Combining climate, land use and water use scenarios to project possible futures of a meso-scale Mediterranean catchment

Hydrological extremes (floods and low flows) are an important societal issue in Mediterranean areas. Dry summers limit the available water resource, while flash floods cause material and human damage. Both extremes are expected to intensify under future climate. There are strong links between changes in the water cycle and changes in human activity in both directions: on the one hand, limitations of water resources constrain societies to adapt their activities and, on the other hand, changes in human activity can further change the pressure on water resources.

It is therefore essential to anticipate how territories should adapt and transform their land uses in response to increasing pressures on water resources. We uniquely combine land use and land cover (LULC) scenarios co-constructed with local stakeholders, hourly climate projections and a spatially distributed hydrological model in order to evaluate the impact of management choices on hydrological variables and the societal implications of these changes.

An interdisciplinary prospective study at 2050 horizon was conducted in the Mediterranean Claduègne catchment (43 km²; Ardèche, France), characterized by rural mixed land use (extensive agriculture, forests, small towns and tourism). This study was based on surveys with 16 key local stakeholders (farmers, elected officials, planners, residents, etc.). Semi-structured interviews and focus groups were used to identify past and current drivers of change, and to explore plausible futures. Three contrasting scenarios were established. These LULC scenarios contain modeled future land cover maps, as well as descriptions of changes in agricultural practices and water management strategies.

The scenarios were fed into a hydrological model along with two contrasting climatic projections, in order to establish six potential future pathways for the middle of the century. We used the spatially distributed process-based hydrological model J2000P at hourly resolution. Besides natural hydrology, the model represents human activity, such as irrigation, livestock farming, and drinking and waste water fluxes, as well as specific water resource management strategies like hillslope reservoirs and external water importations.

We assessed the territorial impact of the three LULC scenarios under climate change, on low flows and contributions of the main hydrological components (overland flow and groundwater). We discuss the capacity of each scenario to sustain diversified agricultural activities with or without external water input to the catchment, and the ability to supply the catchment with drinking water, particularly during critical periods.

The climatic conditions determined the future change of high flow extremes and groundwater contributions, while both climate and LULC significantly impacted low flow extremes, overland flow contribution and the streamflow during the driest period of the year. The capability to sustain agricultural activity in the future depends on additional solutions such as hillslope reservoirs. The extent of this dependency varies across LULC scenarios and climate projections. Additional water importations may be needed some summers in order to sustain drinking water supply for the population. However, these additional solutions rely on management choices that raise economic issues, dependency on infrastructure, and questions of social acceptability.