Climate Change Incidence on the Rhone River Hydrology

TECHNICAL SYNTHESIS

CLIMATE CHANGE INCIDENCE ON THE RHONE RIVER HYDROLOGY

LLANES Alexandre

February 2008

AgroParisTech - Engref à Montpellier B.P.7355_34086 MONTPELLIER Cedex 4 Tél. (33) 4 67 04 71 00 Fax. (33) 4 67 04 71 01 1 RESUME Le changement climatique est aujourd’hui considéré comme une réalité par les scientifiques. Toutefois, les liens de causalité entre changement climatique et modifications des régimes hydrologiques sont difficiles à établir. A cette difficulté perceptible s’ajoute la complexité du fonctionnement du régime hydrologique et climatique du Rhône. Dans ce contexte d’incertitude la société BRL, gestionnaire et distributrice de l’eau du Rhône en Languedoc Roussillon, s’interroge sur la disponibilité et la vulnérabilité de la ressource en eau pour les années à venir.

MOTS CLES : Changement climatique, hydrologie, Rhône, étiages, BRL, vulnérabilité de la ressource.

ABSTRACT Climatic change is widely considered as a reality by scientists. Nevertheless, impacts of hydrological extremes are more difficult to observe and to predict. It is shown how the possible change in the characteristics of precipitations, associated TO climatic change, could alter the river flows in the basin of the Rhone River particularly for summer and the lowest water levels. In this uncertain context, the BRL Company, distributor of Rhone water in Languedoc-Roussillon, investigates the future availability and vulnerability of the resource.

KEY WORDS : Climate change, hydrology, Rhone, lowest water level, availability, growth samples, BRL, vulnerability of the resource.

ABREVIATION IPCC : Intergovernmental Panel on Climate Change MCG : Modèles de Circulation Générale BRL : Bas-Rhône Languedoc (société) MEDD : Ministère de l’Ecologie et du Développement Durable GICC : Gestion des Impacts du Changement Climatique CNRM : Centre National de Recherches Météorologiques CNR : Compagnie Nationale du Rhône SEPALRC : Société d’Etudes et de Promotion pour l’Aqueduc Languedoc-Roussillon- Catalogne ENSO : El Niño et Southern Oscillation MINAN : des débits journaliers minimums annuel VCN : valeur minimale annuelle du débit moyen sur 7 jours ou sur 30 QCN : valeur minimale annuelle du débit seuil sur 7jours ou sur 30 jours ETP : Evapotranspiration Potentielle QMNA5 : débit mensuel minimal dans l’année de période de retour de 5 ans DOE : Débit Objectif d’Etiage DCR : Débits de CRise

2 INTRODUCTION ...... 4

1 GENERAL FRAMEWORKS ...... 5

1.1 GLOBAL CLIMATE CHANGE...... 5

1.2 GENERAL CIRCULATION MODELS...... 5

1.3 CLIMATE CHANGE AND LOW WATER LEVELS ...... 6

2 THE RHONE RIVER ...... 7

2.1 A mode modified during the 20th century ...... 8

2.2 Low water levels of the Rhone river ...... 8

3 IMPACT STUDIES OF THE CLIMATE CHANGE ON THE RHONE RIVER ...... 9

3.1 Retrospective approaches ...... 10

3.2. Prospective approaches ...... 13

4 CONCLUSION ...... 16

3 Introduction

The evolution of the climate related to the human activity is today an established fact (IPCC, 2001). The obvious increase in the concentration of gases and greenhouse effect in the atmosphere has had effects on a certain number of climatic variables, involving for example the increase in the number of hot days and a reduction in the number of very cold days. Precipitation will increase to, at least in the Northern hemisphere, while the snow-covered peaks and the extent of the non-polar glaciers will be in marked regression. Beyond these direct observations, the general circulation models (GCM) show a persistence of these tendencies during the century. Uncertainties which weigh on these projections are very important because the models are conditioned by socio-economic scenarios imagined for the years to come. Moreover, the results can significantly vary from one GCM to another and a certain number of phenomena, acting on intermediate scales, is taken into account in a way very simplified in the computer codes (of which the space resolution is rather coarse) and remains sometimes largely ignored. Thus, in horizon 2100, the increase in the total temperature should range between 1.4 and 5.8 °C. These climatic disturbances should have effects on the hydrological system. However, these consequences are difficult to evaluate, for two main reasons: first of all, a total warming can have antagonistic effects on the flows, by increasing at the same time the evapotranspiration and precipitations. The influence of these two factors also differs according to the regime, nival or rain, of the river. Moreover, the space and temporal scales usable at exit of the MCG do not agree with those of the hydrological model rain/flow. To date, no coherent signal on large scales was detected for the flows, in spite of changes proven at the regional scales (Renard, 2006). Parallel to the context of the total change, the vulnerability link to the extreme hydrological events is in constant increasing, in particular in France. The water resource is requested more and more for the needs for agriculture and industry. These last years (severe low water levels of 2003 and 2005) led to the perception of a recrudescence of the extreme events in France, which is to date has not been established. The water availability is for the managers (producer of electricity, water supply, etc) a dominating character as for the evolution of the plans of stock management. In this context of uncertainty, the BRL company, saddle jib crane of the water of the Rhone river in Languedoc-Roussillon, wonders about the availability and the vulnerability of the water resource for the years to come. This component appears all the more important as a large scale project aiming at extending their distribution network UPTO Béziers and Narbonne was launched in 2005 (figure 1). That implies an additional abstraction at peak Source : Sébastien Chazot, BRL times from 2 to 4 m³/s (BRL, Collectif, 2006). Thus, for the Rhone river subjected to the modifications of weather forcEs, which this share of abstractions represents in hydrologicaimpact l? The absence of a clear answer on behalf of the scientists as for the reality of a climate change affecting the extreme events is partLy due to the methodological difficulty to detect tendencies in this type of hydro- climatic series which, by nature, are subjected to an important natural variability. It is however paramount to be Figure 1 able to answer this interrogation, because as regards prevention of the risks of shortage, the stationnarity of the phenomena in time is an assumption which

