Inter-annual variability of runoff and climate within the Upper Rhine River basin, 1808-2007

Recent climate models project not only an increase in mean air temperature, but also an increase in its year-to-year variability. Knowledge about the underlying mechanism and possible impacts of changes in the variability are important both from a hydrological perspective and for the water management of different regions. This article focuses on seasonal and annual changes in runoff, areal precipitation and air temperature of the Upper Rhine River basin during the period from 1808 to 2007. The changes in the mean behaviour and inter-annual variability of the data were analysed by using moving statistical distributions over 30-years. The most remarkable changes in the distributions were found in winter and spring when runoff, precipitation and air temperature had significantly increased. Except for spring runoff, the year-to-year variabilities have not changed considerably, and changes in the mean behaviour of our data are not related to changes in their year-to-year variability. The distributions of runoff and precipitation are only significantly correlated in winter and spring. A sensitivity analysis has further shown that in winter air temperature is more important for changes in runoff quantity than precipitation, whereas for spring and annual runoff precipitation is the main driving force. For the years 2030, 2050 and 2070 a further increase in winter and spring runoff was estimated, whereas the annual runoff quantities will not change considerably. The results show that more investigations are necessary to understand the mechanism of the year-to-year variability in climatic and hydrological time series: The variability not only fluctuates irregularly within a time series, it also differs among the analysed runoff, precipitation and/or air temperature time series.