Too little rain, too little water in the landscape - so simple, so serious.
Dried out watercourse in the Demnitz Mühlenfließ. | Drone photo: Hauke Dämpfling
Even if the drought is obvious because freshwaters carry less water, plants wither and the soil becomes brittle and cracked, the drying out itself is a complex process in which the spatial context plays an important role. Professor Dörthe Tetzlaff from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) is investigating this interplay of environmental factors, water storage and water flows in the landscape, focussing on the Berlin-Brandenburg region. In the following, she provides an overview of various aspects of the topic.
The North German Plain has recently been experiencing the consequences of climate change, with five consecutive years of negative rainfall anomalies between 2018 and 2022. This means that less rain fell than was lost through evaporation (from soil or water surfaces) or transpiration (from plants).
My research group is investigating the water balance of the landscape in the Demnitz Mühlenfließ catchment area. In this long-term environmental observatory, ecohydrological processes have been quantified on a highly resoluted temporal and spatial scale since 1980. While the main focus used to be on agricultural water pollution, today we focus on ecohydrology and the effects of land use on water security.
Groundwater: Aquifers have a low storage capacity and are vulnerable to human water extraction and climate change
Most aquifers in the Berlin-Brandenburg region that are important for retaining water in the landscape are very close to the surface and have a low storage capacity. This is reflected in the age of this shallow groundwater, which is a maximum of five years old. These water reservoirs are therefore particularly sensitive to human water abstraction and hydroclimatic influences.
Groundwater levels have fallen in recent years. For instance, the net recharge to groundwater in the drought year of 2018 was only one millimetre. By contrast, in the rainy year of 2010, this value was 376 millimetres. Based on water balance data, the storage of groundwater showed a downward trend from 2007 to 2022, declining by an average of around 2.1 millimetres per year. In 2018, the groundwater level fell by 20 millimetres.
One wet year is not enough to replenish the groundwater reservoirs
Even a rainy year cannot compensate for these water deficits. Our team has calculated that, following the many dry periods in recent years, at least two years of evenly distributed rainfall at the average level would be required to replenish the groundwater reservoirs. This has not yet occurred, despite the 'wet summer' of 2024. In fact, spring 2025 was extremely dry again, with several 'negative records'; March, for example, was the driest month on record.
Wetlands and river courses are under threat as superficial aquifers interact strongly with surface water
As surface waters in the Berlin/Brandenburg region are largely fed by groundwater, the state of the groundwater always impacts the state of the region's rivers and lakes. In several studies, we have shown that the watercourses in the Demnitzer Mühlenfließ are drying up, partly because they are disconnected from the groundwater for extended periods. In addition, the water quality has deteriorated.
When heavy rainfall occurs, the sandy soil cannot absorb the water well. The soil structure therefore needs to be improved and the humus content increased
Around 40 per cent of the total inflow to groundwater is lost during the summer months. The water that runs off into surface waters plays only a minor role here. Rather, the water losses can be explained by the high evapotranspiration rates. Around 90 per cent of the total precipitation in the Berlin-Brandenburg region is released back into the atmosphere through evapotranspiration.
As sandy soil is highly permeable to water, it is generally assumed that it can absorb large amounts of water quickly. However, sandy soil is highly hydrophobic, meaning it is water-repellent. During heavy rainfall, therefore, the soil is unable to absorb water effectively. Much of it runs off the surface. Increasing the humus content of agricultural land could improve infiltration rates and help retain precipitation in the soil. However, this is not feasible on a large scale. Another way of stabilising groundwater is through land use.
The way land is used has a decisive influence on the recharge of groundwater and losses through evapotranspiration
The Demnitz Mühlenfließ catchment area is characterised by a mixed land use cover: extensively farmed meadows, wetlands, farmland and forest. These different land uses result in spatially variable patterns of groundwater recharge. Coniferous forests in particular have high evapotranspiration losses, which inhibit recharge. For instance, the rate of groundwater recharge under forests can be 50 per cent lower than under other land uses. In contrast, new agricultural methods such as agroforestry and other mosaic-type land uses mitigate evapotranspiration while simultaneously increasing infiltration and groundwater recharge rates.
Retaining water in the landscape and replenishing groundwater: More moors!
The restoration of formerly drained wetlands also makes sense: for example, we were able to show that renaturation and the re-colonization by beavers along the Demnitzer Mühlenfließ river network have led to an increase in the groundwater level since 2000. Before 2010, less than five percent of precipitation recharged into groundwater. This has increased with restored wetland and the re-colonization by beavers although, the proportion of rainfall recharging groundwater remains below 10 percent.
Wetland restoration, greatly enhanced by increasing beaver populations, resulted in longer water transit times in the stream network, less linear storage-discharge relationships and a loss of daily stream variability. However, rising groundwater levels had to date had less of an impact on stream flow than we had expected. The proportion of rewetted moorland areas compared to agricultural land is probably still too low to significantly affect the discharge of water in the landscape. In addition, several small, rather than larger, contiguous moor areas were rewetted. We therefore recommend rewetting larger former moorland areas and, above all, connected portions. This could improve water retention and storage in agricultural landscapes in the long term.
Our results underscore the importance of developing integrated land and water management strategies in north-eastern Germany, where climate change is expected to lead to a further reduction in precipitation, a rise in temperatures and a decline in groundwater recharge. Successful approaches already exist, but they should be implemented more consistently.
Mai 2025
Zhengtao Ying; Doerthe Tetzlaff; Jean-Christophe Comte; Songjun Wu; Chris Soulsby
Hydrological Processes. - 39(2025)5, Art. e70141
http://dx.doi.org/10.1002/hyp.70141
Februar 2025
Famin Wang; Doerthe Tetzlaff; Christian Birkel; Jonas Freymueller; Songjun Wu; Sylvia Jordan; Chris Soulsby
Hydrological Processes. - 39(2025)2, Art. e70077
http://dx.doi.org/10.1002/hyp.70075
November 2024
Famin Wang; Doerthe Tetzlaff; Tobias Goldhammer; Jonas Freymueller; Chris Soulsby
Journal of Hydrology. - 648(2025)X, Art. 132420
