Assessing long-term performance of a low water level support system under climate change using influenced streamflows models.

The Allier River is a tributary of the Loire, the longest river in mainland France. It covers a watershed of 14,310 km². To maintain an objective flow in the Middle Loire, two reservoirs are used to release water during summer including Naussac on the Allier River (190 hm³). These operations are monitored by the Loire River Basin Authority (Établissement Public Loire). The 2022 and 2023 droughts in France highlighted the vulnerability of Naussac water supply in summer. This risk will be more important in the future, as global warming leads to lower flows in the watershed.

To evaluate the impacts of global change on electricity production across the Loire watershed at Saumur (81,200 km²), a framework that encompasses water usage demand (considering water withdrawals) in hydrological simulations has recently been developed. We propose to complete this framework with the introduction of hydraulic structure management rules (Fig.1), to evaluate the long-term performance of the low water level support system for two future timeframes, 2035-2065 (mid-term) and 2070-2099 (long-term), relative to the current climate (1976-2005).

Our analysis focuses on the management of the Naussac dam and the provision of low-water support for the Allier River [2]. In wet periods, Naussac can be filled in three ways: via natural inflows from the Donozau river, a detour on the Chapeauroux river, and pumping into the Allier. In dry periods, it provides low-water support for the Allier at various strategic points, known as nodal points, to satisfy the multiple uses of water downstream (agriculture, drinking water supply, etc.). Explicit integration of these constraints at nodal points allows for global performance analysis.

The Naussac management model was validated over the historical period, then projected into future climates using 4 climate models from CMIP5, forced by the RCP 8.5 greenhouse gas emissions scenario. The drop in flows forecasted for the end of the century would lead to more frequent interruption in low-water support, that can be highlighted with several indicators analysis [3]. This situation is characterized by a drastic fall in the average stock of the Naussac reservoir over the year by the end of the 21st century (Fig.2). Management rules can be adapted to a certain extent and help reduce vulnerability of the low-water support, but limited by the need to maintain downstream water uses and the good ecological status of the hydro system.

Figure 1: Schematic view of the water management system of Naussac dam.

Figure 2: Model validation over the historical (top) period and visualization of average stock in future (bottom).

References:

[1] Sauquet, E., Robin, Y., Corre, L., Marson, P., Bernus, S., Projections climatiques régionalisées : Correction de biais et changements futurs, Explore2, Recherche Data Gouv, V2, 2022.

[2] Sonnet, O., Etude HMUC : étude d’adaptation du mode de gestion du barrage de Naussac sous l’effet du changement climatique, Phase 1, Technical report, Etablissement Public Loire, 2016.

[3] Francois, B., Thèse : gestion optimale d'un réservoir hydraulique multiusages et changement climatique. Modèles, projections et incertitudes : Application à la réserve de Serre-Ponçon, 2013.