Comparaison des réponses du bilan hydrique de bassins situés en Belgique et en Suisse à un changement de climat

Les impacts possibles d'un changement de climat induit par l'augmentation de la concentration des gaz à effet de serre sur le bilan hydrique ont été simulés sur un ensemble de bassins hydrographiques situés en Belgique et en Suisse. Le modèle hydrologique conceptuel IRMB à pas de temps journalier a été utilisé à cette fin et les paramètres du modèle ont été optimisés sur chaque bassin. Les bassins ont une taille comprise entre 100 km2 et 1200 km2 et couvrent des régions de plaine aussi bien que de moyenne montagne. Un même scénario climatique a été adopté pour tous les bassins. Il est principalement caractérisé par une augmentation de la température de près de 3·C et par une légère augmentation des précipitations annuelles. Cette méthodologie a été adoptée afin de montrer les sensibilités respectives des différents termes du bilan hydrique et de les mettre en rapport avec les caractéristiques des bassins étudiés. L'étude s'est focalisée sur l'évolution de l'évapotranspiration et de l'humidité du sol, de l'enneigement, des débits à l'exutoire et des réserves en eau souterraine. Les impacts sont également abordés en termes d'événements extrêmes. Outre des évolutions qui sont prévisibles pour l'ensemble des bassins étudiés, telles une augmentation de l'évapotranspiration, une diminution légère de l'humidité du sol et une réduction de l'enneigement, les réponses de certains termes du bilan hydrique régis par les caractéristiques du sous-sol des bassins peuvent être sensiblement différentes d'une région à une autre. Ainsi, les bassins caractérisés par une infiltration importante subiraient une évolution favorable de leurs réserves en eau souterraine et des débits de base, alors que les bassins où le ruissellement de surface prédomine verraient une diminution se produire. L'altitude des bassins semble aussi jouer un rôle non négligeable. Tous les bassins de plaine présenteraient une augmentation des débits de crues extrêmes, alors que les bassins de moyenne montagne ne subiraient pour ainsi dire pas ces conséquences négatives.By strengthening the so called greenhouse effect, the rise of the atmospheric concentrations of anthropogenic gases, such as CO2, chlorofluorocarbons (CFCs) and methane, will progressively modify the energy budget of the Earth atmosphere and disturb the climate. Temperature at the soil level will rise. Precipitation and air humidity will be modified, inducing a large perturbation of the water cycle and thus of water availability and hydrological extremes. The last International Panel on Climate Change report (IPCC 1994) states that a rise of 0.3 degrees Celsius per decade could be felt in the mean global temperature in the next century. The conclusions of the Second World Climate Conference (1992) pointed out that among the most likely impacts of climate change will be its effects on the hydrological cycle and water management systems. An increase of the incidence of extreme events, such as floods and drought, would cause increased frequency and severity of disasters.The present paper is a synthesis of several separate published and some unpublished climate change impact studies (Bultot et al., 1988 a and b, 1992, 1994; Gellens, 1991; Schädler et al., 1992; Gellens and Demarée, 1993; Gellens and Roulin, 1996) carried out in Belgium and in Switzerland. All these studies have been done with the same hydrological model and the same climate change scenario. This procedure allows a strict comparison of the sensitivity studies and enables us to identify the common responses and the specific behaviour of the catchments. In this latter case, an attempt to identify the geomorphological origin of the particular responses is possible. A set of eight catchments in Belgium and belonging to the Scheldt and Meuse river basins has been studied. These individual catchments cover areas from 100 km2 to 1200 km2 and are spread over the country in order to represent the main catchment types. Precipitation ranges from 730 to 1160 mm per year. In addition, three tributaries of the Rhine river in Switzerland have been selected in the low alpine altitude (lower than 1500 m a.s.l.). Precipitation rates are higher for these three catchments and range from 1080 to 1300 mm. The adopted IRMB (Integrated Runoff Model - Bultot) hydrological model (Bultot et al., 1976 and 1985) is a daily time step conceptual model. It has been designed at the Royal Meteorological Institute of Belgium to simulate the components of the water cycle in medium- sized catchments, i.e. catchments for which the input data, and in particular the precipitation, can be considered as uniform. The main data needed to run the IRMB model are the precipitation and the potential evapotranspiration. This latter variable is assessed by following the procedure described by Bultot et al. (1983) and requires several climatological data, i.e. the net radiation, the air temperature and humidity, the soil temperature at 10, 20 and 50 cm depth and the wind speed at 2 m above the soil. These data are also taken into account in the snow melting- accumulation simulation.The adopted climate scenario has been constructed from the literature in order to combine the results of various simulations produced by different climate models (Bultot et al., 1988b) in a single set of climate increments. The main characteristics of the climate change scenario is a temperature rise reaching some 3 ¡C, with a higher increase in winter than in summer. Precipitation stress consists of a 50 mm yearly rise distributed as a winter rise (about 10 mm) and a slight summer decrease. Although these scenario increments are small in comparison with the year-to-year variability, they are however large enough to reveal the sensitivity of the water balance to climate perturbations. The detailed description of the algorithms used to apply the monthly increments on the daily time step are presented in Bultot et al. (1988b). After a calibration phase for the present climate conditions, a sensitivity analysis of the water balance of the catchments has been carried out by modifying the input data according to the scenario. This well known if - then - what? method gives the sensitivity of the various terms of the water cycle by comparing their values in the present runs and in the disturbed 2xCO2 runs. For practical reasons, the reference periods of the simulation runs are not the same for all the catchments.The study focuses on the evolution of evapotranspiration and soil moisture, of snow cover, of streamflow at the outlet and of groundwater storage. The impacts are also studied in terms of extreme events.For all the catchments, the analysis shows a rise of the evapotranspiration equivalent to some 7 to 10 percent. A small decrease in soil moisture has also been simulated associated with an increase in dry soil days. Due to the temperature rise a strong depletion of the snow cover might be an economically dominant effect in the low alpine regions where winter sport activities represent a large part of the inhabitants' resources. According to the winter precipitation rise, the monthly streamflows in the cold period are also increased under the 2xCO2 conditions.Besides predictable trends common to all the catchments, the study also shows that some components of the water balance governed by the underground characteristics can present uneven responses. Catchments characterized by strong infiltration could be subject to positive evolution of the groundwater storage and of the baseflow, whereas catchments with predominant surface runoff could exhibit the reverse effect. These effects could be important mainly in summer during the low flow period. The altitude of the catchments also seems to be significant. All the lowland catchments present higher extreme streamflows, whereas catchments in low Alpine regions are spared this negative consequence. While a large degree of uncertainty remains in the assessment of the climate in the next century, this study gives a first insight into the direction of the expected climate change impacts. It also points out the need to analyse the sensitivity of catchments with a particular attention to their characteristics

