Changements sur les caractéristiques des sécheresses en France au cours du 21e siècle

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Effets de scénarios d'adaptation au changement climatique sur les caractéristiques spatio-temporelles perçues des sécheresses

International audienceDrought events develop in both space and time and they are therefore best described through summary joint spatiotemporal characteristics, like mean duration, mean affected area and total magnitude. This study addresses the issue of future projections of such characteristics of drought events over France through three main research questions: (1) Are downscaled climate projections able to reproduce spatio-temporal characteristics of meteorological and agricultural droughts in France over a present-day period? (2) How such characteristics will evolve over the 21st century under different emissions scenarios? (3) How would perceived drought characteristics evolve under theoretical adaptation scenarios? These questions are addressed using the Isba land surface model, downscaled climate projections from the ARPEGE General Circulation Model under three emissions scenarios, as well as results from a previously performed 50-year multilevel and multiscale drought reanalysis over France (Vidal et al., 2010). Spatio-temporal characteristics of meteorological and agricultural drought events are computed using the Standardized Precipitation Index (SPI) and the Standardized Soil Wetness Index (SSWI), respectively, and for time scales of 3 and 12 months. Results first show that the distributions of joint spatio-temporal characteristics of observed events are well reproduced by the downscaled hydroclimate projections over a present-day period. All spatio-temporal characteristics of drought events are then found to dramatically increase over the 21st century under all considered emissions scenarios, with stronger changes for agricultural droughts. Two theoretical adaptation scenarios are eventually built based on hypotheses of adaptation to evolving climate and hydrological normals. The two scenarios differ by the way the transient adaptation is performed for a given date in the future, with reference to the normals over either the previous 30-year window (“retrospective” adaptation) or over a 30-year period centred around the date considered (“prospective” adaptation). These adaptation scenarios are translated into local-scale transient drought thresholds, as opposed to a non-adaptation scenario where the drought threshold remains constant. The perceived spatio-temporal characteristics derived from the theoretical adaptation scenarios show much reduced changes, but they call for more realistic scenarios at both the catchment and national scale in order to accurately assess the combined effect of local-scale adaptation and global-scale mitigation. This study thus proposes a proof of concept for using standardized drought indices for (1) assessing projections of spatio-temporal drought characteristics and (2) building theoretical adaptation scenarios and associated perceived changes in hydrological impact studies (Vidal et al., submitted)

Projections des sécheresses météorologiques, agricoles et hydrologiques en France pour le XXIe siècle

International audienceA 50-year drought reanalysis over France has been performed with the Safran-Isba-Modcou hydrometeorological suite (Vidal et al., 2010). In order to deal with the different types of droughts (meteorological, agricultural and hydrological), three sets of standardized indices have been defined: the commonly used Standardized Precipitation Index (SPI) computed from the Safran 8km atmospheric reanalysis, the Standardized Soil Wetness Index (SSWI) based on soil moisture simulated by the Isba land surface scheme at the same spatial resolution, and the Standardized Flow Index (SFI) based on streamflow computed by the Modcou hydrogeological model at more than 900 locations. These indices enabled to reconstruct past drought events at different time scales and in a consistent way through space as well as through the hydrological cycle. This study presents an assessment of future meteorological, agricultural and hydrological droughts over France. To this aim, an ensemble of climate projections downscaled to the 8km scale have been considered here: (1) a 1961-2100 transient run of version 4.5 of the Arpege General Circulation Model (GCM) under the A2 emissions scenario, downscaled with two statistical methods: a method based on weather types (WT, Boé et al., 2006) and a quantile-quantile method (QQ, Déqué, 2007), (2) 1961-2100 transient runs of version 4.6 of Arpege under the A2, A1B and B2 emissions scenarios, downscaled with the WT method, and (3) projections from 6 CMIP3 GCMs under the A1B scenario for both the 1961-1990 and 2046-2065 time slices, downscaled with the WT method. Isba and Modcou models have been forced by each climate projection, and SPI, SSWI and SFI have been derived by standardizing precipitation, soil moisture and streamflow with respect to the corresponding 1961-1990 GCM control run distributions. 3-month and 12-month indices have been selected here to illustrate the evolution of short-term and long-term droughts. The first result is a dramatic increase in the area affected by drought (index values under a 20% threshold in the baseline period) shown by Arpege transient runs over the 21st century. This increase is much more pronounced for agricultural and hydrological droughts, and is somewhat limited at the end of the century under the B1 scenario. The WT method furthermore suggests stronger short-term agricultural drought changes than the QQ method, mainly due to large changes in the frequency of extreme index values. Besides, a multimodel spatial analysis enabled to identify regions with the largest changes in drought frequency projected for the 2050s: meteorological droughts in the south-west and north of France, agricultural droughts over mountainous regions, hydrological droughts in the groundwater-dominated Seine basin. The above results may inform decisions of long-term water resource planning under the “no-adaptation” assumption. To go further, an on-going analysis is performed to identify potential changes in drought event characteristics (number, duration, magnitude, seasonality) with a non-stationary drought threshold emulating a progressive adaptation to reduced water resources. An assessment of such changes may provide water managers with appropriate information in order to help them build effective adaptation strategies that could also take account of changes in the timing, duration and intensity of drought events

