Dam‐influenced seasonally varying water temperature in the Three Gorges Reservoir

Water temperature is a fundamental parameter for both hydrological and ecological processes in aquatic systems. Usually, a sinusoidal function is used to represent the seasonal variation of water temperature in natural surface water. However, a large number of dams have been built on rivers worldwide, and water mass transport is retarded due to the reservoir impoundment. As a result, the water temperature lags seasonally behind the natural cycle due to seasonal dam‐operation, and the ordinary sinusoidal function is no more valid in describing the thermal process. In this study, a composite cosine function (CCF) is introduced to address the dam‐influenced seasonally varying water temperature pattern, which includes a time varying phase‐lag for temperature variations. The proposed method is applied to the Three Gorges Reservoir (TGR), a major impoundment on the Yangtze River, China. It is found that the annual thermal process of warming and cooling becomes asymmetry during the post‐TGR stage, that is, the seasonally varying temperature follows an asymmetric sinusoidal wave pattern. In frequency spectrum, both annual and semi‐annual components are main contributors to the time varying water temperature. The minimum temperature of TGR's discharge has increased by 1.86°C after the normal operation of the reservoir, and the coldest water arrival timing is postponed by up to 40 days in the dry season. The benefit of using CCF in temperature reconstruction is that it can prevent the mistake of ascribing the semi‐annual component to short‐term fluctuations. The present study can help in improving the understanding of the asymmetric seasonal temperature processes due to dam operation.