Spatio‐Temporal Variability of the Reservoir Thermocline: Insights from Monitoring Data and Numerical Modelling

ABSTRACT A vertical density difference caused by the thermocline during thermal stratification creates contrasting hydrochemical characteristics between the surface and bottom layers, negatively impacting aquatic ecosystems. Although numerous studies have investigated thermoclines, fewer have explored their spatial and temporal variations within the same reservoir using high‐resolution temporal data and a numerical model. This study analysed water temperature (WT) and thermocline variations in the Dalongdong (DLD) reservoir in Southwest China using a numerical model. The results reveal that the surface‐layer water temperature (WT) of the DLD reservoir exhibited a decreasing spatial trend from upstream to downstream. Based on the simulated data, variations in the profile WT, Schmidt stability (SS) and thermocline parameters (thermocline depth [TD], lower boundary depth [TB], thickness [TT] and strength [TS]) were evaluated. It was found that the thermal structure change in the DLD reservoir follows three‐phase patterns: the incubating thermal stratification (ITS) period, the obvious thermal stratification (OTS) period and the fading thermal stratification (FTS) period. During the ITS and FTS periods, the SS exhibits significant positive correlations with the TD, TS and TT, while the correlation between the TD and the TT is weak. In contrast, during the OTS period, correlations between the SS and each parameter of the thermocline weaken, and the TD and TT show a significant negative correlation. These findings suggest that thermocline dynamics are governed by distinct factors at different stages. Specifically, TB primarily controls thermocline changes during the ITS and FTS periods, whereas TD dominates during the OTS period. Additionally, spatial heterogeneity in thermocline parameters is pronounced during the OTS period in the DLD reservoir due to both depth and external recharge water flow. These results provide critical methodologies and perspectives for distinguishing thermocline variations and will contribute to the rational management of reservoir water environments.