Effet de scénarios d'adaptation et d'atténuation du changement climatique sur les caractéristiques spatio-temporelles des sécheresses

International audienceDrought events develop in both space and time and they are therefore best described through summary joint spatio-temporal characteristics, like mean duration, mean affected area and total magnitude. This study addresses the issue of future projections of such characteristics of drought events over France through three main research questions: (1) Are downscaled climate projections able to reproduce spatio-temporal characteristics of meteorological and agricultural droughts in France over a present-day period? (2) How such characteristics will evolve over the 21st century under different emissions/mitigation scenarios? (3) How would perceived drought characteristics evolve under theoretical adaptation scenarios? These questions are addressed using the Isba land surface model, downscaled climate projections from the ARPEGE General Circulation Model under three emissions scenarios, as well as results from a previously performed 50-year multilevel and multiscale drought reanalysis over France (Vidal et al., 2010). Spatio-temporal characteristics of meteorological and agricultural drought events are computed using the Standardized Precipitation Index (SPI) and the Standardized Soil Wetness Index (SSWI), respectively, and for time scales of 3 and 12 months. Results first show that the distributions of joint spatio-temporal characteristics of observed events are well reproduced by the downscaled hydroclimate projections over a present-day period. All spatio-temporal characteristics of drought events are then found to dramatically increase over the 21st century under all considered emissions scenarios, with stronger changes for agricultural droughts. Two theoretical adaptation scenarios are eventually built based on hypotheses of adaptation to evolving climate and hydrological normals. The two scenarios differ by the way the transient adaptation is performed for a given date in the future, with reference to the normals over either the previous 30-year window (“retrospective” adaptation) or over a 30-year period centred around the date considered (“prospective” adaptation). These adaptation scenarios are translated into local-scale transient drought thresholds, as opposed to a non-adaptation scenario where the drought threshold remains constant. The perceived spatio-temporal characteristics derived from the theoretical adaptation scenarios show much reduced changes, but they call for more realistic scenarios at both the catchment and national scale in order to accurately assess the combined effect of local-scale adaptation and global-scale mitigation.This study thus proposes a proof of concept for using standardized drought indices for (1) assessing projections of spatio-temporal drought characteristics and (2) building theoretical adaptation scenarios and associated perceived changes in hydrological impact studies

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

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

Projet ClimSec. Impact du changement climatique en France sur la sécheresse et l'eau du sol. Rapport final