4 is almost systematic. 1 GENERAL FRAMEWORKS

1.1 GLOBAL CLIMATE CHANGE The general operation of the climate of the Earth is based on the concept of balance of the energy assessment: the received total radiation of planet must equalize the emitted total radiation. The distribution of outgoing energy is obviously uniform, neither temporally nor spatially. This imbalance is the cause of the general atmospheric and oceanic circulation. The force of Coriolis, related to the rotation of the Earth on itself, complicates this general outline. There exist thus preferential circulations of energies on the surface of the sphere (dominant winds, oceanic currents). In any general information, the climate is obviously not stationary: it has evolved and has moved since the birth of the Earth and will continue to evolve and move, but one of the major difficulties in the climate study is the existence of variability which have temporal scales (annual, decennial, etc), and regional heterogeneous. This low frequency variability is particularly problematic in the optics of search for evolution. As example, work has put forward oscillations on the strong rains in Spain and the south of France, by using historical data (Llasat et al., 2002), but these oscillatory behaviors are only observed and their constitutive mechanisms remain very largely ignored.

1.2 GENERAL CIRCULATION MODELS The man influences for this complex system was the increase in the gases that caused the greenhouse effect which, in short, limits the heat loss of the terrestrial sphere by trapping the infra-red remission from the ground surface. The increase in these gases does not result formally in warming. Reality is much more complex. The various components of the climate cannot be dissociated from to each other and the modification of one of them will be reflected on the others by feedback phenomenon. It is thus necessary to consider the climatic system as a whole. It is the role of the general circulation models (GCM), which have as a function to recreate a realistic climatic evolution under the influence of various forces (natural and/or anthropic). A GCM can be regarded as a digital representation of the Earth and processes which control its climate. For this purpose the planet is discretized in the form of a space division. These broad total cubes (100 km) or more finely defined regionals (30 km) are characterized by a whole of variables which describe the various components of the climate: pressure, temperature, precipitation, moisture, etc. Starting from initial condition data, the evolution of the climatic conditions under the effect of various forces can be simulated. One of the major difficulties of the MCG is related to this space discretization: indeed of many phenomena have a space scale very largely lower than the cutting suggested by the GCM. To try to describe the operation of the climate and even more its possible evolution under the effect of various forces is an extremely complex task, being given the great number of phenomena and scales implied. All the factors are obviously not controlled, so that the use of GCM is accompanied by several sources by uncertainty: - The measurement of the climatic phenomena themselves; - The structure even of the MCG: difference of resolution, parameter setting, algorithms used, etc; - Data of forcings, more particularly those related to the socio-economic scenarios for the future; - Lastly, the estimate of the impact of the climate change on a variable which is not directly predicted by the GCM: the flows.

5 1.3 CLIMATE CHANGE AND LOW WATER LEVELS Many impact studies of a modification of the climate on the hydrological mode were carried out during these last decades. Arnell (1999) described the hydrology of the future on the scale of Europe on meshs of 0,5° by 0,5° by couplin g various possible future climates and a simplified hydrological model. The results obtained agree on a notable reduction of the surface water resource in the term 2050 on the continental zones on average latitude and to the moderate climate: the low water levels are more critical because the increase in winter precipitations is opposed by an increase in temperatures (and thus of the evapotranspiration) and a reduction in the pluviometric office pluralities in summer period. The GEWEX-Rhone experiment and its prolongation within the framework of the program “Management of the Impacts of the Climate change” of MEDD (GICC) highlight a contrast seizing between the north of the basin of the Rhone which sees its annual flows amplified while in the south, the annual throughputs are reduced until - 40% on the basin of Ardeche (Leblois and Al, 2004). These conclusions must however be relativized taking into account the low capacity of the models to reconstitute the low water levels of the past. On the Adour Garonne basin, the Agency of Water in collaboration with the CNRM engaged a prospective reflexion on the low water levels under climate change (Caballero and Al, 2004). A reduction of 11% of the minimum flows is announced at horizon 2050, consequence of a reduction in precipitations and an increase marked in the temperatures of the air in summer.

These few examples of impact studies of the climate change on the low water levels presented, although characteristic, do not give an account of the diversity of the publications, methodologies, the results and the controversies specific to this subject. It is impossible besides to review so much when the whole of this work is fertile. Figure 2 illustrates this matter well. This multiplicity of data, in turn complementary or contradictory, makes the synthetisation of the results difficult. However, of the whole of these impact studies, some generic elements can be released (Sauquet et al., 2007) : Figure 2 : Evolution of the number of - All the basin slopes do not have the international publications on the set of same sensitivity to the disturbances of themes “Climate change”. Source: Eric climatic forces. Indeed, “the initial state” Sauquet. of the basin (current mode, low water level of summer or winter), the geology and the occupation of the ground are elements which come to moderate the results observed with large scales. - Uncertainties around the predictions are such as a scenario multiplicities must be planned to sit the detected tendencies and to give an account of uncertainties (Prudhomme and Al, 2003). Indeed, the scenarios of Intergovernmental Panel one Climate Change (IPCC), integrated by an expansion of modelings into large scales agree all on a rise of the average temperature on the sphere, but the range of the possible values especially remains broad at horizon 2100. - Distributed modeling, intellectually satisfactory within a framework of comprehension of the basin operation, inevitably does not provide the best results in reconstitution of the low water levels. Consequently, the future trends on this aspect could not be predicted with accuracy. The alternative is given by a simpler conceptual modeling which aims mainly a reconstitution of the mode of the catchment area without seeking to clarify in detail the internal processes. This modeling always remains of topicality in many impact studies to the detriment of heavier physical models. A contrario , the

