## 1 Introduction  
Hydraulic engineering projects aim to achieve efficient utilization of water resources. While generating economic benefits, they also change the natural properties of rivers and have significant impacts on the reservoir ecology [1; 2; 3; 4]. Due to the construction of hydraulic structures, large amounts of water are stored behind dams, which results in deep water with low-velocity flow. These conditions mean that vertical temperature stratification frequently occurs in these deep reservoirs [5; 6; 7]. Water temperature impacts the transportation of oxygen and other nutrients, as well as biological and chemical reaction rates [8; 9; 10]. For example, temperature is closely related to the concentration of dissolved oxygen, nitrogen, and phosphorus in water [11; 12]. Spawning and growing of fish depend on the water temperature also. Thus, cooler temperatures can change or potentially destroy the downstream habitats of fish and other biological groups, pushing them to the verge of extinction [13; 14]. In addition, using cooler temperatures to irrigate can directly reduce the crop yields. Many hydropower engineering projects are affected by low-temperature released water like Glen Canyon Dam and Cherokee Dam in America and the Three Gorges Dam and San Ban Xi Dam in China [15; 16; 17; 18]. Especially in China during the last decade, every large hydraulic project should pass the Environmental Protection Assessment of the China Ministry of Environmental Protection to evaluate the impacts of cool released water from the reservoir on the downstream ecology before its construction. Therefore, increasing the released water temperature (RWT) is extremely significant and imperative for reducing the impacts of hydraulic engineering and promoting efficient utilization of water resources. To resolve this issue, the principle of selective withdrawal was proposed. This involves drawing from a specific layer (thickness determined by a range in depth) of water in the reservoir [19; 20]. From 1966, the U.S. Army Corps of Engineers initiated a laboratory to research the characteristics of the withdrawal zone in stratified reservoirs [21; 22]. Several facilities adopted practices to regulate the outflow temperature, including a stop log gate [15] and side-type orifice outlet [23]. However, these facilities are severely limited and are subject to malpractice. Most importantly, such practices can only be used in the planning or construction stage; once hydropower stations are in operation, reducing the water level in reservoirs seriously degrades power generation and irrigation efficiency. Secondly, facilities such as stop log gates cause the loss of water head [24]. Taking the Guangzhou Hydropower Station as an example, a set of stop log gates cause a 1-2 m head loss, with an annual power loss of about 26 million kW-h [25]. Therefore, as alternative temperature control measures, some flexible structures have recently been adopted in some hydraulic projects. Focusing on a reservoir which has impounded water, the temperature-control curtain (TCC) is an effective facility of selective withdrawal [26; 27; 28; 29; 30]. The TCC is made of an impermeable fabric that can block water flow, forcing water to move above or below it. The TCC can be constructed either on dry land or in a restrained reservoir; moreover, it costs less than other facilities. In summary, compared with other selective withdrawal structures, TCC has the advantages of simpler structure, more convenient construction, and lower requirements for maintenance related to water flow [31]. The previous studies of TCC have showed the effect on raising the RWT [32], and simulated the changed thermal stratification structure of reservoirs influenced by TCC. But most of the previous studies on TCC were based on numerical simulation, resulting in a lack of validation by experimental tests.