WATER TEMPERATURE BEHAVIOR IN THE RIVER DANUBE DURING THE 20TH-CENTURY

Monthly mean water temperatures in the River Danube at Linz, Austria d uring the period 1901-1990 have been investigated in relation to equiv alent information on air temperature and river discharge. Statistical analysis revealed a significant increase in monthly mean water tempera tures of 0.8 degrees C and showed strongest rises in mean values for a utumn and early winter months. No statistically significant trends wer e evident for air temperature or river discharge, and rising water tem peratures are likely to reflect increasing human modification of the r iver system. A strong overall correlation between monthly mean water a nd air temperatures at Linz was made up of a series of more scattered and less steep water/air temperature relationships for individual mont hs, while the influence of snowmelt runoff depressed average water tem peratures in the spring and early summer period by 1.5 degrees C. Mult iple regression relationships developed for individual months from dat a on air temperature, river discharge and time trend during the study period were able to predict monthly mean water temperatures in 1991 an d 1992 with a root mean square error of 0.5 degrees C. These regressio n equations, when combined with scenarios of future changes in air tem perature and river flow as a consequence of global warming, suggest th at only modest rises in monthly mean water temperature will be experie nced in the River Danube by the end of the present century, but that i ncreases of >1 degrees C for all months, and >2 degrees C for the autu mn period of low flows, can be anticipated by the year 2030.