Spatiotemporal evolution characteristics of water temperature structure and lag effect of discharge water temperature in Wudongde Reservoir

Abstract

[Objective] Clarifying the spatiotemporal evolution characteristics and temperature lag effect of high dams and large reservoirs can provide a scientific basis for stacked beam gate scheduling, which is fundamental for alleviating discharge of low-temperature water and protecting the survival and reproduction of fish in the lower reaches. [Methods] Through numerical simulation and based on the measured hydrological data, water temperature data, meteorological boundary data from 2020 to 2022, and design operation data in an average water year, a three-dimensional numerical simulation model of water temperature for the Wudongde Reservoir area was developed. The spatiotemporal evolution characteristics of water temperature structure and lag effect of discharge water temperature after reservoir operation were analyzed. [Results] The result showed that:(1) In spring and summer, a distinct vertical stratification of water temperature was observed in the Wudongde Reservoir, with a temperature difference of 11~12 ℃. After the flood season, as the thermal exchange within the water body increased, the vertical water temperature gradually became uniform. No significant water temperature stratification was observed in autumn and winter.(2) There was a clear issue of low-temperature water discharge in spring and summer, and high-temperature water discharge in winter from the reservoir. Monitoring data from 2020 to 2022 indicated that the discharge water temperature in May was, on average, 2.68℃ lower than the natural water temperature at the dam site, and the discharge water temperature in December was, on average, 4.78℃ higher. Simulation result during the stable operation period indicated that from March to June, the monthly average discharge water temperature was 0.91~4.09 ℃ lower than the natural water temperature at the dam site. From December to February of the following year, the monthly average discharge water temperature was 1.34~4.34 ℃ higher.(3) After the impounding of reservoir, the time for the water temperature to reach 14℃ and 18℃ each year was delayed to varying degrees compared to before the dam construction. The occurrence time of suitable spawning water temperature for fish in the lower reaches was postponed by 30~50 days, with the delay ranked as follows: Rhinogobio ventralis > Jinshaia sinensis, Four major Chinese carps, Common carp, Crucian carp > Lepturichthys fimbriata. [Conclusion] The result show that the stratification of water temperature in the Wudongde Reservoir, along with the low-temperature discharge water it causes, has varying degrees of negative effects on fish reproduction in the lower reaches of the river. Therefore, scientific measures must be implemented to address these issues.

Keywords

Wudongde Reservoir / water temperature structure in reservoir area / discharge water temperature / temperature lag effect / fish reproduction / numerical simulation