ZFirst, the drought on the Po in Italy caused a stir, then France also reported a record drought. And now the levels of the Rhine are falling, and there is no end in sight to the drought in the rivers. The geographer and climate researcher Andrea Böhnisch from the LMU in Munich researches the factors that cause European rivers to dry up.
Ms. Böhnisch, at the end of 2021 you showed in a detailed study on droughts in Europe that we must expect droughts to become more frequent and more intense in the future. The word “drought” is very often heard and read in connection with climate change – can you briefly explain what it means and what forms it can take?
A drought describes an episode of prolonged dryness compared to normal, i.e. an abnormal deviation from the long-term expectation. The latter always depends on the region under consideration. The same amount of rainfall can be perfectly normal for one region while spelling drought for another. A distinction is made between several types of droughts, which are often related to each other: insufficient rainfall (meteorological), insufficient soil moisture (agricultural), insufficient water levels in bodies of water and runoff in rivers (hydrological) and limited access to water (socio-economic).
We are currently experiencing severe droughts in large parts of Europe. The Po recently carried 61 percent less water than usual and triggered a historic water shortage in northern Italy. Now the Rhine also has too little water – why is one of our most important waterways drying up?
The Rhine is a complex body of water: On the one hand, it depends on its many tributaries. A lack of rainfall in France and Switzerland has a negative impact on the discharge volumes of important tributaries such as the Moselle or the Aare and ultimately also on the Rhine, into which they flow. On the other hand, there is no snowmelt from the Alps, which would feed rivers such as the Po or the Rhine at this time of year – because the Alps have been suffering from a historical lack of snow for years. One reason for this is, among other things, the rising temperatures, as a result of which the winter precipitation in the Alps increasingly occurs as rain instead of snow as usual. Because rain seeps away or runs off immediately – in contrast, snow stays put for the time being; it thaws slowly, feeding the rivers in spring. If the snow does not melt, the probability of low water in spring increases.
What happens to the rain that falls in winter? After all, the water cannot vanish into thin air.
This leads to increased discharge peaks in winter, which in turn increases the risk of flooding. When it comes to precipitation, it is not only the amount that is relevant, but also when and how it falls: as snow or rain, in summer or winter, as steady land rain or short-term heavy rain. High variability in precipitation can have dramatic consequences. If heavy rain falls on dry soil, it cannot seep away and drains off on the surface. This water is then not only lacking in soil moisture or groundwater, but can also trigger floods. In fact, the water can actually vanish into thin air: especially at high temperatures, evaporation from the soil increases, which can dry out even more as a result.
