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One of the challenges for hydropower dam operation is the occurrence of supersaturated total dissolved gas (TDG) levels that can cause gas bubble disease in downstream fish. Supersaturated TDG is generated when water discharged from a dam entrains air and temporarily encounters higher pressures (e.g. in a plunge pool) where TDG saturation occurs at a higher gas concentration, allowing a greater mass of gas to enter into solution than would otherwise occur at ambient pressures. As the water moves downstream into regions of essentially hydrostatic pressure, the gas concentration of saturation will drop, as a result, the mass of dissolved gas (which may not have substantially changed) will now be at supersaturated conditions. The overall problem arises because the generation of supersaturated TDG at the dam occurs faster than the dissipation of supersaturated TDG in the downstream reach. Because both generation and dissipation of TDG are functions of water temperature, there is an opportunity to affect the TDG process through selective withdrawal structures at a reservoir. Using a combination of field observations, and hydrodynamic modelling, we analysed the dependence of the water temperature difference on TDG generation from different-elevation release structures of highdam reservoirs. By using of the dissipation model coupled with TDG and temperature, the evolution of supersaturated TDG from different withdrawal structures was simulated and compared in a natural river reach. It showed that warmer withdrawals result in reduced generation of TDG and enhanced dissipation of TDG. INTRODUCTION Water temperature is an amazing controlling factor that affects water quality and aquatic system in many aspects, such as the activity of microorganisms, the mass transfer of dissolved gas, and the suitability of habitats for aquatic organisms. The increasing number of dam constructions and operations are causing distinct spatial and temporal changes in the water temperature (Deng et al. 2011). Temperature stratification is one of the dominant characteristics of large and deep reservoirs. According to the literature (Deng 2003), the temperature difference between the surface and the bottom layer in a reservoir may reach as high as 17°C. To reduce the negative effects on aquatic organisms, various mitigation measures, such as stoplog intake and eco-regulation strategies, have been implemented (Deng et al. 2011, Chen et al. 2016). The temperature-related responses have aroused more public concern, among which is the response of supersaturated total dissolved gas (TDG) to temperature changes. TDG supersaturation (TDGS) is an adverse impact caused by spill discharge from reservoirs, which is widely understood to cause bubble disease and thereby increase fish mortality (Weitkamp et al. 2003, Liang et al. 2013, Xue et al. 2019). Many studies have been performed on the evolution of TDGS and its mitigation strategies (Politano et al. 2012, Urban et al. 2008, Yingzhu et al. 2018). Li et al. (2009) proposed that two processes are involved in the TDGS problem of large dams. The first process is the generation process of TDGS, in which excessive air is dissolved under high pressure. The second consequent process is the dissipation process, in which the supersaturated TDG dissipates from water in the downstream river. Shen (2014) conducted experiments to explore the effect of water temperature on supersaturated TDG dissipation in static and turbulent conditions. Ou (2016) carried out experiments in both a straight flume and a converted flume to study the relationship between the dissipation coefficient and temperature in flowing water. However, before the present work, no investigation has provided insight into the quantitative response of TDGS to the water temperature stratification related to the reservoir operation. Determining the relationship between TDGS and water temperature variation is not only a practical but also a theoretical challenge for improving high dam operations. Nat. Env. & Poll. Tech. Website: www.neptjournal.com Received: 29-07-2019 Accepted: 08-10-2019

Effects of Hydropower Reservoir Withdrawal Temperature on Generation and Dissipation of Supersaturated TDG