6 conceptual models will not be able to follow the possible self-adaptations of the catchment area, not easily appreciable in addition. 2 The Rhone river

The Rhone, by the extent of its catchment area (98 000 km² including 88 000 2 km on the French territory) is subjected to a hydrological mode complexes in connection with the various climates and crossed reliefs (Figure 3) :

1. The Swiss Rhone Figure 3 is an Alpine torrent supplied with water from the close mountains. The mode is marked by low waters of winter and rising of springs and summer due to the snow melt and glaciers.

2. Of its exit of the lake Léman to Lyon the Rhone keeps similar hydrological characteristics thanks to the contribution of the affluents coming from Préalpes and the Jura. Thus the nival regime continues to play a crucial role in the food of the river. The rising of springs are marked, but do not make disappear the influence from the regime of the glaciers on the course of the higher Rhone.

3. In Lyon, the arrival of the Saone river modifies the mode of the Rhone river. Traversing areas of plains subjected to the oceanic rains, the Saone has an opposite river regime to the Rhone at the time of its Lyon crossing: high waters of autumn and winter, low water levels of summer.

4. Downstream from Lyon, thanks to the Saone, the Rhone becomes a powerful and more regular river. However its hydrological mode is modified at the time of the crossing of the Rhone-native furrow with the contribution tributary in nival mode coming again from the Alps.

5. The Cevennes and Mediterranean affluents have an impact only at the time of brief intense episodes of rising generally in autumn. For the Bas-Rhone river, the severe risks of low water levels are centered between August and January with one more critical period between September and October, but the water masses provided by the glaciers of the higher course prevent this tendency to lead to a true shortage.

7 2.1 A mode modified during the 20th century The low water levels which the Rhone can experience are seldom noted. Almost always, the weakness of the contributions of basin part is compensated by relative abundance due to another sector of the basin. The Rhone is indeed subjected to a hydrological mode complex in connection with the various climates and reliefs of the areas coming from its source. Its mode nevertheless underwent some modifications during the 20th century from the construction of the great hydroelectric works and reserves on the mainly alpine Rhone and its affluents (figure 4). The alpine tributary works almost all on the same principle, with the same objective of hydroelectric production: they store water resulting from the snow melt in spring and in summer then release it in winter, at the time of the strongest consumption. For installations of CNR to the current, storage capacities are quasi non-existent. The works disturb the mode of the Petit Rhone.

Figure 4 Profil en long du Rhône

The period of construction began after the Second World War and was to be completed in in the 1970s. The service outputs of the tanks increased. Gradually, the mode of the Rhone changed to reach an operation stabilized in the 1980s. We thus perceive the differences between the “natural” Rhone observed between 1877-1916 and the Rhone controlled by the man of the period 1920-2000. There is no significant change of the volumes forwarded on an annual scale, but a seasonal variation of the flows.

2.2 Low water levels of the Rhone river The last century knew several remarkable years by their low hydraulicity: 1921, 1949, 1976, 1989, 2003, and 2005. Most exceptional is however 1921 by the duration of the low flows of the persistent river throughout the year (Duband, Schoeneich, Stanescu, 2004): the flow threshold of 1000 m³/s. A exceeded in 1921 less than 60 days at the station of Beaucaire, whereas for one normal year, this average threshold is crossed during 275 days. A aqueduct project of between the Rhone and Catalonia was studied at the end of the nineties and for this reason of the studies were carried out on the mode of the minimum flows of the Rhone to Beaucaire, 10 km upstream of the hydrant of BRL (BRL, SEPA LRC, 1999).

8 in Beaucaire The Rhone drains a basin of 95590 km2, of which 85243 km2 are located in France (15% of the French territory). From 1971 and 1997, with on average 54 billion m 3 annually run out, the Rhone presents thus nearly one the third of the metropolitan territory sea contributions. The modes of current low water levels of the Rhone river are well encircled thanks to an exploitation of quality of the hydrometric station of Beaucaire, which can be regarded as presenting the best possible precision in hydrometry of river, that is to say a knowledge with better than 5-10% close to the low water levels. The hydroelectric influences are inventoried, traced exhaustively since 1949 on the basin; methodological difficulties do not allow to desinfluence the hydroelectric use of the flows of the Rhone river in Beaucaire with a step of time lower than the month. Broadly the hydroelectric influence is of annual a net taking away of 95 m3/s (derivation of the Durance towards the pond of Berre). It is a carryforward of the flows from April at August in winter and spring. With the autumn, the influence is neutral in average year, always positive during the marked low water levels (support of low water level from + 50 to + 200 m3/s on all the low water levels of 1971 to 1997). The low water level of Le Rhône à Beaucaire : débits minimums enregistrés de 1970 à 2005 the Rhone is Figure1000 5 Source : Sébastien Chazot, BRL established from 900 August to January, 800 October and 700 September being most 600 critical, more 500 m3/s particularly October for 400 the situations of 300 prolonged dryness 200 (see figure 5). The 100 VC30 (lower medium 0 flow during 30 1/1 1/2 1/3 1/4 1/5 1/6 1/7 1/8 1/9 consecutive days) 1/10 1/11 1/12 (return period: 50 years) is established respectively with 390 m 3/s for the year, 490 m 3/s over August- September; the VC5 (T= 50 years) is 320 m3/s for the two situations.

