A numerical hydrological model has been used with long time series of temperature and precipitation input data to simulate continuously the potential future discharge of a highly glacierized catchment in the Swiss Alps in a progressively warmer climate. The obtained results show first an increase in runoff with a maximum level of discharge during the years 2026 to 2036, followed by a large decrease in discharge where changes in climate are much more significant. From the first decade of the studied timeframe to the last one, discharge at the outlet generally decreases by 28%, whereas the contribution of ice melt to total runoff is not the predominant one anymore. The changes in the melt timing and amount of the Findelen glacier remain the dominant factor that governs flow amount: it first amplifies the discharge and generates shifts in seasonality. Then the fact that the glacier loses 91%
of its surface by the end of the century leads the annual cycle of runoff to a nivo-pluvial dominated regime.
It should be noted here that the SRES A2 scenario used for this study leads to one of the greatest responses in climate projected by the IPCC. One should therefore keep in mind that the runoff trends simulated here depict an extreme outcome of climate change.
Moreover, as mentioned earlier in this paper, only the results of the HIRHAM RCM have been used. A comparison with other scenarios and other models would provide a range of possible outcomes not necessarily as “pessimistic” as the one described here. However, the methodology described in this paper remains simple to implement and the findings give a comprehensive insight of the evolution of discharge throughout the century. A future step would be to open up the present methodology to the entire watershed of the Grande Dixence and to investigate the economic and energy-relevant impacts of climate change on this catchment area.
Changes in discharge amount will have important indirect impacts on the Grande Dixence catchment area as a whole. The consequences will be first positive for hydroelectric potential, as more water will be made available for energy production. Shifts in seasonality and peak discharge may imply, however, rethinking the management and operation of the hydropower infrastructure. Furthermore, the additional melt and rain water in the environment may also require reviews to flood control procedures. On the other hand, at the
end of the analysed timeframe, the flattening and decrease of the discharge curve will have negative repercussions for this exploited watershed, since water will become scarcer, despite a projected slight increase in precipitation. Water shortages could even become problematic, especially during hot and dry summers, as the glacier melt water will no longer be able to sustain discharge. As a result, hydropower utilities will need to consider a contrasted future composed of challenging economic issues where adaptation measures will be inevitable.
Acknowledgments
This work would not have been possible without the help of Grande-Dixence SA and Javier García Hernández from the EPFL for providing meteorological and runoff data, of Daniel Farinotti and Martin Funk for glacier data and of E-dric.ch Sàrl for making the hydrological model available. Part of this work is of relevance to the EU-FP7 “ACQWA” project under contract number 212250 with the European Commission.
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Chapter 6
Conclusion
Despite their massive character, mountains represent very vulnerable environments when confronted with global atmospheric changes. Mountain research at fine spatial scales is necessary to increase our knowledge of complex processes exposed to subtle modifications of climatic conditions. Therefore, the physical response of snow patterns to atmospheric warming in Swiss mountains and of river discharge in a highly glacierized catchment in the Alps are the key elements that have been addressed in this dissertation. As the impacts in the winter tourism sector and in hydropower production may be significant, results presented in previous chapters have enabled a contribution to the understanding of snow behaviour and runoff evolution with climate forcing, using numerical modelling at local scales as a primary investigation tool. A synthesis of the principal findings and of the main insights of the study is provided below and can be considered to be a complement to the detailed discussions given at the end of the four journal papers that make up this manuscript.
The following points highlight the major conclusions that have emerged from this research:
• The GRENBLS surface energy balance model has shown genuine skill in representing current snow amounts and duration at the local scale in different parts of Switzerland for a range of winter types. To model future potential changes in atmospheric conditions, a perturbation method – the so-called “delta disturbance method” – has been developed in order to downscale meteorological outputs of the HIRHAM RCM that are used to drive GRENBLS simulations. The results show a large general decrease of snow depth and duration, particularly marked in the Western regions of Switzerland and in the Eastern Prealps during warm winters. Among parameters that influence the accumulation of snow, this study suggests that air temperature is the dominant factor, in accordance with other independent studies.
