5TH ROSENBERG INTERNATIONAL FORUM BANFF « Managing Upland watersheds in times of Global change » (Sept. 2006, Alberta). Impact of climate change on the management of upland waters : the Rhône river case. BRAVARD J.-P., 41 p.

University Lumière-Lyon 2,  Faculté GHHAT, Département de géographie 5, avenue Pierre Mendès-France, 69676 Bron cédex, France IUF & UMR 5600 EVS
Rhone Watershed Workshop Zone (ZABR)

During the XXth century, the average temperature of the globe increased by 0.6 +/-2°C (IPCC, 2002). The Alps experienced a warming of temperatures comprised between 1° and 2°C. However, more than 1°C out of the strong recent increase, which occurred since 1990 (along with a decrease in precipitations), could be related to positive values of the NAO (Beniston and Jungo 2002).

During the XXIth c., global temperature should increase by 1.4 to 5.8°C (IPCC, 2002). The assessment of climatic change has been traditionally based on general circulation models (GCM) which typically have a resolution of 2.5° latitude and 3.75° longitude. At the basin scale, the GCM projects that the expected climate warming will enhance the hydrological cycle, with higher precipitations in winter, higher rates of evaporation and decreased precipitations in summer and during the fall, and a proportion of liquid to solid relatively greater at high altitude.

In the Swiss Alps, the worst scenario is that winter temperatures could increase by up to 4°C and summer temperatures (July) by 6°C (Beniston et al., 1995). Horton et al. (2005) proposed a scenario of +1°C (expected for 2020-2049) and two scenarios considering two increased green house gas emissions (period 2070-2099: + 2.4 to 2.8 °C and +3.0 to 3.6°C, with rates higher in summer than for annual averages).
In the Swiss Alps, Beniston et al. (2003) have shown that “milder winters are associated with high precipitations levels than cold winters, but with more solid precipitations at elevations exceeding 1,700 – 2,000 m above sea-level, and more liquid precipitations below”. With expected climate warming, the average predicted precipitations would not change, but summer precipitations should decrease, while winter precipitations would increase. Modelling of winter storms suggest a stronger frequency of southern flows from the Mediterranean and heavy storms, like 1999 Lothar storm (Beniston 2004). Also, periods of drought could be more frequent as well as periods of heavy rainfalls. Higher snowfalls at high altitudes would not compensate for increased ice-melting. According to Beniston et al. (1995), winter precipitations would increase by 15% in the Western Alps. The amounts of precipitation are influenced by the Northern Atlantic Oscillation (Beniston 1997).

In France, the ECLAT-2 programme models predicted warming for all the months, but temperature increases were greater from July to September, ranging from 2.5°C to 7.5°C according to the different models tested. A minimum of precipitations in summer months (from -45% to +8%), and increased precipitations in winter, up 5-30% is predicted according to the models. The changes associated with an increase in global temperature are rendered more complex by interactions with the NAO shifts.
Using the ARPEGE-CLIMAT model, an average yearly increase of 2.5°C and an increase in July of 4°C for the doubling of CO2 concentration is predicted.
According to GIEC models applied to France, with the B2 scenario (+ 2-2.5°C in one century), precipitations would increase in the winter, while they would be reduced by 5-25% in the summer. According to the A2 scenario (+ 3-3.5°C in one century), summer droughts would be more severe with a decrease of 20-35% in summer rainfall, associated with severe episodes.

River discharge / regime :
Concerning river discharge, statistical tests applied to 8 gauging stations of the Rhone river downstream Geneva demonstrated that hydrology is stationary. However two types of ruptures are apparent, one locally in 1891, due to artificial developments at the outlet of Lake Geneva, the second one at the end of the 1970’s, with the occurrence of wet decades throughout the basin, following a period (1940-1975) of lull. A new cycle rich in strong floods has occurred in recent years, similar to the late XIXth period, but no effect of global change having been detected yet (Sauquet et Haond, 2003).

In France, a statistical analysis of discharges at 140 gauging stations from 1975 to 1990 show a reduction of snow-melt regimes to the benefit of “transitional” regimes and to a marked irregularity in the seasonality of regimes (Krasovskaia et al., 2002).