Extreme summers impact cropland and grassland soil microbiomes

The increasing frequency of extreme weather events highlights the need to understand how soil microbiomes respond to such disturbances. Here, metagenomics was used to investigate the effects of future climate scenarios (+0.6 °C warming and altered precipitation) on soil microbiomes during the summers of 2014-2019. Unexpectedly, Central Europe experienced extreme heatwaves and droughts during 2018-2019, causing significant impacts on the structure, assembly, and function of soil microbiomes. Specifically, the relative abundance of Actinobacteria (bacteria), Eurotiales (fungi), and Vilmaviridae (viruses) was significantly increased in both cropland and grassland. The contribution of homogeneous selection to bacterial community assembly increased significantly from 40.0% in normal summers to 51.9% in extreme summers. Moreover, genes associated with microbial antioxidant (Ni-SOD), cell wall biosynthesis (glmSMU, murABCDEF), heat shock proteins (GroES/GroEL, Hsp40), and sporulation (spoIID, spoVK) were identified as potential contributors to drought-enriched taxa, and their expressions were confirmed by metatranscriptomics in 2022. The impact of extreme summers was further evident in the taxonomic profiles of 721 recovered metagenome-assembled genomes (MAGs). Annotation of contigs and MAGs suggested that Actinobacteria may have a competitive advantage in extreme summers due to the biosynthesis of geosmin and 2-methylisoborneol. Future climate scenarios caused a similar pattern of changes in microbial communities as extreme summers, but to a much lesser extent. Soil microbiomes in grassland showed greater resilience to climate change than those in cropland. Overall, this study provides a comprehensive framework for understanding the response of soil microbiomes to extreme summers

Robust estimates of climate-induced hydrological change in a temperate mountainous region