However, a direct correlation between the quantities of snow and the mean elevation of an area is not systematic.
• Modifications of snow cover patterns have been explored within selected ski areas as well. A critical altitude has been determined for each ski resort and represents
the highest point beyond which snow is required for the ski lifts to be operational. Here again, the GRENBLS model simulates for a warmer climate a decrease in snow cover in every ski area investigated. The fact that the amount of snow in a resort is dependant not only on its altitude but also on other topographical characteristics is further emphasized: the exposure, localization inland and slope gradient of an area are clearly parameters that also need to be taken into account for assessing snow patterns.
GRENBLS therefore enables to go beyond the simple relation between elevation and amount of snow while at the same time revealing the regional contrasts of such criteria.
Concerning the projected percentage of days during which skiing could be practiced, it is seen from the model projections that the duration of the snow season decreases in all ski resorts. Furthermore, the model results suggest that of 20 resorts sampled, 6 of these may no longer be in a position to offer viable skiing conditions by the end of the century.
• In order to determine whether snow-abundant winters could still exist in a warmer climate, an analysis of joint distributions of temperature and precipitation has been undertaken. These two variables have been shown to be reasonable proxies for underlying synoptic circulations. By this means, it could be first established that the number of snow-abundant winters have in the past been inversely proportional to the number of occurrences of warm/dry (WD) situations as defined by particular combinations of temperature and precipitation thresholds. Since the 1950s, and despite an observed increase in winter temperatures during the last decades, snow-abundant winters have nonetheless occurred, when the WD mode is rare. Using the projections of the HIRHAM RCM, results indicate that in a warmer climate, snow-abundant winters as experienced in recent years may occur less than 5% of the time in the last 30 years of this century, but they will not totally disappear.
• In the third experiment, the current hydrological regime of a small highly glacierized basin, that is part of the Grande Dixence hydropower complex, has been simulated with the hydrological model RS3.0. The model has shown good performance in reproducing observed discharge at the outlet of the Findelen catchment area. These results were obviously the prerequisite for investigating the adaptation of runoff and glacier area/volume to climate maintained under the atmospheric conditions of the baseline period 1961-1990 for over a century. This analysis was done in order to give indications on the hydrological trends that would be expected if the current warming tendency were to cease (i.e., if climate were to remain constant over coming decades) and enabled a decoupling of the glacier reaction from the current climate trend. In order to accomplish this part of the investigations, a random generator of hourly atmospheric data was employed to generate random, long-term series of input that are used as initial conditions for the RS3.0 model. Despite a limited decrease in runoff and glacier surface area (15% and 3.5%, respectively), the results indicate that the system would nevertheless require a little less than half a century to achieve equilibrium with surrounding climate conditions.
• The watershed has also been examined in a context of climate change at high temporal resolution. The large ice-covered fraction of the basin makes it indeed very sensitive to atmospheric warming. In order to simulate future climate conditions, the random datasets of input parameters produced by the weather generator have been perturbed, according to a variant of the so-called “delta disturbance method” used in
Chapter 2 of the study. The changes in the amount and timing of glacier melt remain the dominant factor that governs flows: the general trends of discharge initially exhibit an increase (19.4% in about 60 years) followed by a rapid decrease in runoff by the end of the 21st century. From the beginning to the end of the timeframe studied here, the model simulates a decrease of 28% whereas the contribution of ice melt to total runoff is projected to become of secondary importance. Concerning the seasonality of flows, a general flattening of the discharge curve is noted over coming decades with an earlier and enhanced snow and ice melt in spring. Simulations also indicate that strong discharge will occur less frequently in a greenhouse climate. In addition, the RS3.0 model shows an ice surface loss of 91% from the beginning to the end of the simulation, with no stabilization of the trend.
