The dry seasons that have plagued Central European countries in recent years have several causes. They are most often associated with higher air temperatures, lower precipitation, or their different distribution during the year.
However, the accumulation of snow cover also plays a crucial role, on the changes of which hydrology experts, together with members of the Department of Physical Geography and Geoecology from the Faculty of Science, Charles University, took a closer look.
Central Europe belongs to a typical area with alternating seasons, in which the water in the rivers is largely dependent on the amount of snow cover accumulated during the winter. In areas where the air temperature is around zero, even a slight increase in air temperature can be a problem. In the mountainous areas of the temperate zone (temperate zone of the northern hemisphere), a 1 ° C increase in air temperature shifts the snow line by approximately 150 m above sea level, which can represent a significant reduction in water volume for rivers in spring and summer.
Although it has been clear for some time that this shift is taking place, there are still many questions. In particular, the impact of changes in snow cover on summer flows in different regions and how sensitive individual river basins are to changes. These difficult questions were also asked by a group of hydrology experts. For her research, she chose seven different river basins from three countries – Switzerland, Slovakia and the Czech Republic, and they were divided into three groups according to their altitudes. The choice of river basins was chosen so that all river basins were affected as little as possible by human activity and at the same time had the longest possible data series.
For their research, hydrologists analyzed more than forty years of data starting in 1966 and ending in 2012. The individual changes were examined from both annual, seasonal and monthly perspectives.
During the monitored period, there was a significant increase in air temperature in all monitored river basins (0.7 to 1.3 ° C), most markedly from April to the end of summer. With an increase in air temperature, there was also a decrease in the proportion of snowfall and snow cover thickness, which was observed especially at medium altitudes (1000 to 1500 m above sea level), where the decrease was about 26%. As a result, runoff increased in the winter, and conversely, due to earlier melting, a decrease in flow was recorded later in the spring.
Most dry periods were associated with low temperatures and a decrease in precipitation during the winter or with high air temperatures in the winter, which caused a reduced amount of accumulated snow cover. An important finding from the research of mountain river basins was that the occurrence of drought during the summer is influenced not only by the amount of precipitation and air temperature, but also in a fundamental way by the amount of snow accumulated during the winter. Basins from altitudes of 1000 to 1500 m proved to be the most sensitive.
Although worldwide research shows that the most significant increase in air temperatures is in the highest mountain areas, the most sensitive to changes in water resources are mainly the middle positions, where the air temperature is around zero. In addition to the change in the distribution of precipitation during the year, the change in the volume of snow is a major factor influencing the subsequent dry season during spring and summer.
Source: Kateřina Fraindová, Faculty of Science, Charles University
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