Réf. Jasper & al. 2004 - A

Référence bibliographique complète
JASPER K., CALANCA P., GYALISTRAS D., FUHRER J. Differential impacts of climate change on the hydrology of two alpine river basins. Climate Research, 2004, Vol. 26, p. 113-129.

Abstract: For 2 Alpine river basins, the Thur basin (1700 km2) and the Ticino basin (1515 km2), possible future changes in the natural water budget relative to the 1981-2000 (Thur) and 1991-2000 (Ticino) baselines were investigated by driving the distributed catchment model WaSiM-ETH with a set of 23 regional climate scenarios for monthly mean temperature (T) and precipitation (P). The scenarios referred to 2081-2100 and were constructed by applying a statistical-downscaling technique to outputs from 7 global climate models. The statisticaldownscaling scenarios showed changes in annual mean T between +1.3 and +4.8°C and in annual total P between -11 and +11%, with substantial variability between months and catchments. The simulated overall changes in the hydrological water cycle were qualitatively robust and independent of the choice of a particular scenario. In all cases, the projections showed strongly decreased snowpack and shortened duration of snow cover, resulting in time-shifted and reduced runoff peaks. Substantial reductions were also found in summer flows and soil-water availability, in particular at lower elevations. However, the magnitudes and certain aspects of the projected changes depended strongly on the choice of scenario. In particular, quantitative projections of soil moisture in the summer season and of the runoff in both the summer and autumn seasons were found to be quite uncertain, mainly because of the uncertainty present in the scenarios for P. The findings clearly demonstrate that quantitative assessments of hydrological changes in the Alps using only a small number of scenarios may yield misleading results. This work strengthens the overall results obtained in earlier studies and suggests distinct shifts in future Alpine hydrological regimes, with potentially dramatic implications for a wide range of sectors.

Climate change impact, regional climate scenarios, uncertainty analysis, mountain hydrology, distributed hydrological modeling.

Organismes / Contact
Swiss Federal Research Station for Agroecology and Agriculture (FAL), Air Pollution/Climate Group, 8046 Zürich, Switzerland. karsten.jasper@fal.admin.ch
University of Bern, Institute of Geography, 3012 Berne, Switzerland

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature, Precipitation Snow cover, Soil water content, Rivers    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
Switzerland Alps Thur river basin and the basin of the upper Ticino river   220-3400 m a.s.l. 1981-2000

(1) - Modifications des paramètres atmosphériques
The SD scenarios for the 2081-2100 time slice indicated seasonally varying deviations from the control run. Changes in T were projected to be positive throughout the year. For the Thur basin, the annual 17-scenario mean deviated from the control run by +3.0°C, while the range across the scenarios was between +1.7 and +4.8°C. Corresponding results were found for the Ticino basin. Here the annual increase in T amounted to +2.4°C, whereas the overall scenario set ranged from +1.3 to +3.7°C. In both basins, the largest T anomalies occurred at the end of winter (February and March) and in late summer (August). For the Thur basin, the warming projected by the 17-scenario mean was slightly higher for the winter period (DJF) than for the summer period (JJA) (+3.3 vs +3.1°C); for the Ticino basin, this ratio of winter to summer warming was somewhat different (+2.1 vs +3.0°C).

As opposed to T, projected changes in P were either positive or negative, depending on scenario and season. The mean annual P from the SD scenarios was found to increase by 37 mm in the Thur basin and to decrease by 48 mm in the Ticino basin. In relative terms, these changes were small for both basins (less than 3%). On the other hand, the scenario range was quite large and reflected a high degree of uncertainty in the projections. For the Thur basin the annual changes in P ranged from -72 mm (-5%) to +160 mm (+11%), whereas for the Ticino basin the spread was from -205 mm (-11%) to +112 mm (+6%). Despite the large spread, the average over the scenario ensembles indicated a slight overall increase in winter (DJF) P for both regions (slightly more than +6%), and a significant reduction for the Ticino basin during April to June (about -13% on average). The GP scenarios were more similar to each other than the SD scenarios, with a significant increase in winter P and a marked decrease in summer P.

