Variability of spring riverflow of selected lowland rivers

This paper aims at identification of changes in the spring discharge of six lowland rivers in Poland in 1966-2006. The contribution of the spring flow (February, March, April) in the annual flow was analysed as well as the 0.95 quantile time series of the daily flow. To investigate how the date of the occurrence of the spring riverflow changes, for each hydrological year the day up to which 50% of the annual flow had been reached was determined (CMD, the center of mass data). A test based on linear regression was applied, which determined the direction and rate of the changes. In five rivers, a statistically insignificant tendency to increase the contribution of the spring flow in the annual flow has been observed. A decreasing tendency in the spring discharge of all rivers has been shown; in two cases the changes are statistically significant. The tendencies in the CMD time series are not consistent.