With the sight of these values, the 3 m3/s taking away point under consideration for the extension of network BRL until Narbonne (BRL, Collective, 2006), represent approximately of 1% of the values characteristic of severe low water levels of the Rhone; this is to be added to the current taking away of 15 m3/s, that is to say on the whole, 6% of the values of severe low water levels. 3 Impact studies of the climate change on the Rhone river

The modification of the Rhone hydrological mode induced by a climatic drift is not easily perceptible: the dubious results (sometimes contradictory), the many variables of studies, ambiguities of the exploited models, attest difficulty of this research. Thus, various methodologies of approaches are in the middle of the current thinking since it is of their improvement that the accuracy depends on the future predictions. The methods of investigation propose to explore the recent past to detect possible modifications of the hydrological behavior of the basins ascribable slopes to the increase in gases with greenhouse effect and the future while being based on climatic scenarios to characterize the response of the Rhone river to the modifications of climate forcings.

9 The first point includes/understands analyses of stationariness carried out on the samples of descriptive variables of the low water levels, it is a retrospective approach. The second point falls under the traditional impact studies. This is carried out using total hydrological rain-flow models with a chock optimized on the criteria of resource of the catchment area of the Rhone river. The model rain-flow is a tool which makes it possible to simulate the flows in a point given of a river starting from the knowledge of liquid or solid precipitations on the catchment area corresponding and of the potential return to the atmosphere via the phenomena of evaporation. It is a very simplified representation of the operation of the catchment area and the intrinsic complexity of the hydrological processes. These models are then forced by various representations of the future climate placed at the free disposal of the scientific community. These forces are examined to measure the future sensitivity of the characteristics of low water levels. It is a prospective approach.

3.1 Retrospective approaches These approaches aim at exploring the recent past to detect unstationarities in the flows of the drynesses. Indeed, like any climatic size, the hydrological variables know an intrinsic temporal variability. This variability is usually synthesized by a statistical distribution adjusted on the last achievements (hydrometric data). A climate change would cause a revision of the law established or the use of another. Except major disturbance, the new range of possible values would be for a great part recovering with the old one: the frequent values become a little more or a little less probable. The hydrologist statistician very often seeks a proof of an evolution in negligible variations.

Detection of non-stationarity in low flows A complete sequence of more than 40 years is necessary to integrate a representative sample of the alternation of the dry and wet years (Sauquet & Haond, 2004). Such series are unfortunately rare. The longest chronicles have very often related to basin slopes having undergone human actions for several decades. With the natural signal is grafted a anthropic disturbance which is advisable to identify and to filter before concluding on the origin from a possible drift.

Principle of the test There exist many tests of search for nonstationariness in the hydrological modes, this one was carried out by the research team of the Lyon CEMAGREF associated with CNR (Sauquet et al., 2005). The statistical approach brings a decision-making aid concerning an assumption. The tests rest all on a variable of decision or statistical T which one knows the distribution under the assumption HO. One can then define the terminals of an interval of the values which the statistics can take if H0 is true and to locate the value T at it taken for the analyzed sample. If T is apart from the interval, the value is considered to be not very plausible, therefore H0 is false. The calculations carried out on the samples will result in retaining the assumption of stationnarity HO or its H1 alternative knowing that only one is true (Sauquet et al., 2005). See the table below (source conference E. Sauquet Engref 2007) :

10 Reality H0 H1 Accepted H0 whereas H0 is true Accepted H0 whereas H1 is true

Accepted H1 whereas H0 is true Accepted H1 whereas H1 is true Results of the test the of Results

To defer the results of the test on a diagram (figure 6), one must choose the variables such as : - Z relating to the descriptive variables of the hydrological mode extracted to the long observation series - Evolution in time. One tests the H0 assumption which represents a stationary series.

Z Z t OU The slope t Rupture

time time Figure 6 The difference T between average from and (source : conférence E. Sauquet Engref 2007) of other of a date in supposed rupture

In these two cases, the goal is to test if T ≠ 0, significantly, i.e. apart from a fixed interval of tolerance.

International results In Canada, Zhang et al. (2001) insist on the existence, the sign and the origin of the drift of the monthly medium flows: the rise of the temperatures of winter end involves earlier snowy coat, from where flows increased March and April and reduced in May and June. For the United States, there exists a consensus on the low water levels which from now on would be marked (Douglas et al , 2000). The detected tendencies could be allotted to the persistence of the same phase of the Atlantic Northern Oscillation during the last years. In Oceania, the difficulty comes from the marked influence of the phenomenon ENSO, that prints a strong interannual variability on the flows and masks the possible drifts. The low water levels less marked during the three last decades observed in the Island of the South of New Zealand would be primarily due to a greater frequency of the phase El Niño over this period (Mac Kerchar and Henderson, 2003). In Europe, Hisdal et al. (2001) are interested in the low water levels, but do not detect anything really tangible, because of the anthropic disturbances, of the short observation periods and natural variability. A total conclusion is obviously not allowed taking into account the diversity of the examined variables, the operated pretreatments, the periods considered and the tests applied.