Le projet CLIMSEC mené par la Direction de la Climatologie de Météo-France avec le soutien de la Fondation MAIF a permis de caractériser la typologie des sécheresses en métropole sur la période 1958-2008 et d'établir un diagnostic sur leurs évolutions attendues au cours du XXIe siècle. L'étude en climat actuel s'est appuyée sur une modélisation rétrospective de l'humidité du sol sur 50 ans avec la chaîne hydrométéorologique Safran-Isba-Modcou (SIM). La réanalyse SIM établie dans le cadre du projet a fait l'objet d'une évaluation particulière pour vérifier son homogénéité temporelle et a permis d'établir une climatologie à haute résolution de l'humidité des sols en métropole. Les différents types de sécheresse (météorologique, agricole, hydrologique) ont été caractérisés à partir d'indicateurs standardisés. Ces indicateurs permettent à la fois de garantir la cohérence spatiale des analyses à différents pas de temps et la détermination des caractéristiques des évènements: début, fin, sévérité, magnitude. Cet outil a été d'abord utilisé à des fins de description comparative des épisodes marquants de sécheresse rencontrés au cours des 50 dernières années (tels que l'été 1976, les années 1989-1990, l'été 2003) en termes de caractéristiques moyennes (durée et surface moyennes, magnitude totale) mais aussi de déficits records sur la période considérée. Une climatologie spatialisée des sécheresses en métropole a ensuite été établie dont la valeur et l'originalité ont été notamment reconnues par l'attribution du prix Norbert Gerbier Mumm de l'OMM en 2010. Elle a mis en évidence les disparités régionales notamment en termes de durée, de sévérité moyenne puis a permis une qualification des événements extrêmes observés sur la période considérée. La deuxième partie de l'étude a été consacrée à l'évaluation des caractéristiques futures des sécheresses sous changement climatique à l'aide des indices standardisés développés dans le projet et en s'appuyant sur les différentes projections climatiques régionalisées disponibles en métropole et issus des travaux de l'AR4 du GIEC. La caractérisation des sécheresses a pris particulièrement en compte les différentes sources d'incertitude, liées aux modèles climatiques globaux, aux scénarios socio-économiques et aux méthodes de régionalisation. La pertinence des jeux de données et des différentes méthodes utilisées, notamment en contexte climatique non stationnaire, a fait l'objet d'une attention particulière. Les incertitudes sur l'évolution des caractéristiques des sécheresses ont été décrites à la fois dans leurs dimensions moyennes mais aussi en terme de variabilité régionale et saisonnière. Au final, les convergences des résultats issus des différentes projections climatiques permettent d'apporter un diagnostic complet sur la compréhension des éléments majeurs de l'aggravation des sécheresses au cours du XXIe siècle et une mise en perspective dans le cadre des démarches pour l'adaptation au changement climatique

Data-Based Comparison of Frequency Analysis Approaches: Methodological Framework and Application to Rainfall / Runoff Data in France

Frequency analysis (FA) is one of the cornerstones of hazard quantification and risk assessment. Its basic objective is to estimate the distribution of some environmental variable X, e.g. annual maximum of the areal rainfall over some catchment, annual maximum flood, etc. This distribution can be used to estimate the exceedance probability of a given value of X (often expressed in terms of return period), or alternatively, to estimate the p-quantile of X, i.e. the value having an exceedance probability equal to 1-p. The estimation of quantiles is of primary importance since they are used to design civil engineering structures (e.g. dams, reservoirs, bridges) or to map hazard-prone areas where restrictions may be enforced (e.g. building restrictions in flood zones). FA has been the subject of extensive research, yielding an abundance of approaches. In practice, FA users and practitioners may feel lost facing such an abundance of methods. Consequently, several initiatives aimed at assisting users in realizing their analyses using best-practice methods. In addition to these end-user-oriented guideline documents, a large number of comparisons between competing methods have been reported in the research literature. The French National research project EXTRAFLO aims to perform a thorough comparison between FA approaches currently used in France, based on an extensive dataset of long series of rainfall and runoff. This poster provides a detailed description of the methodology used to perform the comparison, and presents preliminary results of its application to large rainfall and runoff datasets. More precisely, the following topics are presented: 1. Presentation of the datasets, including more than 1000 series of daily runoff and more than 2000 series of daily rainfall 2. Decomposition of the datasets into calibration/validation sub-samples 3. The issue of scrutinizing uncertainty estimates is discussed, and a method based on the concept of predictive distribution is proposed in order to compare the reliability of competing uncertainty estimates. 4. Reliability indices are derived in order to compare the performances of competing methods on an objective basis. 5. This methodological framework is applied to the datasets and preliminary results are discussed