Diurnal variation characteristics of thermal structure in a deep reservoir and the effects of selective withdrawal

Thermal structure significantly impacts the reservoir ecology and environment. The diurnal temperature vari-ation (DTV) influences the photosynthesis and respiration of phytoplankton in day and night, and meanwhile changes the vertical density convection and current. Selective withdrawal is practical to control the withdrawal elevation and outflow temperature, and can also influence the thermal structure of reservoir area. Previous research mainly focusses on the long-term change pattern of thermal structure in reservoir, while the diurnal thermal dynamics is less studied, and the effects of selective withdrawal on the DTV is still unknow. Hence, this paper aims to illustrate the diurnal variation characteristics of thermal structure in reservoir, and moreover to reveal the corresponding effects of selective withdrawal. Taking the Sanbanxi Reservoir as study case, a hydrodynamic-temperature numerical model with hourly simulating resolution was built and validated using measured temperature profile during August 2015 to August 2016, based on the CE-QUAL-W2. The effects of selective withdrawal schemes, including different withdrawal elevation and withdrawal types, were illuminated. The discrete Fourier transform (DFT), energy spectrum and statistical analysis were used. This paper showed the following: (1) In the reservoir area, there were three main regions with significant DTV, i.e., the surface layer, the 10-m water layer and the 60-m water layer (withdrawal layer). For the baseline (existing intake), the average DTVs were 0.657, 0.497 and 0.174 degrees C, respectively, at the surface layer, the 10-m-depth layer and the 60-m-depth layer. (2) From the upstream to the downstream channels of reservoir, the DTV kept unchanged at the surface layer, and increased at the 10-m-depth layer and 60-m-depth layer. (3) A higher withdrawal elevation and the internal weir schemes (stoplog gate/temperature-control curtain) could increase the epilimnetic DTV and energy density, and were suggested to mitigate the hypoxia and nutrient enrichment, compared with a lower withdrawal elevation and the multi-level intake scheme. The results could provide technical support for the ecological management of reservoir and engineering design of the selective withdrawal.

Automated summary

This paper investigates the diurnal variation characteristics of thermal structure in reservoir and the effects of selective withdrawal. It reveals that there are three main regions with significant diurnal temperature variation in the reservoir area, namely the surface layer, the 10-m water layer, and the 60-m water layer. The diurnal temperature variation remains unchanged at the surface layer from the upstream to the downstream channels of the reservoir, but increases at the 10-m depth layer and 60-m depth layer. Higher withdrawal elevation and internal weir schemes are suggested for mitigating hypoxia and nutrient enrichment.