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
The climate scenarios were based on outputs of coupled atmosphere-ocean general circulation models (AO-GCMs). Scenarios derived by means of "statistical downscaling" (Giorgi et al. 2001) of GCM outputs to the regional scale were mainly used. The models were calibrated for 1951-1999 using the sea-level pressure (SLP) data set of Trenberth & Paolino (1980, updated) and the near-surface temperature (NST) data by Jones et al. (2001). Changes in long-term mean T and P during 2081-2100 relative to the baseline 1961-1990 were obtained by applying the canonical correlation analysis (CCA) models to GCMsimulated anomalies of SLP and NST. The authors derived a total of 17 different climate scenarios based on the changes simulated by 7 GCMs and 4 emission scenarios. They will refer to the statistical-downscaling scenarios for both T and P as "SD", for the subset of the statistical-downscaling T or P scenarios as "SDT" and "SDP", respectively.

Additional P scenarios were constructed only for a subset of 3 selected SDT scenarios. These were the scenarios CSIRO_B2 (1.8°C), CSIRO_A2 (2.4°C), and HadCM3_GS (3.4°C). They will be referred to as "SDT-low", "SDT-medium" and "SDT-high", respectively. The authors constructed the "dry-summer" scenarios by simply combining the SDT-low, -medium and -high scenarios with the P changes simulated by the respective GCMs. For this purpose, they considered the average signal from selected grid points (GP) in the vicinity of the European Alps (for details, see Gyalistras 2002). The resulting set of combined scenarios was named SDT_GPP. In addition, they combined the SDT-low, -medium and -high scenarios with an idealized, uniform 15% increase in P throughout the year. This set of combined scenarios was named SDT_P15.

For the projection period (2081-2100), the monthly 5 km SD scenario grids were interpolated onto the 1 km grid of the hydrological model using bi-cubic spline interpolation. Thereafter, these high-resolution scenarios were linearly interpolated in time. In result of this procedure, daily 1 km gridded scenarios of T and P were obtained.

(2) - Effets du changement climatique sur le milieu naturel
Unless otherwise specified, all hydrological model results presented here were based on ensemble simulations using the SD scenarios (SDT_ SDP).

Evapotranspiration and soil water content
ET was found to increase significantly in all experiments and for both regions. The 17-scenario mean showed an increase in the annual ET rate of about 16% relative to the control runs. The wetness index ET/PET showed significantly lower values in the projection time slice than in the control run. This is particular true for the summer period (JJA). Like the future evolution of ET/PET, the projections of soil water content (SWC) indicated decreasing values for both basins over most of the year, in particular during the vegetation period. The relative reduction in SWC was 7.2% (Thur basin) and 7.7% (Ticino basin) on a yearly average, and 9.7% (Thur basin) and 16.2% (Ticino basin) when averaged over the vegetation period from April to August.

Snow cover and snow-water equivalent
For the Thur and Ticino basins, the 17-scenario projections revealed a decrease in the annual mean snowwater equivalent (SWE) between 73% (Thur basin) and 69% (Ticino basin) as compared with the control run. Averaged over the winter period (DJF), SWE was calculated to diminish by 68% (26 mm) in the Thur basin and by 57% (66 mm) in the Ticino basin. The lifetime of continuous snow cover was shortened by about 1 to 3 months, while the snow-line was raised by some 300 to 600 m. The occurrence of snow-free conditions (SWE < 1 mm) was advanced by about 5 to 8 weaks in spring, as opposed to a postponement of only about 1 to 3 weaks for the begin of the snow season. In both basins, large differences across the scenario members occurred in particular during the spring season.

Winter (DJF) runoff increased on average by 14% (Thur basin) and 31% (Ticino basin), whereas summer (JJA) runoff reduced by 16% (Thur basin) and 33% (Ticino basin). The annual mean runoff was reduced by about 6% (Thur basin) and 10% (Ticino basin) relatively to the control run. Note that, depending on the selected scenario, the projection results differed strongly; for the annual mean runoff, the changes were between -17 and +7% for the Thur basin and between -22 and +4% for the Ticino basin. In general, significant changes in runoff were projected in spring (early summer), mainly due to clearly changed snow conditions. As a consequence, the projected runoff peaks occurred earlier and substantially reduced relative to the peaks in the control runs. Note, furthermore, that changes in runoff were largest for areas where runoff is most strongly controlled by snow dynamics.

