Simulation of thermal stratification and water temperature dynamics in the Joumine reservoir (Tunisia)

Thermocline stratification has become increasingly important under climate change conditions, impacting the water bodies' quality, by changing the epilimnion thickness, particularly the biological quality related to phytoplankton communities. Advanced modeling techniques based on the new Derived EOLE Joumine Model (DEOLE-J) and metaheuristic approaches were used to model thermocline stratification in the Joumine reservoir in the North of Tunisia. Relative Water Column Stability (RWCS) and thermocline parameters such as thermocline depth and strength index (TSI) were used to assess the water temperature profile and the impact of the thermocline on the phytoplankton community distribution and abundance. Monthly samplings were conducted at eight gauging stations from May 2021 to August 2021. Water samples were collected to measure physical and biological parameters. Joumine's thermal stratification can be divided into three periods: Mixing, Formative, and Stable. During the Mixing period, TSI and air temperature had a significant negative correlation. Similarly, significant negative correlations were observed between TSI, air temperature, and RWCS during the Formative period. Our results reveal that weaker stratification in spring is primarily driven by increased inflow discharge, while summer stratification intensifies, creating sharp thermal gradients. The model successfully captures seasonal thermocline fluctuations and shows that wind speed plays a critical role in regulating vertical mixing. However, moderate wind speeds typical of the Joumine region have limited impact on the deeper layers of the reservoir, particularly during summer. A comparison of model estimates and measured data indicates a bias due to distant meteorological stations and the exclusion of horizontal fluxes, such as water withdrawal and throughflow. Despite these limitations, the DEOLE-J provides valuable insights into the thermal dynamics of reservoirs, showing that prolonged stratification periods reduce vertical mixing and nutrient circulation, potentially degrading water quality. These findings have significant implications for water quality management, particularly in the context of climate change, where extended stratification periods are expected leading to exacerbate water quality issues. Future research should explore two-dimensional models to enhance temperature estimation accuracy and include horizontal fluxes. Keywords: DEOLE-J, Thermal stratification, Joumine reservoir, Thermocline strength, Thermocline depth, phytoplankton distribution, water temperature.