11 Rhone river flows The French part of the Rhone was the subject of several investigations of the Lyon CEMAGREF and CNR concerning the stationnarity of the flows (Sauquet & Haond, 2004). The stations selected for this examination are famous being slightly influenced, at least for high waters, nothing is not sure concerning the low water levels. The stations of the high Rhone present a similar behavior: a reduction of the daily outputs minima annual (MINAN) and an increase in the annual minimal value of the medium flow (VCN, over 7 days or 30 days) and of the annual minimal value of the flow threshold (QCN, on 7jours or 30 days). It is necessary to notice the coherence of the dates of ruptures located essentially in the years 1970. Downstream, it is the station established in Perrache (Lyon), which is characterized by the number of stationnarities. The stations of the Bas-Rhone (Valence and Beaucaire) seem to post a stationary mode. Of these analyses, the absence of detection of rupture at the known dates marking the installation of the great works and derivations is a surprise. These disturbances are added or neutralized those possibly generated by an evolution of the climate. In the North of the Alps, in winter, the flows are low by retention of water in the form of snow. Nonstationarities concentrate in this sector with a significant evolution towards less severe low water levels in winter. Is it necessary to see there the consequence of an increase in the temperatures of the air? Nothing is still acquired, it should as a preliminary be made sure that the flows indeed natural or are influenced very little. For the southern part of the prospected zone, anomalies could be identified for certain stations and on certain variables, but they are not sufficiently many and coherent to conclude (Sauquet et al. 2005).

In short, it comes out from this work an absence of real sign of disturbance on the French part of the Rhone. Tests applied to several variables characterizing various aspects of the hydrological mode made it possible to confirm international results. Anomalies could be identified for certain variable stations and unquestionable, but they are not sufficiently many and coherent with supposed dates of beginning of the climate change to charge them to the intensification of the greenhouse effect. Moreover, the evolution that they print registers in a natural variability of the hydrological mode. Lastly, they can be explained by human actions.

Scandinavian Approaches applied to the Rhone river Other research, aiming at apprehending the signature of the climate change in the Scandinavian hydrological modes, was undertaken on the seasonal variation of the flows subjected to climatic forcing (Krasovskaia, Gottschalk, 2002). According to atmospheric forcing, a hydrological mode can change and present a rather different seasonal diagram, but the intensity of the awaited disturbance depends on the sensitivity of the hydrological mode concerned. Under the conditions prevailing in Scandinavia, the fluctuations of the average annual temperature, even modest (±1°C), are reflected at the same time in the type of mode and its hydrological stability in approximately 20% of the studied cases (Krasovskaia, Gottschalk, 2002). The modes of flow can be considered under the angle of their intrinsic dimensions and fractales, i.e. seen like as many characteristics and components subjacent with a given hydrological mode (size, geological nature and slope of the catchment area, precipitation, etc). These dimensions give a convenient characterization of the stability of each hydrological mode: the most stable types for example (presenting little interannual variability), are those having the lowest dimensions. These Scandinavian approaches were applied to the Rhone (Krasovskaia, Gottschalk and Leblois, 2002). This methodological study aims at detecting an echo of the climatic fluctuations in the flow seasonal variations, through a sample of 140 chronicles of flow to steps of monthly times of 1975 to 1990 (thus 2240 year-stations of twelve values each one). For this study, only the oceanic and nival rain modes hydrological, whose seasonal behavior is opposed by the high ones and low waters, were considered.

12 Without extending on methodological complexity from the space and statistical processing from various dimensions from the Rhone hydrological mode, it should be retained that the comparison of the results for the various periods observed watch a clear evolution of the mode towards a greater irregularity in the seasonal variation of the flows.

The modes of transition, in very large proportion already, increased towards the end of the year 1980, with depend on the nival modes. The increase in the modes of transition (pluvio-nival mode and mode Mediterranean characterized by autumnal secondary high waters) means concretely that maximum and the minimum annual ones of the flows could be observed more frequently than maintaining at other periods of the year than those awaited. As the flows of transition are of greater dimension (unstable) that rain or nival modes pure, there could be a difficulty increased with the forecast of the modes of future flows. In short, we will witness a gradual reduction in the nival modes to the profit of the rain modes. These results are coherent with those obtained for Scandinavia (Krasovskaia, Gottschalk and Leblois, 2002).

3.2. Prospective approaches

GICC Rhône Project The project of Management of the Climate change Impacts (GICC) gathers several scientific partners, organizations and industrialists: BRGM, CEMAGREF, CNRS, EDF, Weather- France. The objective is to give a lighting on the impacts to be envisaged on the French part of the Rhone river basin, because of climate change resulting from a doubling from CO2 in the atmosphere (probable expiry 2050). The project was declined in two work phases: - Climatic scenarios and their hydrological consequences - impacts relative to various physical, biological and socio-economic fields. The first phase of project GICC the Rhone river began in 1999 (Leblois et al., 2004). For this study, six configurations resulting from 4 MCG were considered (LMD of IPSL, ARPEGGIO of Weather-France, HC of Hadley Center and UR of the University of Reading). These configurations correspond to simulations with CO2 doubling under assumption of 1% of annual growth. This climatic impact study is based on the achievements of the co-operative project the Gewex-Rhone that led to the installation of an unequalled database on the basin and to the development of a hydro-meteorological modeling of increasing complexity over one long period of time (1981-1996) (Etchevers et al., 2001). This base constitutes the reference of the climate present to evaluate the disturbances of the various compartments of the hydrosystem under assumption of the climate to modify. The 6 configurations are divided into 4 configurations “low resolution” (2 or 3 points MCG only on the basin) and 2 configurations “high-resolution (between 10 and 28 points on the basin). Generally, the MCG have a good capacity to simulate the monthly cycle of the temperature of the air, but have difficulties in represent monthly average precipitations with monthly variations being able to be important. From the results of these MCG, the coupling of a model calculating evapotranspiration (ISBA) and of a model rain flow (MODCOU), makes it possible to clarify a certain number of impacts of the climate change on the cycle of water.

