Climate change will lead to more frequent and intense droughts and extreme heat episodes in mainland France. This will have negative consequences on ecosystems and biodiversity, particularly concerning aquatic environments. Additionally, the presence of numerous small water reservoirs, whether subjected to extractions (irrigation, industry, drinking water) or not (recreation, fishing, hydroelectricity production, abandonment), creates cumulative impacts on these environments. These impacts depend on various factors, including the type of water inflow into the reservoirs and the mode of water release back into the river (Carluer et al., 2016).

The aim of this thesis is to characterize the impact of reservoirs on the hydrological and thermal regimes of headwater rivers a identify the factors related to reservoir design that may modulate these impacts. The operational goal is to support watershed managers in their land planning processes (standardization, dam removal, reservoir creation) to best reconcile good ecological status of aquatic environments with human activities. The thesis is structured around two main axes:

1 - Experimental Monitoring
2 - Modeling

The experimental monitoring focuses on observing water temperatures and volumes around three small reservoirs (with a volume of tens of thousands of m3) presenting different design configurations, located in the Yzeron catchment (69). These observations will help identify the processes at play in each type of reservoir, and their hydrological and thermal impacts on the receiving river, depending on climatic conditions and water extractions. Figures 1, 2, and 3 illustrate the measurement configurations for one reservoir, with two examples of the obtained data.

Once these processes are characterized, a specific reservoir module (representing the hydrological and thermal balance of the reservoir, considering its characteristics and the incoming hydrological and thermal flows) will be integrated into an existing hydrological model. The ultimate goal is to explore, through numerical modeling, various reservoir management scenarios to propose solutions that reduce their cumulative impact on the hydrological and thermal regimes of rivers.

Reference

Carluer, N., Babut, M., Belliard, J., Bernez, I.I., Burger-Leenhardt, D., Dorioz, J.-M., Douez, O., Dufour, S., Grimaldi, C., Habets, F., Bissonnais, Y.L., Molenat, J., Rollet, A.-J., Rosset, V., Sauvage, S., Usseglio-Polatera, P., Leblanc, B., n.d. Cumulative impact of reservoirs on the aquatic environment. 2016. Joint scientific appraisal. 146 pp.doi : 10.17180/zwsy-3sc20 ⟨hal-
02604911)