A sustainable water resources management depends on sound information about the impacts of climate change. This information is, however, not easily derived because natural runoff variability interferes with the climate change signal. This study presents a procedure that leads to robust estimates of magnitude and Time Of Emergence (TOE) of climate-induced hydrological change that also account for the natural variability contained in the time series. Firstly, natural variability of 189 mesoscale catchments in Switzerland is sampled for 10 ENSEMBLES scenarios for the control (1984-2005) and two scenario periods (near future: 2025-2046, far future: 2074-2095) applying a bootstrap procedure. Then, the sampling distributions of mean monthly runoff are tested for significant differences with the Wilcoxon-Mann-Whitney test and for effect size with Cliff's delta d. Finally, the TOE of a climate change induced hydrological change is determined when at least eight out of the ten hydrological projections significantly differ from natural variability. The results show that the TOE occurs in the near future period except for high-elevated catchments in late summer. The significant hydrological projections in the near future correspond, however, to only minor runoff changes. In the far future, hydrological change is statistically significant and runoff changes are substantial. Temperature change is the most important factor determining hydrological change in this mountainous region. Therefore, hydrological change depends strongly on a catchment's mean elevation. Considering that the hydrological changes are predicted to be robust in the near future highlights the importance of accounting for these changes in water resources planning

MontanAqua : Wasserbewirtschaftung in Zeiten von Knappheit und globalem Wandel. Wasserbewirtschaftungsoptionen für die Region Crans-Montana-Sierre im Wallis

Das nationale Forschungsprogramm NFP 61 «Nachhaltige Wassernutzung » des Schweizerischen Nationalfonds hat sich zum Ziel gesetzt, wissenschaftliche Grundlagen zur nachhaltigen Wasserbewirtschaftung in der Schweiz zu liefern. Als Teil dieses Forschungsvorhabens wurde im Rahmen des Projektes MontanAqua die Wasserbewirtschaftung der Region Crans-Montana-Sierre (Wallis) untersucht. Es ging dabei darum, in enger Zusammenarbeit mit den in der Region betroffenen Akteuren nachhaltige Wassernutzungsstrategien für die Zukunft zu entwickeln. MontanAqua hat sich vertieft mit den bestehenden Systemen der Wasserbewirtschaftung auf der regionalen Skala (11 Gemeinden) auseinandergesetzt. Dazu wurden die zukünftigen Auswirkungen der klimatischen und sozioökonomischen Veränderungen einbezogen. Das Forschungsteam analysierte die aktuelle Situation anhand von quantitativen, qualitativen sowie kartografischen Methoden und kombinierte diese mit Modellberechnungen. Für die Modellierung der Zukunft wurden regionale Klimaszenarien und vier mit lokalen Akteuren entwickelte sozioökonomische Szenarien verwendet. Dieser Überblick fasst die Resultate des Projektes MontanAqua zusammen. Fünf wesentliche Fragen werden beantwortet und fünf Kernbotschaften erläutert. Zudem sind Empfehlungen für die Verantwortlichen der regionalen und kantonalen Wasserbewirtschaftung formuliert

Climate change and mountain water resources: overview and recommendations for research, management and policy

Mountains are essential sources of freshwater for our world, but their role in global water resources could well be significantly altered by climate change. How well do we understand these potential changes today, and what are implications for water resources management, climate change adaptation, and evolving water policy? To answer above questions, we have examined 11 case study regions with the goal of providing a global overview, identifying research gaps and formulating recommendations for research, management and policy. <br><br> After setting the scene regarding water stress, water management capacity and scientific capacity in our case study regions, we examine the state of knowledge in water resources from a highland-lowland viewpoint, focusing on mountain areas on the one hand and the adjacent lowland areas on the other hand. Based on this review, research priorities are identified, including precipitation, snow water equivalent, soil parameters, evapotranspiration and sublimation, groundwater as well as enhanced warming and feedback mechanisms. In addition, the importance of environmental monitoring at high altitudes is highlighted. We then make recommendations how advancements in the management of mountain water resources under climate change could be achieved in the fields of research, water resources management and policy as well as through better interaction between these fields. <br><br> We conclude that effective management of mountain water resources urgently requires more detailed regional studies and more reliable scenario projections, and that research on mountain water resources must become more integrative by linking relevant disciplines. In addition, the knowledge exchange between managers and researchers must be improved and oriented towards long-term continuous interaction