Maximum Runoff of the Rivers of the Prut and Siret Basins Within Ukraine and Its Long-Term Spatial and Temporal Variability

Abstract

The article studies the maximum annual runoff of the rivers of the Prut and Siret basins and its spatiotemporal variability. According to the orographic conditions, the studied territory belongs to the mountain and piedmont region. Therefore, a source data bank was formed for the maximum daily average for a year and the corresponding maximum urgent water discharges for all the rivers of the studied basin from the beginning of observations to 2016 inclusive (12 gauging stations). The relationships between the maximum urgent and maximum daily average water discharges are established and the spatial generalization of the relationship between them is carried out. Correlation relationships between the two maxima are quite close (approximation coefficients R2 = 0,59 – 0,95), but the ratio of the maximums on the rivers of the studied catchments of the Prut and Sireta basins are different. It is revealed that the highest ratios between the maximum urgent and maximum daily average water discharges are observed in mountain catchments with average altitudes of 1000-1200 m abs. – an average of 1,7-1,9 times the maximums, which are recorded in the observation time, exceed the average daily maximum. From watersheds with average altitudes of 400 m abs. such ratios are reduced to 1,4 -1,6. The homogeneity of the time sequences is estimated, and the statistical distribution parameters are calculated. The results of the estimation of homogeneity by the criteria of Student, Fisher and Wilcoxon showed that the series of maximum water discharges are homogeneous, both in terms of norm and in terms of variance. Statistical parameters have shown that the coefficients of variation in the vast majority vary within 0,8 -1,0; the asymmetry coefficients have positive values and are generally in the range of 1,8-2,5. The analysis of the long-term variability of the maximum runoff of the rivers of the Prut and Siret river basins was carried out using the total integral curves, difference integral curves and moving averaging. Long cycles could not be identified. In sequences of maximum water flow, cycles lasting 5-7 years are traced.