Streamflow trends in Switzerland

Mean daily stream flow records from 48 watersheds in Switzerland with an undisturbed runoff regime are analysed for trends with the Mann-Kendall nonparametric test in three study periods (1931-2000, 1961-2000, 1971-2000). The statistical significance of trends is tested for each station on an annual and seasonal basis and for different stream flow quantiles. The field significance of trends is tested by a bootstrap procedure. Identified trends in stream flow are examined together with changes in precipitation and air temperature, and correlated with watershed attributes. Complex changes in the stream flow regime in Switzerland especially in the more recent periods are demonstrated. The main identified trends are an increase in annual runoff due to increases in the winter, spring and autumn season runoff, an increase in winter maximum stream flow (at more than 60% of the stations) and an increase in spring and autumn moderate and low flows. The behaviour in the summer period is different, with both upward and downward trends present in moderate and low flow quantiles. Many of the trends are field significant. Changes in precipitation are not sufficient to explain the observed trends in stream flow. Air temperature, most notably a substantial increase in the number of days with minimum daily temperature above 0 degrees C, may explain some of the observed increases in winter and spring season runoff. Correlation analyses reveal a strong relationship between stream flow trends and mean basin elevation, glacier and rock coverage (positive), and basin mean soil depth (negative). These relationships suggest that the most vulnerable environments from the point of view of stream flow change are mountain basins. (c) 2005 Elsevier B.V. All rights reserved.