Hydrological impact on the basin of the Rhone starting from model ISBA-MODCOU Model ISBA-MODCOU is distinguished from hydrological models simpler by a description rather fine of the processes of surface, in particular related to the resolution of the diurnal cycle of the energy assessments and hydrous of surface. This model highlights the effects of the climate change on the assessments of surface and each effect starts various processes closely connected :

13 1. 1. The increase in the winter rains stronger than the increase in the ETP increases the water contents of the ground, therefore streaming of surface and drainage. In mountainous area, the simultaneous increase in the temperature of the air is accompanied by a reduction in snow-covered precipitations and consequently by a reduction of winter accumulations. 2. The reduction in estival precipitations increases the hydrous deficit of the ground which is reinforced by the increase in the ETP (increase in the temperature of the air), which involves a desaturation of the grounds much earlier and more severe low water levels in autumn. 3. The increase in the temperature of the air involves an increase in real evaporation during the winter and spring because the grounds are well fed out of water. On the other hand, real evaporation tends to be reduced in summer and autumn by effect of hydrous stress. The significant heat flow towards the atmosphere is then reinforced. 4. Snow-covered surfaces decrease on average by 25% to 40% following the configurations. The snowy coat for high altitudes is a little less affected because the average temperature is largely lower than the melting point there. The anomalies of the snowy coat are likely to very strongly impact the mode of the alpine rivers. The early nival cast iron involves a peak of raw in May, in approximately one month advance. It is remarkable to note that this shift is present in all the scenarios. It follows low water levels much more marked from there in July and August.

Results The regional variations of the hydrological impacts can be strong within the basin of the Rhone. The impact on the flows is very variable between North (the Saone) and the South of the basin (Ardeche, Durance): increase in the annual throughput of the Saone of approximately 10%, stronger reduction on the basins of Ardeche and the Durance (reaching 15% for this last). These contrasted regional answers are compensated on the level of the discharge system of the Rhone to the Fish ponds which presents a weak reduction of the annual volume of water (approximately 2%). The real evapotranspiration is strongly increased on the zones of relief because of the reduction of snow-covered surfaces. Evaporation increases also slightly in North and is reduced much more strongly (5%) in the South under the effect of the hydrous stress of the ground. The water contents are decreased overall on the whole of the field (because of the increase in evaporation), manifestly on the basins of the South accentuating their current vulnerability thus (Leblois et al., 2004).

To conclude on the results from project GICC, the impact of the climate change is adjusted seasonally. The flows on the Rhone river basin are likely to decrease by May to November. The surfaces covered with snow during the winter decrease on average by 25 to 40% following the scenario. The cast iron of the snowy coat occurs earlier and snow-covered precipitations decrease. The strong spring flows are generally reduced and they appear earlier (1 front month). The winter flows increases appreciably (more winter rains), whereas the summer flows are reduced by 50%. These general tendencies are reproduced by all the scenario, to differing degree, but no precise prediction of quantity could be advanced. The methodological uncertainties and limits are still too important for the tangible assertion of quantity of water run out by 2050.

Extrapolation with other basins This model is currently places from there on the basins Adour-Garonne and the Seine, on which hydrological simulations of the current period are satisfactory with the decadal steps of time. These two basins have hydrological modes rather different from those of the Rhone which can lead to more marked vulnerabilities vis-a-vis the climate change. A study on the basin Adour-Garonne, watch which the hydrological sensitivity to the climate change can be in particular increased for the modes of low water levels (Caballero and Al, 2004). Indeed,

14 the nival component would be more affected than on the basin of the Rhone because of the lower altitude of the Pyrenees combined with a more moderate increase in winter precipitations. For the small basins, the problem is more difficult still because of the difference in space-time scales between scenarios MCG and these small basins (a few km2) in time of prompt response (a few hours). For the basins of the Mediterranean coastline, one can imagine that the climate change will be accompanied by winter and autumnal risings stronger (the increase in the intensity of the rains is all the more appreciable as the size of the basin is low) and of periods of dryness more marked even (Redaud et al., 2002).

Limits of the study Like any scientific approach, the GICC-Rhone study highlighted some limits of its own methodology which are as many future ways of research. Method employed lack of relevance for the extremes, this being less the fact of the limitation of adopted hydrological modelings, that initial choice of the technique of adaptation of scales of the atmospheric scenarios which appear privileged the average flows. Indeed : - They are the fluctuations of the monthly median values which are deferred on the chronicles of the atmospheric variables to the steps of time tri-schedules or days laborer. - the MCG are recognized like imperfect, but their sensitivity to the CO2 doubling is regarded as Juste. These two effects combine to make debatable the fluctuations of the variables modelled, in particular of the hydrological variables and their extremes (Leblois et al., 2004).

Risque-Décision-Territoire 2003 Project The purpose of this approach does not call into question the results of project GICC, but is to refine the conclusions of the impact studies on the low water levels of the Rhone and to open a parallel way with different tools and data to appreciate uncertainties while following a specific objective; a diagnosis on the threats which weigh on the surface water resource (Sauquet et al., 2005). These studies were undertaken on basins slopes feeding the French part of the Rhone-native basin. - It is a standard step of impact study to the regional scales (Casing et al., 1994): - Choice and chock of a model rain-flow on several affluents of the Rhone river under the last conditions. - Extraction of climatic variables of the future years and selection of the relevant data for the Rhone sector and its affluents. It is the description of the climate resulting from the MCG. - Construction of future time serieses starting from the monthly anomalies delivered by the climatic models. - Application of the model rain-flow selected supplied with the series representative of the possible future climates; - Examination of the modifications generated on the hydrological mode by comparison with the observations.

