Water temperature behaviour in the River Danube during the twentieth century

Monthly mean water temperatures in the River Danube at Linz, Austria during the period 1901–1990 have been investigated in relation to equivalent information on air temperature and river discharge. Statistical analysis revealed a significant increase in monthly mean water temperatures of 0.8 °C and showed strongest rises in mean values for autumn and early winter months. No statistically significant trends were evident for air temperature or river discharge, and rising water temperatures are likely to reflect increasing human modification of the river system. A strong overall correlation between monthly mean water and air temperatures at Linz was made up of a series of more scattered and less steep water/air temperature relationships for individual months, while the influence of snowmelt runoff depressed average water temperatures in the spring and early summer period by 1.5 °C. Multiple regression relationships developed for individual months from data on air temperature, river discharge and time trend during the study period were able to predict monthly mean water temperatures in 1991 and 1992 with a root mean square error of 0.5 °C. These regression equations, when combined with scenarios of future changes in air temperature and river flow as a consequence of global warming, suggest that only modest rises in monthly mean water temperature will be experienced in the River Danube by the end of the present century, but that increases of > 1 °C for all months, and > 2 °C for the autumn period of low flows, can be anticipated by the year 2030.