Comparison of the water balance responses of belgian and swiss catchments to a climate change
By strengthening the so called greenhouse effect, the rise of the atmospheric concentrations of anthropogenic gases, such as CO2, chlorofluorocarbons (CFCs) and methane, will progressively modify the energy budget of the Earth atmosphere and disturb the climate. Temperature at the soil level will rise. Precipitation and air humidity will be modified, inducing a large perturbation of the water cycle and thus of water availability and hydrological extremes. The last International Panel on Climate Change report (IPCC 1994) states that a rise of 0.3 degrees Celsius per decade could be felt in the mean global temperature in the next century. The conclusions of the Second World Climate Conference (1992) pointed out that among the most likely impacts of climate change will be its effects on the hydrological cycle and water management systems. An increase of the incidence of extreme events, such as floods and drought, would cause increased frequency and severity of disasters. The present paper is a synthesis of several separate published and some unpublished climate change impact studies (Bultot et al., 1988 a and b, 1992, 1994; Gellens, 1991; Schädler et al., 1992; Gellens and Demarée, 1993; Gellens and Roulin, 1996) carried out in Belgium and in Switzerland. All these studies have been done with the same hydrological model and the same climate change scenario. This procedure allows a strict comparison of the sensitivity studies and enables us to identify the common responses and the specific behaviour of the catchments. In this latter case, an attempt to identify the geomorphological origin of the particular responses is possible. A set of eight catchments in Belgium and belonging to the Scheldt and Meuse river basins has been studied. These individual catchments cover areas from 100 km2 to 1200 km2 and are spread over the country in order to represent the main catchment types. Precipitation ranges from 730 to 1160 mm per year. In addition, three tributaries of the Rhine river in Switzerland have been selected in the low alpine altitude (lower than 1500 m a.s.l.). Precipitation rates are higher for these three catchments and range from 1080 to 1300 mm. The adopted IRMB (Integrated Runoff Model - Bultot) hydrological model (Bultot et al., 1976 and 1985) is a daily time step conceptual model. It has been designed at the Royal Meteorological Institute of Belgium to simulate the components of the water cycle in medium- sized catchments, i.e. catchments for which the input data, and in particular the precipitation, can be considered as uniform. The main data needed to run the IRMB model are the precipitation and the potential evapotranspiration. This latter variable is assessed by following the procedure described by Bultot et al. (1983) and requires several climatological data, i.e. the net radiation, the air temperature and humidity, the soil temperature at 10, 20 and 50 cm depth and the wind speed at 2 m above the soil. These data are also taken into account in the snow melting- accumulation simulation. The adopted climate scenario has been constructed from the literature in order to combine the results of various simulations produced by different climate models (Bultot et al., 1988b) in a single set of climate increments. The main characteristics of the climate change scenario is a temperature rise reaching some 3 ¡C, with a higher increase in winter than in summer. Precipitation stress consists of a 50 mm yearly rise distributed as a winter rise (about 10 mm) and a slight summer decrease. Although these scenario increments are small in comparison with the year-to-year variability, they are however large enough to reveal the sensitivity of the water balance to climate perturbations. The detailed description of the algorithms used to apply the monthly increments on the daily time step are presented in Bultot et al. (1988b). After a calibration phase for the present climate conditions, a sensitivity analysis of the water balance of the catchments has been carried out by modifying the input data according to the scenario. This well known if - then - what? method gives the sensitivity of the various terms of the water cycle by comparing their values in the present runs and in the disturbed 2xCO2 runs. For practical reasons, the reference periods of the simulation runs are not the same for all the catchments. The study focuses on the evolution of evapotranspiration and soil moisture, of snow cover, of streamflow at the outlet and of groundwater storage. The impacts are also studied in terms of extreme events. For all the catchments, the analysis shows a rise of the evapotranspiration equivalent to some 7 to 10 percent. A small decrease in soil moisture has also been simulated associated with an increase in dry soil days. Due to the temperature rise a strong depletion of the snow cover might be an economically dominant effect in the low alpine regions where winter sport activities represent a large part of the inhabitants' resources. According to the winter precipitation rise, the monthly streamflows in the cold period are also increased under the 2xCO2 conditions. Besides predictable trends common to all the catchments, the study also shows that some components of the water balance governed by the underground characteristics can present uneven responses. Catchments characterized by strong infiltration could be subject to positive evolution of the groundwater storage and of the baseflow, whereas catchments with predominant surface runoff could exhibit the reverse effect. These effects could be important mainly in summer during the low flow period. The altitude of the catchments also seems to be significant. All the lowland catchments present higher extreme streamflows, whereas catchments in low Alpine regions are spared this negative consequence. While a large degree of uncertainty remains in the assessment of the climate in the next century, this study gives a first insight into the direction of the expected climate change impacts. It also points out the need to analyse the sensitivity of catchments with a particular attention to their characteristics.