Thermal-effect of the upper Yangtze reservoirs and countermeasures

In this paper, we use observational data to reveal that water temperature (T) discharging from the Three Gorges reservoir (TGR) downstream has been increased in excess of 5.3℃ in winters, and delayed by 30-43 days with the pool T exceeding the local air by 10℃ when comparing with the pre-impoundment situation. This fact is likely caused by the thermal effect of TGR and other reservoirs upstream, aggravation with the increasing impoundments upstream. Due to the enormous flow discharge, storage and well-mixed in the warmed pool, quantity of the modified heat is so huge that T change can extend over 690 km. Two effects should be listed as prior environmental issues currently: 1) the effect on dissolved oxygen (DO) in the pool and downstream, which is probably lowered by a magnitude comparable to the level of the chemical oxygen demand of the Yangtze and 2) the effect of the additional heat released from the pool and discharged downstream of TGR in winter, which is estimated to be 243GW (241 W/m2 in density) and 173 GW in average in Nov.-Jan. (2014-2016). In addition, another 200 GW heat is going to be trapped by pools that are planned to be impounded before 2030 upstream in the hot-valleys through water absorption of solar radiation, which will also a stressing forcing of warming for the TGR in the future. DO is known as a key eco-property for surface waters.At present, harmful algal blooms constantly occur in the tributary pools, and hypoxia has already been observed in 2014 in the main pool of the TGR. The declining environmental status warns us to take the increasing pool T and associated DO depression as prior issue in the Yangtze River environment protection. On the other hand, as the Yangtze is unique in world large rivers that originates from warmer and intensive solar radiation valleys to an artificial mainstream mega pool in dimming and humid basin with the storing and detention of huge heat induced from the upstream pools on water quality and local evapotranspiration, soil moisture and climate should thus deserve to be listed as key investigation. As a countermeasure, we suggest to restore the increasing thermal effect by installing pool photovoltaic power (PPV) in upstream hot-valleys. Except for inhibiting additional heat absorption, the PV in combination with the installing power and regulation advantages of the cascaded hydro power in southwest China can also produce significant renewable electricity and regulation facilities that are urgently desired by the coal dominant power system of China.