Numerical models For these studies, the basin is regarded as a homogeneous entity. Four versions of models rain-flow were tested and fixed on phenomena observed: GR4J, HBV, TOPMODEL and IHACRES. Four instrumented basins of the French part of the Rhone little influenced by snow were studied: Eyrieux, Drome, Roubion and Azergues. The models are fed by chronicles of rain of basin, obtained by simple arithmetic mean of the actual values at the points of measurement. There is no universal model which is appropriate for all the basins, thus each basin has a preferential model. These models rain-flow are then forced by the MCG relative to the various climatic scenarios, i.e. by disturbed series (Sauquet et al., 2005).

15 Impacts on the mode of low waters One of the assumptions is that the transformation rain-flow will be modified little by 2080 (absence of adaptation of the area catchment in terms of occupation of the ground, vegetation and evolution of the taking away of water). The future characteristic flows were extracted: Q90 (flow exceeded 90% of time) and QMNA5 (minimal monthly flow in the year of period of 5 years return). Indicators of brittleness of the low water levels and critical point of the system were worked out for this research (Sauquet et al., 2007). These indicators all are with the fall whatever the scenario, this fall is sometimes drastic. A more operational manner to exploit these simulations is to evaluate the perenniality of the system by applying current rules of management to the future context (Sauquet et al., 2007). The crisis situations (failures of the system) correspond to sequences of flows lower than specified values for which a priorisation of the taking away is necessary. Within the framework of a plan of management of the low water levels, these thresholds could be defined by the DOE (Objective Flows of Low water level) or the DCR (Flows of Crisis). To establish a diagnosis, one can be based on the concepts of reliability and impact strength which respectively qualify the probability of observing the system in a satisfying state and the capacity of the system to return in a satisfactory state (Loucks, 1997). These indicators must be used for comparing various policies of management of water and appreciating the durable character of the system in time (Peters et al , 2005). Within the framework of this study, these indicators were included for the basin of the Rhone. All in all, Sauquet et al. (2007) note very little difference between the studied basins. Thus, management will be problematic and this in spite of the conservation of the same flow of objective low water level. The crisis situations could be observed more than 10% of time. The durations of the episodes would be lengthened (an anticipation of the crises would be thus more necessary). Finally, the performances would be reduced with stronger deficits.

Here still, only of the tendencies can be released. Whatever the climatic scenario, a tendency to an increased severity of the low water levels seems to take shape. The potentialities of refill supported by more abundant, beneficial winter precipitations for the river in low water level, are neutralized by a rise of the estival temperatures. Finally the assessment is negative for the river in term of resource. However, the ranges of reduction are very diverse according to the basin considered. 4 Conclusion

What is necessary to conclude from it? Such as they are, the various climatic scenarios cannot answer the impact studies alone. The hydrologists must ensure a day before on these scenarios, but the current results are still, and for a long time, too dispersed to establish solids conclusions in quantitative terms for the future. Moreover, there does not exist any coherence with large scales. For the catchment area of the Rhone, no conclusion relating to the quantities of water available in the future, nor even concerning the general response of the basin to the climate change, can be advanced. Various approaches were developed to apprehend the incidence of the climate change on the hydrology of the Rhone: - First of all, retrospectively, by statistical analysis, the search for variability in the low water levels observed since approximately a century. It arises, in spite of the anomalies of certain stations, an absence of real sign of disturbance. With a methodological approach different from the treatment of long series of flows, other research concludes on a tendency to a greater irregularity in the seasonal variation from the flows. - Then, approach GICC the Rhone insists on the regional variations of the hydrological impacts within the basin of the Rhone. Indeed, the impact on the flows is very variable between North and the South of the basin and the increase in the

16 evapotranspiration overall will decrease the quantities of water to the discharge system. - Lastly, the researchers working on project RDT 2003, use a certain number of flow indicators characteristic and conclude with the drastic fall of those. The clarification of these results and the prediction of the quantities available in the future depend closely on methodologies used, in particular of the improvement and the refinement of the MCG, but also of the choice of a method adapted to a local context. It then appears essential to specify that, in the actual position of knowledge, no figure can be advanced as for the low water levels future and only of the tendencies can be released. These tendencies themselves always do not go in the same direction, which pushes us to conclude on the difficulty, even impossibility of answering the problems evoked in introduction. The climatic risk is, with final, not easily appreciable, it must thus be supplemented by a concept of vulnerability to establish independently or almost evolutions of the climatic scenarios, on each basin or under-basin. This concept of vulnerability of the low water levels, or indicator of brittleness of the low water levels, the competence of the hydrologist must be a signature of the revealing basin of the process operating the basin in terms of transformation rain-flow. Moreover, the behavior of the basin around its point of current operation is what interests the hydrologist and the manager for an effective management in the medium term. It is a question of being centred on studies of sensitivity and of concentrating on joint realistic evolutions of the key variables conditioning the low water levels. A new project carried out by the CEMAGREF and CNR will make it possible to establish a typology of basins for which the contributions are most likely to be decreased and for which the associated risks of shortage of water are increasing. They also plan to work on a scale them under-basins to try to isolate the local sensitivity of the sector from that of the sectors upstream. A reactualization of the scenarios and a better strategy of space-time disintegration are to be considered.

17 Bibliography

ARNELL N.W., 1999. The effect of climate change on hydrological regimes in Europe: a continental perspective. Global Environmental Change, 9 , p. 5-23.

