Multidecadal changes in the river ice regime of the lower course of the River Drava since AD 1875

- Freeze-up became ∼9 days later and break-up became ∼10 days earlier since AD 1875.
- The duration of ice-cover decreased by 14 days on a century average.
- The number of total ice-affected days decreased by 31 days on a century average.
- Drava river ice is a powerful proxy for the winter temperature of Central Europe.

SummaryRiver ice is principally controlled by atmospheric conditions, especially temperature, so these records provide useful information on the climate in general. A more than 130-year-time-series of ice-on and ice-off, and freeze-up and break-up dates was analysed from 4 selected hydrological stations along the lower course of the River Drava since the beginning of river ice observations the start- and end-dates of ice phenomena on Drava River have displayed a significant trend. Freeze-up dates have shifted to ∼9 days later, and break-up dates to ∼10 days earlier. A similar trend is present in the dates of ice-on and ice-off; these dates have shifted to ∼23 days later and ∼17 days earlier per 100 years on average. These changes have resulted in a pronounced reduction in the ice-covered and ice-affected seasons, too. The duration of ice-cover has decreased by ∼14 days and the total number of ice-affected days has decreased by ∼31 days over a century on average on the lower course of the Drava. Interannual variability was compared to local and regional instrumental temperature records. The strongest correlation was found between ice-off and January-February mean temperatures (r = 0.81, p < 0.05), and between the total number of icy days and the mean winter temperature (r = 0.88, p < 0.05). Statistical evidence indicated, however, that the subdued climate control from the 1970s is probably due to anthropogenic intervention in the upper course (e.g. reservoir construction, hydropower management). Spatial correlation analysis revealed that the temperature signal carried by the river ice records of the Drava prior to the anthropogenic disturbance seems to be a powerful proxy for the winter temperature of Central Europe.