Hydrological response to additional precipitation scenarios
The projection results for the SDT_GPP scenarios tended to intensify the hydrological changes derived from the SD scenarios. With respect to the wetness-index ET/PET and soil moisture, smallest changes were projected when using the SDT_P15 climate scenarios to drive the hydrological simulation. In this case, the uniform all season increase in P (+15%) almost compensates the potential T-induced decrease in ET/PET and SWC. As opposed to the results of the SDT_GPP and SDT_SDP scenarios, the SDT_P15-driven projections showed positive runoff changes not only for winter but also for summer, independently of the choice of scenario.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
The areas selected for this study were (1) the Thur river basin (tributary of the Rhine) in northeastern Switzerland, and (2) the basin of the upper Ticino river on the southern slopes of the Alps. The Thur basin represents a typical catchment of the northern Alps and their forelands. Midland character (areas lower than 1000 m) is dominant and the basin is primarily used by agriculture. Loam has been identified as the prevailing soil texture. The upper Ticino basin, is characterized by extreme topographic conditions; the mean slope of this basin exceeds that of the Thur basin by a factor of 3. More than 80% of the Ticino area is located higher than 1000 m a.s.l. and forested and rocky areas are dominant. The prevailing soil texture is loamy sand.

Meteorological observation data were made available by MeteoSwiss. The data series covered a 10 yr period for the Ticino basin (1991-2000) and a 20 yr period for the Thur basin (1981-2000), respectively. The selected stations can be divided into automatic stations (hourly values), conventional stations (3 readings/day), and simple rain gauges (daily sums).

Simulations were carried out using the grid-based catchment model WaSiM-ETH (Water Flow and Balance Simulation Model; Schulla 1997, Schulla & Jasper 2000). The soil model in WaSiM-ETH calculates the infiltration of water into the soil and the surface-runoff generation using the Green-Ampt approach, with estimates of the saturation time after Peschke (1987). A discrete formulation of the Richards equation (Richards 1931) is used to simulate the vertical flow of water in the unsaturated soil zone. Interflow is generated as lateral water flow in predefined soil layers, depending on the hydraulic pressure, the drainable water content, the hydraulic conductivity and gradient, as well as the flow density. Baseflow is determined as exfiltration from the groundwater into the surface river system. The simulation of discharge routing within the river channels based on hydraulic calculations of the flow velocities (kinematic wave approach). The WaSiM-ETH model was calibrated and validated by continuous observation-driven runoff simulations.

The parametrization of WaSiM-ETH was based on information derived from 3 gridded GIS data layers: maps of elevation, land surface and soil characteristics.

(3) - Effets du changement climatique sur l'aléa

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(4) - Remarques générales

(5) - Syntèses et préconisations

Références citées :

Giorgi F, Hewitson B, Christensen J, Hulme M et al. (2001) Regional climate information—evaluation and projections. In: Houghton JT, Ding Y, Griggs DJ, Noguer M, van der Linden PJ, Dai X, Maskell K, Johnson CA (eds) Climate change 2001: the scientific basis. Contribution of Working Group I to the Third Assessment Report of the IPCC. Cambridge University Press, Cambridge, p 583–688

Gyalistras D (2002) An uncertainty analysis of monthly temperature and precipitation scenarios for Switzerland. Internal report, Climatology and Meteorology Research Group, University of Bern

Jones TC and 11 others (2001) Anthropogenic climate change for 1860 to 2100 simulated with the HadCM3 model under updated emission scenarios. Hadley Centre Technical Note 22, Met Office, Bracknell

Peschke G (1987) Soil moisture and runoff components from a physically founded approach. Acta Hydrophys 31(3/4): 191–205

Richards LA (1931) Capillary conduction of liquids through porous mediums. Physics 1:318–333

Schulla J (1997) Hydrologische Modellierung von Flussgebieten zur Abschätzung der Folgen von Klimaänderungen (Hydrological modelling of river basins for estimating the effects of climate change). Zürcher Geographische Schriften 69, Swiss Federal Institute of Technology (ETH), Zürich

Schulla J, Jasper K (2000) Model Description WaSiM-ETH. Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zürich

Trenberth K, Paolino DA (1980) The northern hemisphere sea-level pressure data set: trends, errors and discontinuities. Mon Weather Rev 108:855–872