Ultimate Limit of Cumulative Effect of Cascade High Dam Reservoirs on Water Temperature

The construction of reservoirs has brought us tremendous benefits, but at the same time, the impact on the environment has become increasingly significant. Especially, the change of natural water temperature caused by reservoir discharge directly disrupts the natural rhythm of reproduction and growth of downstream aquatic animals and plants, and cascade reservoirs undoubtedly magnify this impact. In order to study the ultimate effect of cascade high dam reservoir on water temperature, a virtual reservoir, Alpha, is constructed in this paper, and the head and tail of Alpha reservoir are connected indefinitely. The water temperature of each reservoir is simulated by using a laterally-averaged two-dimensional mathematical model. The results indicate that there is a ultimate limit to the cumulative effect of water temperature caused by river cascade development, and the stratification of water temperature is greatly weakened by cascade development, but the stratification structure will not disappear. After 20 reservoirs, the temperature of discharged water is no longer changed relative to the temperature of incoming water. At the same time and at the same relative location, both the surface and the bottom water temperature are the same as the upstream cascade, and the flow field in the reservoir is the same as the upstream reservoir. Reaching a cumulative effect limit indicates that the inflow heat and outflow heat are completely balanced. The results can provide a theoretical basis for mitigating the cumulative impact of cascade reservoir development.