Evolution of low flows in the Czech Republic

摘要

This study investigates the temporal evolution of annual and seasonal low-flow regimes of 144 Czech river basins for the 1961-2005 period. Summer and winter low-flow seasons are identified and delimited. The gauging stations are classified into two categories, mountain and lowland, according to their low-flow regime. The standard and modified versions of the Mann-Kendall trend test were used in the study to detect trends in several low-flow variables. These include 7-day annual, summer and winter low flows, number of days with discharge under two low-flow thresholds and deficit volumes delimited with the same two low-flow thresholds. At majority of stations, trends in low-flow variables are not significant. Among significant trends, decreasing summer low flows, increasing winter low flows and increasing drought durations as well as deficit volumes are observed. Such combination of trends is typical for the north-eastern part of the Czech Republic and if it continues in the future this area might become drought-prone. It was found that proportions of positive and negative trends between mountain and lowland stations are similar. Thus the low-flow evolution is irrespective of the basins altitudes and climates. Dates of low flows were evaluated to study the evolution of the timing of summer and winter low flows. Non-significant trends prevail for the timing of seasonal low flows, but at a number of stations the dates of summer minima shift towards earlier dates. This can be explained by an earlier onset of the spring flood caused by warmer winters.