Changes in the depth and duration of snow cover :
Beniston (1997) has correlated thick snow cover and long duration in the Swiss Alps with high NAO index because during these episodes, winter temperatures shift toward higher values (« the frequency of temperatures exceeding the freezing point is more than doubled above 1000 m, thus enhancing the potential for early snowmelt »).

The depth of snow cover is influenced by temperature. At Portes Pass (Northern French Alps, alt. 1,320 m), snow depth from February 11th to 20th has decreased during the last 40 years. The strong reduction in the last ten years is “probably related to climate warming” (Etchevers & Martin 2002). Using satellite imagery, Baumgartner & Apfl (1994) observed a reduction of snow cover by 3-4 weeks during the late 80’s and the early 90’s.

Glaciers and permafrost :
As a result of climate change, glaciers have already retreated because they stand close to the freezing point. Haeberli (1994) considers that past and present fluctuations of glaciers and pergelisol are proofs of past and present climate changes through the changes in energy balance. Due to the green house effect, the velocity of observed changes exceeds the changes monitored during the Holocene. Haeberli (1995) and Haeberli & Beniston (1998) have shown that "the glaciers of the European Alps have lost about 30 to 40% of their surface and about half of their volume".

According to Vincent (2002), glaciers of the French Alps retreated during two periods :
-From 1942 to 1953, due to low winter snow falls and to a high rate of retreat in summer
-From 1982 to 1999, due to a high level of summer ablation (from 1.9 m to 2.8 m at 2800 m at the elevation of 2800 m). This is due to a strong increase of the energy balance. The difference in mass balance between 1800-1850 and 1970-1980 is comprised between 0.50 and 1.00 m in water equivalent for the glaciers of the French Alps.

Changes in the depth and duration of snow cover :
An average increase of 4°C in temperatures, forecasted by several regional models for this area of Europe, would reduce the volume of snow by ca 50% in the Swiss Alps. For every °C increase in temperature, the snow line will rise by about 150 m so  that “regions where snowfall is the current norm will increasingly experience precipitation in the form of rain. » (Beniston, 1997).

According to the scenario of Météo-France (Martin & Durand, 1998), assuming an increase in temperature of +1.8 C°, at an elevation of 1,500 m, the average length of snow cover, presently comprised between 160 and 180 days in the Northern French Alps, could decrease down to 125-135 days. In the Southern Alps, it could decrease from 130-100 down to 80-55 days/yr. This means one month less of snow cover that today (SAFRAN-CROCUS snow model, in French ARPEGE GCM - Equipe Climate Modelling and Global Change). According to the GICC-Rhône study, the depth may be reduced by 50% at low altitudes, but is less affected at higher altitudes (1800-2000 m). In the different scenarios, the areas covered by snow decrease by 25-40% (Etchevers & Marti., 2002).

According to Abegg and Froesch (1994), an increase of temperature of 2-3°C by the year 2050 would adversely affect low altitude (below 1,200-1,500 m). Warmer winters will bring less snow at these altitudes, and snow will melt faster.

Glaciers and permafrost :
30-50% of existing mountain glacier mass could disappear by 2100 if global warming scenarios in the range of 2-4°C indeed occur. With an upward shift of 200-300 m in the altitude of the line of equilibrium , the reduction in ice thickness could reach 1-2 m per year (Maisch, 1992). The sensitivity of the line of equilibrium to temperature is between 60 and 120 m/°C according to different authors (Green et al., 1999; Maish, 2000; Vincent, 2002).

Six et al. (2002) proposed that the mass balance of alpine glaciers could be negatively correlated to the oscillations of NAO index, as Beniston et al. (1995) proposed for periods of warm temperature and low precipitations.

Vegetation, soils and water balance in mountain ecosystems :
At the basin scale, the GICC study predicts that the pattern and the spatial extension of natural vegetation would not change significantly, so hydrology would not be affected by this parameter. However, on the long term, vegetation will colonize the upper slopes of the Alps. In the Southern regions, the decrease of water content in soils and vegetation will increase the stress on vegetation, may induce a higher sensitivity to fires during the driest periods of the year, and increase exposition to soil erosion (IPCC, 2001).

