Réf. Bîrsan & al 2003 - P

Référence bibliographique complète
BÎRSAN, M-V., MOLNAR, P., BURLAND, P. Streamflow trends in Switzerland. In: V.R.Thorndycraft, G. Benito, M. Barriendos and M.C. Llasat (Eds). Palaeofloods, Historical Floods and Climatic Variability: Applications in Flood Risk Assessment, Proceedings of the PHEFRA Workshop, Barcelona, 16-19th October, 2002, p. 249-254.

Abstract: Mean daily streamflow data from undisturbed independent watersheds in Switzerland were analysed for trends with the Mann-Kendall nonparametric trend test. Based on seasonal analyses of streamflow quantiles, it was found that streamflow has been increasing in the winter period, especially the winter annual maximum (at about 60% of the stations); and streamflow has been decreasing in the summer period, in particular the low streamflow quantiles. The trends are statistically significant and point to a substantial change in the streamflow regime.

Mots-clés
 Streamflow, seasonal trends, Mann-Kendall test, Switzerland.

Organismes / Contact
Institute of Hydromechanics and Water Resources Management (IHW), Swiss Federal Institute of Technology Zürich (ETHZ); ETH Hönggerberg, HIL G 32.3, CH-8093 Zürich, Switzerland.
birsan@ihw.baug.ethz.ch

(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
  Streamflows    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland         1931-2000

(1) - Modifications des paramètres atmosphériques
Reconstructions
  
Observations
  
Modélisations
  
Hypothèses
  

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstructions
  
Observations
First, streamflow has been significantly increasing in the winter period, especially the winter annual maxima (about 60% of the stations). To a lesser degree this increase affects other quantiles as well, especially in the latter period of study. Looking at the whole period 1931-2000, we see that upward and downward trends are balanced in the low quantile range.

Second, streamflow has been significantly decreasing in the summer period, in particular in the summer low streamflow quantile range. In this case, the whole period 1931-2000 shows the most substantial change in the streamflow regime. In the 70-years analysis, downward trends are dominant, while for the 30 and 40-years periods, upward trends are dominant. The spatial distribution of trend significance for the recent period 1971-2000 for winter maximum streamflow are shown.

Spring and autumn act as transition periods between winter and summer. They show a change from the downward trends of 70-year period to predominating upward trends of the recent period. Spring trend results are most similar to the annual case. Autumn appears to be more influenced by summer in the 70-year period (almost only descending trends) and by winter (almost only increasing trends) in the other two periods.

On an annual basis, the seasonal trends compensate each other, which results in more upward trends in lower quantiles and more downward trends in maximum and upper quantiles. The percentages of significant annual trends are decreasing with shorter record lengths.

The trend results for streamflow in this study are in agreement with recent studies regarding precipitation trends in Switzerland conducted by Widmann & Schär (1997) and Frei & Schär (2001), where significant increases in winter precipitation were observed. However, it appears that changes in precipitation can only partly explain the observed trends in streamflow.
Modélisations
  
Hypothèses
  

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

The data used in this study are mean daily streamflow of 54 currently operating gauging stations in Switzerland, with continuous records which are not affected by major anthropogenic influences and can be considered independent in space (from base network of Pfaundler, 2001). The analysis was conducted for three periods of study: 1971-2000 for 54 sites (70 years), 1961-2000 for 35 sites (40 years), and 1931-2000 for 17 sites (30 years) of the base network.

Mean daily streamflow data were analysed for trends with the Mann-Kendall nonparametric trend test. To find shifts in the distribution of daily streamflow, a range of quantiles on annual and seasonal bases were studied for three different observation periods. Two approaches in the trend analysis have been used: the trend analysis was conducted on the original data series; and series with positive lag-1 serial correlation were prewhitened by applying a first order autoregressive filter to the data prior to trend analysis.

(3) - Effets du changement climatique sur l'aléa
Reconstructions
  
Observations
  
Modélisations
  
Hypothèses
  

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

(4) - Remarques générales
 

(5) - Préconisations et recomandations
 

Références citées :

Frei, C. & Schär, C., Detection probability of trend in rare events: Theory and application to heavy precipitation in the Alpine region, J. Climate, 14, 1568-1584, 2001. - [Fiche biblio]

Pfaundler, M., Adapting, analysing and evaluating a flexible index flood regionalisation approach for heterogeneous geographical environments. IHW Pub. 008, 189 pp., 2001.

Yue, S., Pilon, P. & G. Cavadias, Power of the Mann-Kendall and Spearman’s rho tests for detecting monotonic trends in hydrological series. J. Hydrol., 259, 254-271, 2002.

Widmann, M. & C. Schär, A principal component and long-term trend analysis of daily precipitation in Switzerland. Int. J. Climatol., 17, 1333-1356, 1997.