Caractérisation des sécheresses et de l'humidité du sol sous changement climatique : résultats et applications opérationnelles du projet ClimSec en France

Abstract

International audienceEvolution of water resource is one of the main stakes of climate change in France. The ClimSec project (2008-2011) focused on the impact of the climate change on drought and soil moisture over France by using a high resolution climatological reanalysis since 1958 based from a Soil Vegetation Atmosphere Transfer model, the Safran/Isba/Modcou suite. The high-resolution 1958–2008 Safran atmospheric reanalysis was used to force the Isba land surface scheme and the hydrogeological model Modcou. Meteorological droughts are characterized with the Standardized Precipitation Index (SPI) at time scales varying from 1 to 24 months. Similar standardizing methods were applied to soil moisture and streamflow for identifying multiscale agricultural droughts – through the Standardized Soil Wetness Index (SSWI) – and multiscale hydrological droughts, through the Standardized Flow Index (SFI). Based on a common threshold level for all indices, drought event statistics over the 50-yr period – number of events, duration, severity and magnitude – have been derived locally in order to highlight regional differences at multiple time scales and at multiple levels of the hydrological cycle (precipitation, soil moisture, streamflow). Results show a substantial variety of temporal drought patterns over the country that are highly dependent on both the variable and time scale considered. Independent spatio-temporal drought events have then been identified and described by combining local characteristics with the evolution of area under drought have finally been used to compare past severe drought events, from multi-year precipitation deficits (1989–1990) to short hot and dry periods (2003). Results show that the ranking of drought events depends highly on both the time scale and the variable considered. This multilevel and multiscale drought climatology served as a basis for assessing the impacts of climate change on droughts in France. At the end of the project, the new drought indices were adapted for the operational hydrological monitoring and used for the qualification of drought event in real time, namely for the 2011 spring drought. These indices were also calculated in future climate from the various regionalized climatic projections available over France. Three particular experiences in socioeconomic scenarios, climatic models and downscaling methods have been run to estimate the relative importance different uncertainties for drought evolution. In the same time, a diagnosis on drought evolution can be established with a schedule much shorter and more intense for the agricultural drought linked to the deficit of soil wetness than for meteorological drought in relation with the precipitation. The climatic projections suggest that France could know at the end of the XXIth century a quasi-continuous drought with a strong intensity, totally unknown in the present climate