Popular Science: Snow cover, tell me how much water will there be in the summer?
Central Europe belongs to a temperate zone with seasonal changes in hydroclimatic variables. The local rivers largely depend on the amount of snow cover accumulated during the winter. In areas where the air temperature is often close to melting point, even a slight increase in air temperature can be a problem. It is stated that in the mountainous areas of the temperate zone when the air temperature increases by 1 °C, the snowline rises by about 150 m above sea level, which causes a significant reduction in the amount of water volume for rivers in spring and summer.
Although this shift has been observed for some time, many questions remain. In particular, the effect of changes in snow cover on summer flows in different regions and the sensitivity of individual river basins are the main unknown. These difficult questions were also asked by a group of experts in hydrology. For their research, they chose seven different river catchments from three countries – Switzerland, Slovakia and the Czech Republic. The catchments were divided into three groups according to altitude. The river catchments were chosen so that all river basins were affected as little as possible by human activity and at the same time had the long-term hydroclimatic data series.
The study included more than forty-year data series starting in 1966 and ending in 2012. The individual changes were analysed annually, seasonally and monthly. It was not a big surprise that an increase in air temperature was registered in all monitored river basins (0.7–1.3 °C), mostly from April to the end of summer. With the increase in air temperature, there was also a decrease in the proportion of snowfall and snow cover thickness, which was observed especially at elevations from 1,000 to 1,500 m a.s.l., where the decrease was about 26%. As a result, runoff increased in the winter, and conversely, a decrease in flow was recorded later in the spring due to earlier melting.
Most of the hydrological droughts were connected to either low air temperatures and a decrease in precipitation during the winter or to high air temperatures in the winter, which caused a reduced amount of accumulated snow cover.
One important result of the research in mountain catchments was that the occurrence of summer droughts is affected not only by the amount of precipitation and air temperature, but also by the amount of snow accumulated during the winter significantly. The catchments at elevations from 1,000 to 1,500 m a.s.l. were the most sensitive.
Although studies worldwide show that the most significant increases in air temperatures occur in the highest elevations, the catchments most sensitive to changes in water resources are in middle elevations where the air temperature is often close to the melting point. In addition to the change in the distribution of precipitation during the year, the changes in snowpack are a major factor influencing the subsequent dry periods during spring and summer.
Kateřina Fraindová
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