Understanding the Temperature Variations and Thermal Structure of a Subtropical Deep River-Run Reservoir before and after Impoundment

A two-dimensional hydrodynamic CE-QUAL-W2 model was configured for a deep subtropical river-run reservoir, the Xiluodu Reservoir (XLDR), in China to simulate water temperature in the first two years of impoundment (2013–2014) using measured data as model input. It was calibrated using observed temperature profiles near the dam and the outflow temperatures. Observed daily temperatures at four gauging stations upstream or downstream of XLDR before (2000–2012) and after the impoundment (4 May 2013) were analyzed and fitted with a sine function representing seasonal temperature variation. The fitted annual temperature phase shifts showed no phase delay in XLDR area before the impoundment but revealed a phase delay about 17 days between outflow and inflow after the impoundment, which was not caused by the air temperature variation. The simulated temperatures verified a similar phase delay after the impoundment. The simulated temperatures, water ages, and vertical temperature gradients demonstrated an average metalimnetic deepening rate of 0.49 m/day (average inflow ~4500 m3/s) while the largest rate due to massive inflow (~15,000 m3/s) was 1.67 m/day. The W2 model was run under hypothetic scenarios of different inflow/outflow rates and outflow withdrawn elevations. The results revealed that greater inflow/outflow rate could lead to higher metalimnetic deepening rate and smaller outflow phase delay, while deeper outflow withdrawn could lead to deeper metalimnion and larger epilimnetic depth.