Last year and at the beginning of this year, it rained a lot in Berlin and Brandenburg. This is a positive change for the soils, lakes and rivers in the region after the dry previous years. Professor Dörthe Tetzlaff from IGB and Humboldt-Universität zu Berlin is investigating the water balance of the landscape taking the Demnitzer Mühlenfließ (area tributary of the Spree) in Brandenburg as an example. Their data show that although lots of water has reached the ground, the groundwater has not yet returned to the state it was in before the dry year of 2018. Moreover, in 2023, a year with high precipitation, a particularly large amount of water evaporated due to high temperatures. So, no reason to sound the all-clear?
The Drought Monitor of the Helmholtz Centre for Environmental Research (UFZ) on the state of soil moisture in Germany has shown little yellow, orange or red in recent months. According to the report, it was not even abnormally dry in large parts of Germany and the categories of moderate, severe, extreme or exceptional drought did not occur on a large scale.
"It is very positive to see that the water deficit in our soils has been replenished by the rainfall last year and at the beginning of this year. In some regions of Germany – including here in Berlin/Brandenburg – the soils are completely saturated," said Dörthe Tetzlaff. The "soil water deficit" is an important input parameter for the UFZ Drought Monitor. The water content in the upper soil layers down to a depth of 1.80 metres is an indicator of the water in the landscape.
Groundwater reservoirs not yet replenished after more than five years of drought
However, the deeper groundwater is not covered by this approach. "Our current data show that the groundwater levels in the very dry Berlin-Brandenburg region have risen, but have not yet reached the 'normal values' from before the 2018 drought," explained Dörthe Tetzlaff.
In a study from 2021, she and her team had already calculated for the 60-square-kilometre catchment of the Demnitzer Mühlenfließ that it would take up to four years of long-term average precipitation distributed regularly throughout the year to recover after an extreme drought like the one in 2018/2019. This does not yet consider the extreme drought of 2022. "One wet winter is therefore not enough to replenish the groundwater and overall water deficits in our landscapes. The water has to infiltrate several meters of unsaturated soil before it reaches the aquifers, and also should not be lost via surface runoff on saturated fields" said Dörthe Tetzlaff. The more than 30 years of long-term IGB data from the Demnitz Mühlenfließ show an overall downward trend in the long-term groundwater level since 2011, which has intensified since 2018.
At high temperatures, a large amount of water evaporates from the soil and plants
The researcher also points out that it is not only precipitation that plays a role in the water balance, but also the proportion of water that evaporates again. "Evapotranspiration is actually the most important component of the water balance in Berlin/Brandenburg – especially if we assume rising temperatures as a result of climate change, as we recorded in 2023, for example. This will lead to significantly higher evaporation via soil and transpiration via plants. Spring times with above average warm temperatures, as we see it currently, result in high evaporation rates already, in particular in areas with still bare soils. In Brandenburg, the annual average potential water transpiration and evaporation via plants and soil is in most years higher than the annual precipitation," said the ecohydrologist.
When groundwater levels are low, the connection to surface waters is lost
Less groundwater and higher evapotranspiration also lead to lower water levels in surface waters: Compared to the period from 2001-2017, the discharge of the Demnitz Mühlenfließ, for example, was reduced by an annual average of 64 per cent in the years from 2018-2022. More often, the river carried no water at all – sometimes for several months. "This was because the connection between groundwater and surface water was interrupted due to the low groundwater level," explained Dörthe Tetzlaff. However, this was mainly the case in the non-restored, straightened sections of the stream network. Stream sections with larger floodplain or moorland areas showed greater resilience to drought.
Allowing rivers to meander to maintain the important connection between groundwater and surface water
This is because the restoration of streams promotes water retention in the landscape: if the water has a longer, meandering course, does not run off so quickly above ground and can therefore infiltrate better, the groundwater is also fed. The creation of floodplains and wetlands along the Demnitzer Mühlenfließ has had a positive effect on the groundwater: Following restoration measures in parts of the catchment in 2001, the groundwater level rose significantly in the years 2001-2003. And is still higher there today.
Due to the groundwater table close to the ground surface in wetlands and floodplains, the river can stay connected and flow for longer. The groundwater then accounts for more than half of the inflow. Conversely, the water table in the wetlands remains very close to the ground surface even in the summer months when the river is not flowing. "These results clearly show how the water flow between groundwater and surface water can stabilise itself in a restored stream system and is also less susceptible to intermittency during droughts," said Dörthe Tetzlaff.
"Planting" water: Forest conversion to more mixed forests
A lot can also be done to retain water in the landscape through sustainable land use management: As Dörthe Tetzlaff's team has shown in another study, different forms of land use have a decisive influence on water absorption and release. For example, a mixed land use mosaic contributes overall to more water infiltration into the soil, reaching the groundwater and less evaporation. In addition, the cultivation of particularly water-intensive crops in our region must be critically assessed. Forest conversion to more mixed forests instead of coniferous forest monocultures is also urgently needed.
A recent study by Dörthe Tetzlaff based on modelling and field studies in the Demnitzer Mühlenfließ shows that replacing conifer forests with uneven-aged mixed forests with younger broad-leaved trees has the greatest potential for reducing total evapotranspiration and increasing groundwater recharge. For forests, a 50% increase in the proportion of broad-leaved trees was projected to result in an 11% increase in groundwater recharge across the catchment. The mixed-forest management alternatives also reduced groundwater turnover times, which would support more rapid recovery of soil moisture and groundwater stores following droughts.
"We have to learn from the dry years, urgently rethink and implement the existing climate-resilient adaptation strategies now, because after the drought is before the drought," emphasized Dörthe Tetzlaff. Long-term planning and sustainable management of our landscapes and the water bodies embedded within is most important in times of global change.
