3 2 1 0 8 7 6
Figure 4.6. Average daily discharge for the control run (black curve), the decrease period (dotted curve) and the statibilization period (grey curve).
4.4. Conclusions and Outlook
Despite the complex topography of the Alps, the hydrological model RS3.0 has demonstrated its ability in simulating water discharge. Not only the favourable efficiency criteria, but also the visual adequacy of the observed and simulated runoff curves attests to the quality of the model runs for the highly glacierized Findelen basin. These results were naturally the prerequisite for exploring the implications for runoff and glacier area/volume if the atmospheric conditions of the baseline 1961-1990 climate were to last for more than one century.
The weather generator used for this purpose has generated a long-term series of temperature and precipitation that have successfully reproduced not only the general statistical properties of the observed weather distributions but also those of the water discharge quantities. RS3.0 was therefore run with stochastic generated sequence of meteorological data over more than a century and has shown consistency throughout the long period of simulation. The results indicate that discharge would decrease during the first four to five decades before stabilizing to the end of the studied time period. The glacier surface follows this same general trend. This methodology enables therefore a quantification of the response time for Findelen Glacier to find its balance with the climate if the atmospheric conditions remain stable. In such a context, it would need a little less than half a century to achieve equilibrium between runoff or glacier volume and climate.
However, a limited decrease in runoff of 15% is observed and the glacier loses only 3.5% of its surface. The simulations also indicate that discharge resulting from glacier melt is rarely in equilibrium with the current climate.
The findings presented here contribute to the foundation of a companion paper (Uhlmann et al., this issue) that investigates the impacts of climate change on runoff and glacier evolution in the Findelen catchment area. The same kind of study applied to the entire basin of the Grande Dixence, at an hourly resolution for runoff and taking into account the impacts of extreme events could be an area of future investigation to provide further information of use to water managers and hydropower utilities.
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Chapter 5
Modelling runoff in a Swiss glacierized catchment – Part II: daily discharge and glacier
evolution in the Findelen basin in a progressively warmer climate
Bastienne Uhlmanna, Frédéric Jordanb and Martin Benistona
aInstitute for Environmental Sciences, University of Geneva, Switzerland
bEngineering firm e-dric.ch, Lausanne, Switzerland
Abstract
In the context of global warming, hydrological regimes in mountain areas are likely to be modified and will therefore result in significant impacts to the supply mechanisms for hydro-power and other end-uses of water, both in the mountains themselves and in the lowland regions downstream. The main objective here is to attempt a fine and continuous analysis of the impacts of such changes on runoff and glaciers in a largely ice-covered catchment in the Swiss Alps: the Findelen watershed. The simulated daily discharge values have been obtained with a semi-distributed hydrological model called Routing System 3 (RS3.0). A stochastic weather generator has been used to generate a 110-year sequence of meteorological input data that have been further perturbed in order to simulate a progressively warmer climate by the end of the century. The amplitude of atmospheric change is suggested by regional climate model simulations, under the SRES A2 greenhouse-gas emissions scenario. Results show at first an increase of discharge (19.4%
in about 60 years) due to the rapid melt of the glacier, followed by a large decrease in runoff at the end of the period (-28% from the beginning to the end of the studied timeframe), primarily due to the depletion of the solid water reservoir. The glacier is indeed projected to lose 91% of its surface area. In parallel, a seasonal shift in flow patterns is also observed:
the discharge values are high earlier in spring due to advanced snow and ice melt, while the rest of the curve flattens out.
Submitted to International Journal of Climatology
5.1. Introduction
Switzerland is often considered as the water tower of Europe, as 56% of the Swiss electricity generation comes from hydraulic power plants, mostly located in the Alps (Swiss Federal Office of Energy, 2009). As a large part of precipitation is stored in the higher parts of the mountains in the form of snow and ice, water resources in this environment is very sensitive to climate variability and change (Beniston, 2000). Moreover, the role of dams and reservoirs as a mean of flood control will also be affected by atmospheric modifications (Beniston, 2010). It is therefore of prime importance to analyse the potential impacts of global warming on hydrological regimes in the Alps, since shifts in the seasonality of runoff and changes in discharge amount will have numerous implications for water management.
The topic of shifts in water regimes has been explored at international level, for instance by Jha et al. (2006) for the Mississippi River basin, or Hagg et al. (2007) for central Asia. In the Alps, potential changes in runoff in a warmer climate have also been analysed (Westaway, 2000; Middelkoop et al., 2001; Etchevers et al., 2002; Jasper et al., 2004). The evolution of discharge in highly glacierized watershed areas related to climate warming has also been addressed in a few studies (e.g. Braun et al, 2000; Huss et al., 2008; Kobierska et al., 2011). However, these works generally use a doubling of atmospheric CO2 or linear shift forcing in weather data records for particular time snapshots (typically a mean of the 2071-2100 period) as a reference change of climate. Nevertheless, the day-to-day and year-to-year fluctuations in terms of meteorological variables can considerably influence glacier-fed runoff. The present paper aims therefore at investigating the evolution of water discharge for a warming climate at a higher temporal resolution and with a continuous simulation through to the end of the century.
Among the widely exploited and thus exposed watersheds in the Swiss Alps, this study focuses on one of the numerous highly glacierized sub-basins which constitute the Grande Dixence hydroelectric complex; the site of Findelen serves as a case study area. It is a particularly suitable catchment for such an analysis since the large ice-covered fraction of the area makes it very sensitive to atmospheric warming; indeed, in this region, temperatures have increased by over 1.5 °C since 1900 (Beniston, 2003).
A conceptual hydrological model named Routing System 3.0 (RS3.0) is used to simulate water discharge. The calibration of the model and a detailed analysis of the simulated discharge of RS3.0 in a stable climate have been presented in a companion paper (Uhlmann et al., this issue). In order to undertake future projections of the various components of the water cycle, stochastic input data, created by a random weather generator, have been perturbed with the so-called “delta disturbance method” (discussed further in this paper) to simulate climate change. This methodology enables an uninterrupted estimation of runoff throughout the century, as mid- and long-term values can also have considerable impacts on the hydraulic energy supply system. It also provides an investigation of the distribution of runoff in the course of the year, of flow amount and of changes in ice melt contribution to total runoff. Since the hydrological model takes into account changes in glacier mass, the reduction of the ice thickness and surface areas are examined as well.