cascade reservoir from an undammed river to the regul ated flow from the hypolimnion of the upperlying reservoir, while the meterological 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 cascad e reservoir is based on hydrologically similar conditions. It is concl uded that the change in the inflow temperatures manifests itself in th e change from a lower degree of stratification and an accumulation of cold water near the bottom in the solitary reservoir to a more pronoun ced stratification in the cascade reservoir. Comparing years with diff erent RT, in a dry year (longer RT) the stratification is more pronoun ced both in a solitary and a cascade reservoir, the high flows in a we t year (shorter RT) producing much more rapid heating of the deep laye rs. The surface temperatures are lower in the cascade reservoir, too, being affected by mixing of the inflowing water with the surface strat a. 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 produce d in the cascade reservoir under conditions of the same retention time , than in the solitary reservoir. The outflow temperature of the casca de reservoir is lower than the temperature of the solitary one. Anothe r 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 ca scade reservoir due to sedimentation, phosphorus uptake and organic ma tter decomposition in the upperlying reservoir, causing lower turbidit y, less phytoplankton and less colour.