River / torrent discharge :
A detailed study has been performed on the potential impacts of climate change on the runoff regimes of 11 small catchments having glacier surfaces comprised between 0 and 50%, at altitudes ranging between 1340 and 2940 m, under different hydrological regimes (Horton et al., 2005 ; Schaeffli, 2005). Predictions were developed for a scenario of +1°C (expected for 2020-2049) and two scenarios considering two increased green house gas emissions (period 2070-2099: + 2.4 to 2.8 °C and +3.0 to 3.6°C, with rates higher in summer than for the average). The conclusion are the following for the +1°C scenario :
- A decrease of annual precipitations
- An increase of winter precipitations, with the risk of higher flood peaks
- A decrease of summer precipitations
- A strong decrease of ice-covered areas, due to the strong increase of summer temperatures. The regimes will be mainly driven by snow-melt during the Late XXIth c.
- A decrease in the amplitudes of discharge 
- A significant decrease of annual discharge (5-15% for the +1°C scenario) due to the reduction of precipitation, the increase of evapo-transpiration, the long term decrease of glacier surface and discharge.

Horton et al. (2005) predicted “a significant decrease of the total annual discharge and a shift in the monthly maximum discharge to earlier periods of the year due to the temperature increase and the resulting impacts on the snow melt processes”. At lower altitudes, “the influence of precipitations is more pronounced and the variability of the predicted climate change impact is mainly due to the large range of predicted regional precipitation change”.

The coupled ISBA-MODCOU model was used in three sub-watersheds and on the entire Rhone basin for a selected warm year, then tested for the prediction of change (Noilhan et al., 2000; Etchevers et al., 2001; Etchevers & Martin, 2002; Leblois, 2002; Leblois & Gresillon, 2005) :
- In the Doubs basin, the snow-rain regime shifts to rain regime with an increase of discharge in December and January, and a decrease in spring, without a significant change of the total yearly discharge.
- In the Saône basin (Mâcon), the rain regime remains the same, but discharges decrease in summer.
- In the Isère basin (Northern Alps), the maximum shifts from April to March, the winter maximum increases, and the summer minimum decreases by 50%.
- In the Southern Alps, during contemporary dry years, the Durance basin experiences a “precocious and excessively rapid snow melt… resulting in an early peak and correspondingly very weak summertime flows”. The simulated change forecasts “an annual reduction of river discharge and of the soil moisture, decreasing by as much as  30% below the present values”.
- However, if “the reduction of snowfall and earlier snow melting (increased air temperature) induced a decrease of the average snow depth by 50% and of the snow duration by more than one month”, snow pack at high altitude is less affected because even with the air warming, the average air temperature would remain below 0°C.
- The Ardèche river basin experienced a “significant reduction in summer flow” and a strong reduction of the soil water content, … “reflecting the heavy reduction of precipitation in that area”

The GICC-Rhone programme extended these conclusions drawn from sub-waterheds to the larger area of the French part of the Rhone basin (Leblois et al., 2005):
- Average yearly discharge and low flows decrease (from May to November), but high discharges increase. Low flows may be reduced by 40-50% close to the outlet of the Rhone.
- Spring flow related to snow-melt decreases since the warming of the climate reduces snow depth and the duration of snow cover, and snow melt occurs one month earlier. 
- The behaviour of rivers in the winter depends on the different scenarios, but generally the increase of winter rainfall induces an increase of winter discharges.

Vegetation, soils and water balance in mountain ecosystems :
Changes in direct water consumption by existing vegetation will occur. They will be due to changes in forest cover and to changes in the amount of evapotranspiration. If an increase in water consumption can be predicted, then a decrease of river flow is logical.

Regimes of mountain rivers :
The specific annual discharge of mountain rivers is higher than the specific discharge of extended watersheds including lowland areas. This results from higher precipitations, low evaporation rates, and by conditions favouring runoff. “The hydrological regime is strongly influenced by water accumulation in the form of snow and ice and the corresponding melting processes resulting in a pronounced annual cycle of the discharge. A modification of the prevalent climate and especially of the temperature can therefore considerably affect the hydrological regime and induce important impacts on the water management” (Horton et al., 2005). The recent increase in temperatures has probably already had consequences on river regimes.

