Cross Wavelet Analysis on the Influence of the Three Gorges Dam Impounding on the Reservoir Precipitation

How the Three Gorges Dam impounding impact on local climate environment is a hot issue in science and engineering communities. Since the dam worked in 2003, the reservoir water level changes from 66 m up to 175 m in three stages, providing a perfect period observing the coherence of water level and rainfall. In order to diagnose their interaction in multiple periodic windows, historical water level changes of the Three Gorges Reservoir and the daily precipitation data recorded at four national weathering stations are analyzed with temporal full-scales spectra by Morlet cross wavelet analysis with power spectra, coherence spectra, and phase spectra. The results show that the obvious impact of water impounding on the reservoir rainfalls, of which the strongest impacting period occurred at the first impounding stage, causing reservoir rainfall reduction of one year period and increment of high frequencies. Historical reservoir precipitations have premier period 1.0 a, and multiple secondary longer periods, such as 2.0 a and 4.0 a. The premier period keeps after three stages, but the secondary period, 2 a, at dam area change obviously. There are two influences of reservoir impounding on reservoir rainfall in phase spectra:Artificial control and natural regulation. The former is completely antiphase at the premier period 1.0 a, which is obviously different from the inphase before the dam construction. The latter is reflected in the anti-phase of water level and rainfall in hi-frequency components at zone (1/128-1/32 a), the scale effect increases with the increasing reservoir water level. The noted fact is that the coherence 3.5 a period at Badong and Jingzhou stations antiphase, possibly indicating the water cycle changed spatially. Besides, water-level change corresponds to rainfall in antiphase annual variations of dam's function of water control, different from their variations in-phase before the dam construction.