Climate and hydrology model simulations, 2054-2064 evolution (2CO2), French Rhone basin, surface budgets, snow cover

Météo France/CNRM/CEN/ ENSMP/CIG, pierre.etchevers@meteo.fr, joel.noilhan@meteo.fr

Air temperature, long wave radiation, precipitation

Watersheds

French Rhone river basin

Jura, French Alps

900-3600 m

Within the framework of the Global Energy and Water Cycle Experiment (GEWEX) / Rhone project, a system has been built to estimate the hydrological budget of the Rhone. The methodology is based on 3 models, one for each component of the hydrometeorological system: the atmospheric parameter analysis and the snow cover; the surface water and energy budget; the underground transfer and discharge estimation.

The results of the ARPEGE-Climat general circulation model (GCM) have been used to estimate the climate of the Rhone catchment in 60 years (2054-2064). The perturbation of the air temperature and precipitation has been taken into account but vegetation and soil structure are supposed to be identical to current values. The river discharge and soil water resources under the climatic scenario have been compared with the results of the actual simulation. The results under the assumption of the CO2 concentration doubling have been compared with the values under the actual climate; analysis mainly focuses on the monthly discharge of the basin big rivers and on the water surface budget evolution. The uncertainties in the results have been estimated by analysing the model sensitivity to different simple climatic scenarios. In particular, the analysis brings into light the impact of the downscaling of the GCM results.

The ratio between future and actual mean annual discharges presents a strong northward gradient, identical to the precipitation anomaly. In the North, the discharge increase lies between 10 and 50%. In the central part of the basin and over the Alps, the ratio is close to 1. In the south, the streamflow diminishes by 20 to 40%.

In the northern part, the discharges are stronger in winter (because the liquid precipitation increases) and spring. They reach values comparable to actual ones at the end of spring and during summer and are moderately lower during autumn. In the southern part, the low altitude rivers are influenced by a general reduction of the discharge during the whole year, and especially during autumn and spring.

Precipitation, evaporation and runoff increase (respectively of 13, 10 and 16%), but this average evolution is very contrasted between north and south, and depending from seasons. Runoff increases in the north (+50 to +150 kg m-2 per year) and decreases in the south (-50 to -150 kg m-2 per year).

Evaporation anomaly is more homogeneous and less strong (-50 to +50 kg m-2 per year). The higher increase in evapotranspiration occurs in the Alpine catchments, where it reaches 20% (reduction of snow cover duration due to higher air temperature and reduction of snowfall ranging between -18 and -36%).

The snowpack is the most sensitive hydrological component to the atmospheric warming.

The snowpack is strongly reduced in the southern part of the Alps, the maximum snow water equivalent (SWE) and snow cover duration diminish by 50%. In the northern Alps, the SWE reduction is weaker (-20%). The snowpack anomalies strongly depend on the elevation and are significant below 1000 m. The mean snow depth diminishes by 25 cm under 2000 m (-31%) but only by 16 cm at 3000 m (-11%). The contrast is particularly strong between 1500 and 2400 m. The reduction of the snow cover lifetime is 50 days (-26%) under 2000 m versus 40 days (-3%) at 3000 m.

For the Durance at La Clapière (2150 m), the snowpack appears a month later and disappears a month earlier. The SWE reaches a maximum value reduced by 30% and begins to decrease in March instead of April. The winter discharges are not modified, but the melt flash floods differ completely, occurring a month earlier and their amplitude diminishes by 30%. The summer discharges are lower (stronger evaporation). The same evolution affects the Buech river (in the southern Alps) and the Isere, but for the last, the amplitude of flash floods is unchanged (increase of spring rainfall).

(4) - Remarques générales

Strong contrasts in the hydrological response of the catchment are noticeable, depending on the location of subcatchments and the amount of precipitation. In the northern part, the relative increase of the discharge (about +40%) is bigger than the relative augmentation of the precipitation (about +25%) because of the relatively weak increase of evaporation (+13%).in the southern part, the river discharge decreases by 10 to 30%.

Snow cover is the most sensitive hydrological component to the air temperature increase (especially at low and medium elevations), and the snowy river regimes are strongly modified. The spring flash floods occur a month earlier and the annual discharge generally diminishes as the evaporation increases. Considering the soil water content, it appears that the north part of the domain stays quite wet, whereas drying is enhanced in the south.

Note that strong uncertainties affect the results.