Summers are simply summers – that was the case in Germany for years. For some time now, however, there have also been repeated heat waves, droughts and water shortages in this country. Time for a differentiated view.
When the ground is so bone dry that it cracks, the word crust takes on a whole different meaning. In Germany, too, drought is becoming more and more common, with rivers running out of water and crops starving in the fields. Experts distinguish four types of drought, as climate researcher Harald Kunstmann from the Karlsruhe Institute of Technology (KIT) and the University of Augsburg says. The lack of precipitation compared to a reference period is referred to as meteorological drought. That could be 30 years, for example, as Kunstmann explains. “That can vary from country to country.” In some regions there is not enough data about such long time series.
According to Kunstmann, the three other droughts are primarily about their effects: in the case of hydrological drought, for example, the effects of insufficient snowmelt on the levels of lakes and rivers are considered – a phenomenon that can be seen on the Po in Italy. According to the definition of the German Weather Service (DWD), one speaks of hydrological drought when it is at least four months drier than usual. There is talk of agricultural drought after just two months. Then there is a risk of crop losses. For the weather service, the socio-economic drought occurs after one year of water shortage, which slows down the manufacturing economy. Basically, these are the consequences of the other types of drought, for example on the supply of vegetables, fish and hydropower, says Kunstmann.
This also raises the question of how humans can intervene – for example using drought-resistant seeds or irrigating fields. However, the expert makes it clear that a deficit in precipitation does not automatically result in a deficit in soil moisture. “It’s not that easy.” A little bit of rain every day is best for the soil. During dry periods, it always depends on how the precipitation falls at different times and how the plants grow. “That can mean something different for corn than for rapeseed.”
Cereals generally need less water than vegetables, says Frank Glante, head of the “Soil Condition, Soil Monitoring” department at the Federal Environment Agency (UBA). Farmers could also plant catch crops on their fields to reduce evaporation. Because wind contributes significantly to drying out, explains the expert. The condition of the soil is also important: the richer in humus and clay the soil is, the better it can store water in the upper layers, says Glante. Silt soil stores the most so-called plant-available water, as Florian Stange from the Federal Institute for Geosciences and Natural Resources (BGR) explains. The particles in the soil are medium-sized – between sand and clay.
Loess soil, which was voted “Soil of the Year” in 2021, is a very good water reservoir, says Stange. While sandy soil needs a shower every week, loess soils can go six weeks without rain. Loess consists mainly of silt, sometimes also of fine sand. With an optimal pore structure, it stores water – unlike clay – in such a way that plants can tap it easily. According to the Federal Institute, it occurs in the Jülich-Zülpicher Börde on the northern edge of the Eifel, through the Magdeburg Börde to the Leipzig lowland gorge and the Saxon hill country. In southern Germany, the loess was deposited in the areas north of the gravel fields of the Alpine rivers, in the Upper Rhine Valley and in the low mountain range in the wide valleys and basin landscapes.
According to Stange, organic matter in the soil, i.e. humus, is important. On the one hand, it stores water itself, as moors and peat soils show. On the other hand, it forms structures in the soil that help to store water. However, this only reduces the problem and does not solve it: “Humus does not turn sandy soil into silt,” says Stange. Therefore, good loess soils are usually used as arable land. Forest tends to grow on poorer soil. “That’s why we now see the problems with the drought in particular there,” explains the expert. And the issue goes deeper: Even a heavy downpour would not be enough to fill up the groundwater reservoirs, which have dried up in many places after several dry years, as Glante from the Federal Environment Agency emphasizes. “It’s such a long dry spell that it would take several weeks of rain to fill it up.”
With its drought monitor, the Helmholtz Center for Environmental Research (UFZ) simulates a soil moisture index from various parameters down to a depth of around 1.80 meters and up to 25 centimeters. The federal states have their own and – typical of federalism – partly different approaches. There is no claim to overriding validity, says Kunstmann. From his point of view, it is not crucial that all scientists agree on the exact number of meters in which, for example, soil moisture is considered. “The basic assumptions are the same.”
