The Rhine river basin 1 plays a significant role across Europe due to the interconnected challenges that affect it: flood protection, resource available during low-water periods2, usage (navigation, hydroelectric production, etc.). Currently, management of the Rhine basin - specifically scaling development projects such as dikes and dams - is linked to river flow thresholds that correspond to current conditions. Will these reference levels and their corresponding developments be adapted to future flows that are likely to shift with climate change?
This is a crucial question for managers and political leaders who must decide how to manage the rivers in their regions and more specifically, the investment needed to adapt their structures. The answer requires a detailed evaluation of the impact of climate change (temperature and rainfall changes) on river flows. For this reason, Irstea's experts in hydrological modeling joined forces with managers of the Rhine (DREAL Grand Est, Rhin-Meuse Water Agency) to lead the MOSARH21 project3. Their aim was to evaluate the impact of climate change on the flow of French tributaries to the Rhine (Vosges, Sarre and Moselle rivers), using the latest scenarios produced by the Intergovernmental Panel on Climate Change (5th IPCC report, 2014) and French regional projections from the Drias portal.
The study was built around three phases:
- simulating flows for a past reference period, specifically 1970-2000,
- simulating flows for future periods using rainfall and temperature data predicted by climate models using IPCC scenarios,
- calculating then comparing monitoring indicators for each period for the available resource (average flow), high-water periods (maximum daily flow for a 10-year return period) and low-water periods (minimum monthly flow for a 5-year return period).
"Although this approach provides an up-to-date estimate of the impact of climate change, the study uniquely focused on the combined use of two hydrological models converting rainfall into flow (used separately in previous studies); one designed by Irstea (GRSD) and the other by a consultancy participating in the project. Doing this allowed us to standardize our methods and data, so that our results were more reliable," specified Guillaume Thirel, project leader and researcher at the Irstea Antony Center.
Overall results that match preceding studies
By using the various scenarios developed by the IPCC based on the evolution of greenhouse gas emissions (RCP scenarios), the research was able to reveal three significant trends for the evolution of flow in the Rhine's tributaries:
- slight increase in average annual flow, which could be significant in case of a large increase in greenhouse gas emissions (RCP 8.5).
- an increase in flood intensity in the near future (2021-2050), with uncertain changes in the distant future (2071-2100).
- a decrease in low-water period flows in the near future, with uncertain changes in the distant future.
These results confirmed previous estimates and show that even a relatively humid region such as the east of France could suffer water shortages with weaker low-water periods leading to resource shortages.
Detailed decision support tools
Beyond these main trends, the MOSARH21 project created tools specifically designed for managers and decision-makers: summary pages presenting the future developments of climate, high and low-water periods for each sub-basin draining into the study area (a total of 70 sub-basins), as well as data on monitoring indicators for relevant high and low water periods. "Now updated, these tools should make it possible for each manager/decision maker to understand the evolution of their basin using the various climate scenarios available and to plan for the adjustments needed according to the scenario they select. In addition, this data should encourage debate with the countries neighboring the Rhine (particularly to standardize the reference levels) and maintain an organised management system throughout this European basin," concludes Guillaume Thirel.
For more information
- Article. Modeling river flows freely with airGR software
Consult the web pages of the HYDRO team and the Irstea Antony Center
1- A river basin is an area limited by the highest topographical lines, which drains each drop of water it receives towards an outlet, river or the sea. 2 - Low-water periods for a river. 3 - MOSARH21 (2015-2017). Partners: Irstea Antony (project leader), Irstea Lyon, DREAL Grand Est, Rhine-Meuse Water Agency (funding), Université de Lorraine, HYDRON (engineering consultancy, Germany).