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Water Stress Assessment By 2050 Under Human And Climate Changes

Congress: 2015
Author(s): Julie Fabre, Denis Ruelland, Alain Dezetter, Benjamin Grouillet
CNRS - HydroSciences Montpellier1, IRD - HydroSciences Montpellier2

Keyword(s): Sub-theme 17: Climate change, impacts and adaptation,
AbstractIntroduction
In recent decades, climatic and human pressures on water resources have increased in the Mediterranean region, and the area could experience higher water stress over the 21st century. In this context integrated modeling approaches are needed to build water allocation scenarios and assist water resources planning. This study aims to assess water stress by 2050 under climatic and socio-economic scenarios in river basins facing increasing climatic and human pressures, and to compare the impacts of a wide range of possible future climates and socio-economic trends. Such information will be extremely useful to policy makers when designing water plans for coming decades.

Methods
A modeling framework integrating hydro-climatic and human dynamics and accounting for interactions between resource and demand at a 10-day time step was developed and applied in two basins of different scales and with contrasted water uses: the Herault (2 500 km2, France) and the Ebro (85 000 km2, Spain) basins. Natural streamflow was evaluated using a conceptual hydrological model. A demand-driven reservoir management model was developed and applied to the largest dam in the Herault basin and to 11 major dams in the Ebro basin. Urban water demand was estimated from population and monthly unit water consumption data. Agricultural water demand was computed from irrigated area, crop and soil data, and climate forcing. The percentage of water demand unsatisfied by lack of water availability was computed for each type of demand at a 10-day time step.

Four indicators comparing water supply to demand at strategic resource and demand nodes were computed. The frequency of shortage (F) was expressed through the frequency of years including at least one time-step with limitations on irrigation water withdrawals exceeding 50% of demand. Maximum shortage (MS) was defined by the maximum simulated annual irrigation water shortage rate and reliability (Rel) was considered to be the rate of occurrence of satisfactory years, i.e. years with an annual irrigation water shortage below 50%. The frequency of occurrence of water sharing conflicts (C) was indicated. For each indicator, an acceptable range of values was defined according to management criteria of the local basin agencies.

This framework was applied under four combinations of climatic and water use scenarios to differentiate the impacts of climate- and human-induced changes on water supply capacity (see Table 1). 18 climate scenarios by 2050 were built based on 9 GCMs under RCPs 4.5 and 8.5. A baseline water use scenario for 2050 was built based on demographic and socio-economic trends.

Table 1. Four combinations of climatic and water use scenarios tested in the Herault and the Ebro basins.

Results and discussion
Temperature projections show a clear increasing trend, particularly marked in the summer. Projections for precipitation are more uncertain and differ among the 18 scenarios considered. However, a decrease in spring and summer precipitation could occur in both basins. These climatic trends could result in changes in natural discharge: while scenarios diverge in fall, winter and spring, all 18 scenarios result in a decrease in summer low flows. Tables 2 and 3 show the hydroclimatic and water demand changes by 2050 in the Herault basin as an example of these results.

Table 2. Hydroclimatic changes by 2050 in the Herault basin under 18 climate change scenarios.
Table 3. Agricultural Water Demand (AWD) and Urban Water Demand (UWD) changes by 2050 in the Herault basin under 18 climate change scenarios and a water use trend scenario.

Three sub-basins of the Herault appeared to be vulnerable to climate and/or anthropogenic change: the Salagou dam, the Herault at Gignac, and the Herault at Agde (see Table 4). The area supplied by the Salagou dam is vulnerable to climate change and sensitive to its uncertainties: while the water use scenario has no impact on water demand satisfaction, the wide range of potential climate variations induces an uncertainty in the filling of the dam and consequently on the reliability of water supply. In the Gignac area, planned efforts in demand management will bring the system into balance under current climate variability. Under future climate scenarios, these efforts could offset most impacts of climate change. In the downstream area of Agde the water use trend scenario remains in balance with water availability under current climate variability. However under climate change scenarios (which do not affect the balance between water availability and current water uses), the capacity of water supply to meet demand becomes sensitive to climatic uncertainties.

In the Ebro basin (not illustrated here), current water uses could be affected by climate change in two of the largest irrigation systems, in the right bank semi-arid sub-basins and in the most downstream areas. Only two irrigation systems are significantly affected by the water use trend scenario with no climate change. However the water use trend scenario could induce a higher sensitivity of the water balance to climatic uncertainties.

Table 4.Changes in the balance between water availability and demand by 2050 in the Herault basin under 18 climate change scenarios and a water use trend scenario.

Conclusion
This study points out the areas most vulnerable to climate- and/or human-induced changes by 2050 in two Mediterranean basins. In at least one area in each basin, projected water uses are not sustainable under climate change scenarios. Although water uses in the Herault basin are currently mostly in balance with water availability, they do not appear less sensitive to human- and climate-induced changes than in the Ebro. Adaptation measures will be necessary to achieve a sustainable balance between water use and availability over the long term. 1. Dezetter, A., Fabre, J., Ruelland, D. and Servat E. (2014). Selecting an optimal climatic dataset for integrated modeling of the Ebro hydrosystem. In: Hydrology in a changing world: environmental and human dimensions (Proc. 7th FRIEND Int. Conf., Montpellier, France, 7–10 Oct. 2014). IAHS Publ., 363, 355360.
2. Fabre, J., Collet, L., Milano, M., Ruelland, D., Dezetter, A., Ardoin-Bardin, S. and Servat, E. (2014). Assessing the long-term evolution of water supply capacity: comparison of two Mediterranean catchments. In: Hydrology in a changing world: environmental and human dimensions (Proc. 7th FRIEND Int. Conf., Montpellier, France, 7–10 Oct. 2014). IAHS Publ., 363, 203208.
3. Fabre, J., Ruelland, D., Dezetter, A. and Grouillet, B. (subm.). Simulating long-term past changes in the balance between water demand and availability and assessing their main drivers at the river basin management scale. submitted in Hydrol Earth Syst Sc.
4. Grouillet, B., Fabre, J., Ruelland, D., Collet, L. and Boyer, J.-F. (2014). Assessing past and future water demands under climate change and anthropogenic pressures on two Mediterranean basins. In: Hydrology in a changing world: environmental and human dimensions (Proc. 7th FRIEND Int. Conf., Montpellier, France, 7–10 Oct. 2014). IAHS Publ., 363, 185190.
5. Grouillet, B., Fabre, J., Ruelland, D. and Dezetter, A (subm.). Historical reconstruction and 2050 projections of water demand under anthropogenic and climate changes in two contrasted Mediterranean catchments. submitted in Appl Geogr.

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