BRL, SEPA LRC, 1999 . Etudes des étiages du Rhône. Influence de l’augmentation du débit prélevé par l’aqueduc LRC sur le régime hydraulique du Rhône aval, en période d’étiage . Nîmes, Rapport final, tome 1, 110 p.

BRL, Collectif, 2006. Satisfaire les besoins en eau du Languedoc-Roussillon tout en respectant les milieux aquatiques . Aqua 2020 volet « Ressources,Version 12. Nîmes, 137 p.

CARTER T.R., PARRY M.L., HARASAWA H., NISHIOKA S., 1994. Technical Guidelines for Assessing Impacts of Climate Change . Geneva, Intergovernmental Panel on Climate Change (IPCC), WMO and UNEP, 59 p.

CABALLERO Y., HABETS F., NOILHAN J., MOOR J.F., 2004. Le changement climatique : étude de l’impact sur les ressources en eau du bassin Adour-Garonne. Revue de l’Agence Adour Garonne, 88 , p. 5-11.

DUBAND D., SCHOENEICH P., STANESCU V.L., 2004. Exemple de l’étiage 1921 en Europe (Italie, France, Roumanie, Suisse,…) : climatologie et hydrologie. La Houille Blanche n° 5-2004 , p 18-29.

DOUGLAS A., VOGEL R.M., KROLL C.N., 2000. Trends in floods and low flows in the United States : impact of spatial correlation. Journal of Hydrology 240 (1-2) , p.90-105.

ETCHEVERS P., GOLAZ C., HABETS F., 2001. Simulation of the water budget and the river flows of the Rhône basin from 1981 to 1994. Journal of hydrology n° 244 , p. 60-85.

FORTIN L.G., TURCOTTE R., PUGIN S, CYR J.F., PICARD F., 2007. Impact du changement climatique sur les plans de gestion des lacs St François et Aylmer au sud du Quebec. Canadian Journal of Civil Engineering, vol. 34 , p.934-945.

HENDRICKX F., 2001. Impact hydrologique d’un changement climatique sur le bassin du Rhône. Hydroécologie Appliquée, tome 13, vol. 1 , p.77-100.

HISDAL H., STAHL K., TALLAKSEN L.M., DEMUTH S., 2001. Have streamflow droughs in Europe become more severe or frequent ? International Journal of Climatology 21(3) , p. 317- 333.

IPCC, 2001. Climate Change 2001 : Impacts, Adaptation, and Vulnerability . McCarthy J.J., Canziani O.F., Leary N.A., Dokken D.J. et White K.S. (Eds). Cambridge University Press, Cambridge, U.K, 1032 p.

KRASOVSKAIA I., GOTTCHALK L., 2002. River flow regimes in a changing climate. Hydrological Sciences Journal, 47(4), p. 214-232 .

KRASOVSKAIA I., GOTTCHALK L., LEBLOIS 2002. Signature of changing climate in river flow regimes of Rhône-Mediterranean-Corsica region. La Houille Blanche n°8 , p. 25-30.

LEBLOIS E. et al., 2004. Projet GICC-Rhône. Rapport final, octobre 2004 . Paris, Ministère de l’Ecologie et du Développement Durable, 184 p.

18 LLASAT M.D., BARRIENDOS M., RIGO T., 2002. Frequency and temporal analysis of mediterranean heavy rainfalls producing severe floods in Spain and southern France. Houille Blanche-Revue Internationale de l’Eau , p. 134-144.

LOUCKS P., 1997. Quantifying trends in system sustainability. Hydrological Sciences Journal, 42(4) , p. 513-530.

MAC KERCHAR A.I., HENDERSON R.D., 2003. Shifts in flood and low-flow regimes in New- Zealand due to interdecadal climate variations. Hydrological Sciences Journal 48 (4) , p. 637- 654.

PETERS E., VAN LANEN H.A.J. TORFS P.J.J.F. et BIER G., 2005. Drought in groundwater- drought distribution and performance indicators. Journal of Hydrology, 306 , p. 302–317

PRUDHOMME C., JAKOB D. et SVENSSON C., 2003. Uncertainty and climate change impact on the flood regime of small UK catchments. Journal of Hydrology, 277(1-2) , p. 1-23.

RENARD B., 2006. Détection et prise en compte d'éventuels impacts du changement climatique sur les extrêmes hydrologiques en France. Thèse de doctorat. INP Grenoble, 361 p.

REDAUD J.J., NOILHAN J., GILLET M., HUC M., BEGNI G., 2002. Changement climatique et impact sur le régime des eaux en France . Paris, Ministère de l’Ecologie et du Développement Durable. Paris, 41 p.

SAUQUET E., HAOND M., 2004. Examen de la stationnarité des écoulements du Rhône en lien avec la variabilité climatique et les actions humaines. Journal of Hydrology 279 (1-2) , p. 261-270.

SAUQUET E., LEBLOIS E., RENARD B., HAOND M. et JOUVE D., 2005. Examen de la vulnérabilité de la ressource en eau vis-à-vis des fluctuations climatiques. Application aux étiages du bassin du Rhône . Rapport final, pré-projet Risque Décision Territoire. Lyon, 58 pages.

SAUQUET E., LEBLOIS E., HAOND M., JOUVE D., 2007. Examen de la vulnérabilité de la ressource en eau vis à vis des fluctuations climatiques : application aux étiages du Rhône . Lyon, Pré-projet MEDADD, Cemagref, CNR, 14 p.

ZHANG X., HARVEY K.D., HOGG W.D. YUZYK T.R. 2001. Trends an Canadian streamflow. Water Resource Research 37(4) , p. 987-998.