Temporal variation of spring flood in rivers of the Baltic States.

Extreme floods can be caused by various combinations of hydrological and meteorological factors and river basin conditions that have not been observed for a long time. Long-term observational series permit estimation of both the frequency and variation of spring floods - the key issues of protection systems. Fortunately, Baltic States have a long-term record of hydrological data for the last 80 years. In this research, spring flood parameters (maximum discharge, height of maximum discharge and its timing) for the Baltic countries were assessed for four periods (1922-2008, 1941- 2008, 1961-2008 and 1991-2008). In total, 70 hydrological data series of spring flood parameters were used. To detect trends in time series for these periods, the Mann-Kendall test and the nonparametric Sen's method for the magnitude of the trend were used. The index flood method was used to estimate the maximum discharge in ungauged catchments. The results showed that maximum discharges and heights of spring floods decreased over a longer period. Spring flood peaks took place on earlier dates. Only some significant trends of maximum discharges and their timing were found in the last time period (1991-2008). All these changes could be caused by the increasing ambient temperature and precipitation in the later decades. [ABSTRACT FROM AUTHOR]