In Switzerland, “shifts in snow-pack duration and amount will be crucial factors in water availability » for runoff according to Beniston et al. (2003). The increase in winter temperatures will have clear consequences on the beginning of snowmelt and on the reduction of flow during the spring at low altitudes and on summer flow at the highest altitudes. The rarefaction of snow cover below 1000 m will reduce runoff. These shifts will affect river regimes with higher winter discharges. However, increased evaporation in winter may partly reduce runoff and river discharge.

Climate warming will increase the average discharge of rivers flowing from glaciers at first during the period of retreat, but then will decrease summer discharge, as rivers will progressively lose their glacial-type hydrological regime.

With the warming of climate, “minimal and maximal discharges will be observed more frequently than in present times during other periods of the year than it is presently expected”. In others words, prediction will be more difficult and the authors recommend the adoption of a probabilistic approach (Krasovskaia et al., 2002). However specialists consider that discharge regimes have not changed enough to justify any change in the policy of dam management (D. Duband, oral comm.).

Floods :
The statistical study of river discharges in France did not detect any significant change in the number and the intensity of floods since the mid-XXth c. Also, it is impossible to confirm any change in low discharges, mostly because of heavy human impacts on rivers (Lubès-Niel & Giraud, 2003; Lang et al., 2005).

In the last 15 years, severe floods occurred in the Upper Rhone downstream Geneva (1990 was the 1 on 100 years flood), and in the lower Rhone (for instance: 1993, 1994, 2003). As stated above (Sauquet & Haon, 2003), they may be just a cycle of high discharges as many occurred in the past. Also, they may be the first signals of changed climate towards higher peak floods. Anyhow, they revealed the strong vulnerability of the Rhone valley to flooding.

Forest fires :
For instance, the 2003 summer drought provoked several fires in the Vercors, a wet massif of the Northern Prealps, which had not experienced any fire during the last decades. 

Floods :
The major apparent risk is linked to increased flood hazards. If winter floods occurring on rivers in Switzerland have negative influences on discharges in downstream countries, then these countries may ask for improved retention in the Swiss lakes and reservoirs, along with political consequences (Schädler, 2003).

Interactions between sediment supply and floods :
Considering winter peak flows, they should interact with changes in sediment fluxes and, locally, with the hydraulic geometry of rivers, increasing waterborne risks. The increased elevation of pergelisol due to increased temperatures will decrease the cohesiveness of soils, and trigger mass movements (Haeberli et al., 1990).
Extreme rainfalls and increased average winter temperatures, increased alternations of freezing and warming in weak rocks, will increase landslides and rockfall hazards. However, recent catastrophic events in the Mattertal (Valais region) in 1987, 1993, and 2000, and above-average concentration of events have been proved to be caused by insufficient and short archival data (Stoffel et al, 2005). These changes in slope processes will increase sediment inputs into rivers, will induce deposition and will raise the level of floods, interacting with land occupation issues along valley floors. This trend could affect northern regions of the basin, as predicted by Beniston et al. (1995).

In 1995, the French government launched a large study called “Global Rhône study”, combining hydraulics, sediment transport and land occupation, as these different topics having been recognized as complementing each other. The 2003 flood, approximately the 1 in 100 years flood for the downstream gauging stations, motivated the French government to launch the so-called “Rhône Masterplan” (2005) which includes a series of measures to mitigate the human consequences of flooding, the reduction of hydrological hazards being recognized as quite impracticable. The expected risk explicitly refers to the largest past floods (1856), to extremal scenarios combining several meteorological origins (the so-called “general flood” in the sense of Pardé, 1925), and to the negative impacts of the occupation of the floodplain. It is thus worth noting that the possible effects of climate change on the intensity of large flood is not taken into account, despite the possible increase in extreme winter events. Also, to face the expected changes, the French Ministry of Environment and Sustainable Development recommended to extend the number of the “Plans de Prévention des Risques” and to improve forecasting procedures (Redaud et al., 2002).