The methodology used in this research project has enabled to provide valuable insights into the potential repercussions of a greenhouse climate on fine-scale features of Swiss mountain environments. As snow-reliability is a key element in the ski industry, the decrease in snow depths simulated in the present case by GRENBLS and less frequent snow-abundant winters will necessarily lead to a complete rethinking of strategies related to ski tourism. Large impacts of ski resorts on local economies will certainly need to be considered. The present study provides an overview of potential snow patterns, physical bases on which possible adaptation measures could be assessed and implemented. In addition, this investigation emphasizes the variable character of snow cover, depending on a number of geographical factors. For this reason, a resort-specific analysis is required for a more complete perception of all local components involved.
Similarly, the selected methodology has given the opportunity of continuous simulations of water discharge and glacier surface through to the end of this century, with contrasted results. We have first seen that with a stationary climate, runoff and glacier would need time to reach equilibrium with atmospheric conditions. This lag highlights the fact that, even though if measures were taken to abate GHG emissions in order to stabilize global climate trends, adaptation measures would nevertheless be necessary at the local scale. This fact needs to be taken into account when regulating water resources on the long-term in an exploited catchment area. Furthermore, climate warming in such highly glacierized environments implies major changes, for instance in the quantity and seasonality of runoff, altering the glacial hydrological regime of the watershed into a more nivo-pluvial regime. In the first instance, not only the additional melt and rain water in the environment will lead to a rethinking of the management of the hydropower facilities but it will also require reviews to flood control procedures. In a second phase, the conditions will be reversed: the large decrease in runoff within the hydroelectric complex at the end of the analysed timeframe could result in potential water shortages and thus energy shortfalls if no adaptation measures are envisaged.
6.1. Outlook
Despite the fact that the simple methodology developed in this research has provided significant outcomes on the effects of global warming on snow and water, these topics certainly warrant further investigation. For instance, the response of snow and water to
climate change examined in the present study is the product of one of the most “worst-case”
projections among the SRES scenarios. It is likely that simulations produced on the basis of other IPCC storylines could enhance the significance of such research and modulate our results. In the same direction, outputs from several GCMs such as drivers for the HIRHAM RCM and comparison with other local snow or hydrological models could give a more comprehensive view of the potential future of mountain environments. In addition, the application of such methodology on other national and international study sites could constitute further research topics. In particular, the modelling of the entire Grande Dixence watershed including monitoring civil engineering infrastructure could improve future water management.
Further improvements can be made towards a more detailed description of snow cover with GRENBLS. For example, the division of the snow pack into several superposed layers with individual parameterizations, or the integration of results over the entire surface of the ski resort, could certainly increase the level of confidence of the modelling results. Similarly, modelling processes related to slope exposure or improved formulations of the evolution of the surface area of glaciers in the snow and ice sub-model of RS3.0 could be other paths worth exploring.
The random weather generator used in this study could also take advantage of some development, specifically in terms of extreme events. The stochastic production of atmospheric values is currently based on existing situations. Both “delta methods” enable a differential disturbance of the parameters according to the histogram of their distribution (i.e., not the same deltas disturb mean and extreme values of temperature and precipitation). However, the creation of random weather series based on data whose properties exceed those of the observed bounds could refine the projections. In addition, an analysis of future potential snow-abundant winters has been undertaken in the present study, but a more comprehensive examination of extreme events could provide significant insights into extreme processes under conditions of global warming. For instance, a review of the return period of floods in the catchment area could complement the present research fields.
The coupling of both local models with economic tools would represent the most logical advance of this research. On the one hand, useful material could be drawn from the interaction between the environmental results obtained here and projected tourism or technical parameters for a given resort (e.g., accommodation capacity, quantitative information concerning transport and snowmaking facilities, or the size of the population basin). On the other hand, the future of surface discharge could also be processed with
The coupling of both local models with economic tools would represent the most logical advance of this research. On the one hand, useful material could be drawn from the interaction between the environmental results obtained here and projected tourism or technical parameters for a given resort (e.g., accommodation capacity, quantitative information concerning transport and snowmaking facilities, or the size of the population basin). On the other hand, the future of surface discharge could also be processed with