Accounting for hydro-climatic and water use variability in the assessment of past and future water balance at the basin scale

International audienceThis study assesses water stress by 2050 in river basins facing increasing human and climatic pressures, by comparing the impacts of various combinations of possible future socio-economic and climate trends. A modelling framework integrating human and hydro-climatic dynamics and accounting for interactions between resource and demand at a 10-day time step was developed and applied in two basins of different sizes and with contrasted water uses: the Herault (2500 km2, France) and the Ebro (85 000 km2, Spain) basins. Natural streamflow was evaluated using a conceptual hydrological model (GR4j). A demand-driven reservoir management model was designed to account for streamflow regulations from the main dams. Urban water demand was estimated from time series of population and monthly unit water consumption data. Agricultural water demand was computed from time series of irrigated area, crop and soil data, and climate forcing. Indicators comparing water supply to demand at strategic resource and demand nodes were computed. This framework was successfully calibrated and validated under non-stationary human and hydro-climatic conditions over the last 40 years before being applied under four combinations of climatic and water use scenarios to differentiate the impacts of climate- and human-induced changes on streamflow and water balance. Climate simulations from the CMIP5 exercise were used to generate 18 climate scenarios at the 2050 horizon. A baseline water use scenario for 2050 was designed based on demographic and local socio-economic trends. Results showed that projected water uses are not sustainable under climate change scenarios

Historical reconstruction and 2050 projections of water demand under anthropogenic and climate changes in two contrasted Mediterranean catchments

This paper presents an integrative conceptual framework developed to simulate the spatial and temporal dynamics of water demand caused by human influences and climate change, at the river basin management scale. The past dynamics of urban, agricultural, and industrial components of water demand were simulated at a 10-day time step for the period between 1970 and 2009. The same model was used to forecast water demand at the 2050 horizon under water use scenarios based on local projections, trends observed in the past, and climate scenarios. Climate prediction uncertainties were taken into account using a wide range of climate scenarios downscaled from 9 IPCC-AR5 GCMs under RCP8.5. To test how widely and easily our approach can be applied, we tested it in two river basins facing different kinds of human pressure and different water management issues: the Herault basin (2500 km(2)) in France and the Ebro basin (85,000 km(2)) in Spain. Results showed that water demand has increased significantly in the last 40 years in both the Herault basin (+29%) and the Ebro basin (+57%), and revealed spatially heterogeneous variations in water demand. Identifying the main drivers of water demand and their past dynamics enabled us to build water use trend scenarios at the 2050 horizon. Simulations of water demand under anthropogenic and climate trends in 2050 revealed a significant increase in total water demand in both basins (Herault +38% to +50%, Ebro +35% to +58%). These projections show that changes in the pressure of human activities will influence variations in water demand more than climate change. The broader aim of this research is to assess the balance between water demand and supply through a comprehensive modeling framework to evaluate the sustainability of water uses in a changing environment

Evolution des déséquilibres quantitatifs, adaptation aux changements climatiques et anthropiques au sein de deux hydrosystèmes méditerranéens : l'Hérault et l'Ebre

International audienceLes évolutions climatiques et anthropiques récentes et à venir dans la région méditerranéenne posent la question de l’équilibre entre usages et ressources en eau dans les bassins versants. Cet équilibre dépend de l’évolution conjuguée de l’état quantitatif des ressources en eau et des usages anthropiques. Le travail présenté vise à comprendre les interactions entre usages et ressources en eau au sein d’hydrosystèmes anthropisés, en tenant compte des dynamiques spatio-temporelles de variables physiques et socio-économiques, et à analyser l’influence d’un large spectre de futurs possibles sur les déséquilibres quantitatifs et les stratégies d’adaptation envisagées

Simulating past changes in the balance between water demand and availability and assessing their main drivers at the river basin scale

In this study we present an integrative modeling framework aimed at assessing the balance between water demand and availability and its spatial and temporal variability over long time periods. The model was developed and tested over the period 1971-2009 in the Hérault (2500 km(2), France) and the Ebro (85 000 km(2), Spain) catchments. Natural streamflow was simulated using a conceptual hydrological model. The regulation of river flow was accounted for through a widely applicable demand-driven reservoir management model applied to the largest dam in the H,rault Basin and to 11 major dams in the Ebro Basin. Urban water demand was estimated from population and monthly unit water demand data. Water demand for irrigation was computed from irrigated area, crop and soil data, and climatic forcing. Water shortage was assessed at a 10-day time step by comparing water demand and availability through indicators calculated at strategic resource and demand nodes. The outcome of this study is twofold. First, we were able to correctly simulate variations in influenced streamflow, reservoir levels and water shortage between 1971 and 2009 in both basins, taking into account climatic and anthropogenic pressures and changes in water management strategies over time. Second, we provided information not available through simple data analysis on the influence of withdrawals and consumptive use on streamflow and on the drivers of imbalance between demand and availability. Observed past variations in discharge were explained by separating anthropogenic and climatic pressures in our simulations: 3% (20%) of the decrease in the H,rault (Ebro) discharge were linked to anthropogenic changes. Although key areas of the H,rault Basin were shown to be highly sensitive to hydro-climatic variability, the balance between water demand and availability in the Ebro Basin appears to be more critical, owing to high agricultural pressure on water resources. The modeling framework developed and tested in this study will be used to assess water balance under climatic and socioeconomic prospective scenarios and to investigate the effectiveness of adaptation policies aimed at maintaining the balance between water demand and availability

On the Use of Hydrological Models and Satellite Data to Study the Water Budget of River Basins Affected by Human Activities: Examples from the Garonne Basin of France

Natural and anthropogenic forcing factors and their changes significantly impact water resources in many river basins around the world. Information on such changes can be derived from fine scale in situ and satellite observations, used in combination with hydrological models. The latter need to account for hydrological changes caused by human activities to correctly estimate the actual water resource. In this study, we consider the catchment area of the Garonne river (in France) to investigate the capabilities of space-based observations and up-to-date hydrological modeling in estimating water resources of a river basin modified by human activities and a changing climate. Using the ISBA-MODCOU and SWAT hydrological models, we find that the water resources of the Garonne basin display a negative climate trend since 1960. The snow component of the two models is validated using the moderate-resolution imaging spectroradiometer snow cover extent climatology. Crop sowing dates based on remote sensing studies are also considered in the validation procedure. Use of this dataset improves the simulated evapotranspiration and river discharge amounts when compared to conventional data. Finally, we investigate the benefit of using the MAELIA multi-agent model that accounts for a realistic agricultural and management scenario. Among other results, we find that changes in crop systems have significant impacts on water uptake for agriculture. This work constitutes a basis for the construction of a future modeling framework of the sociological and hydrological system of the Garonne river region