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Angelina Tittmann

Rainy summer, dry soils

July and August brought heavy rain and flooding in some places. More than enough precipitation to make up for the recent dry years, one might think. But it is not that simple! Taking a sub-catchment of the Spree – a drought-sensitive region in north-eastern Germany – as an example, IGB researchers show that although precipitation in recent weeks has been higher than the long-term average, groundwater levels and soil moisture remain below normal.

What the scientists see in their data is consistent with general observations: Drying soils, creeks that no longer flow, wells that are empty and lower crop yields are easily recognisable signs that water storage is insufficient to support groundwater recharge and evapotranspiration. | Photo: Hauke Dämpling/IGB

The investigated region in Eastern Brandenburg suffers from drought: In 2018, 30 % less precipitation fell compared to the long-term average. In the two following years 2019 and 2020, it was 10 to 15 % less in each year. And in the current year, too, there was too little rain until June. But how do periods of drought affect our water resources? And how much precipitation is needed to compensate for the shortage?

In order to assess these questions, IGB researchers are investigating how water is distributed across the landscape, how it runs off and how much of it can be stored. Since 2018, the research group led by hydrologist Professor Dörthe Tetzlaff has been analysing soil water samples from a 66 sq km groundwater-dominated lowland catchment of the Demnitzer Millcreek, where there are various forms of land use. The scientists are focusing primarily on the vegetation period – i.e. those months in which plants are actively growing. With the help of stable water isotopes and modelling, they quantify groundwater recharge, surface runoff and the evaporation rates of soils and vegetation, and thus also determine the flow paths and age of the available water. In this way, they find out how much water is stored where and for how long in the landscape.

 

Figure precipitation, groundwater and shallow soil moisture

To date, the recent dry years have had an impact on groundwater levels and shallow soil moisture. The data were collected in the catchment area of the Demnitzer Millcreek, a sub-catchment of the River Spree. They show the divergence from the long-term average. | Figure: Aaron Smith/IGB

 

As the data show, groundwater recharge occurs with a time lag. For example, the groundwater level reached its lowest level in 2020 after the drought summer 2018. It was more than 20 % – or 40 cm – below the normal groundwater level. Even today, despite the increased precipitation in the past two months, groundwater levels are still too low. The situation is similar for upper soil moisture: recent rains have not resulted in the soils being able to absorb enough water. Compared to the average of the last 13 years, about 15 % is missing.

“We would need at least four years of average rainfall, or about 600mm per year in this region, for groundwater levels to recover to pre-drought levels, and one year of average precipitation to replenish soil water storage,” predicts Dörthe Tetzlaff. With increasing extreme events such as droughts, sustainable land management strategies would therefore be needed that are adapted to water availability and increase resilience towards climate change.

Selected publications
Contact person

Lukas Kleine

Doctoral Student
Research group
Landscape Ecohydrology
Projects

VeWa

Vegetation effects on water flow and mixing in high-latitude ecosystems
Contact person
Dörthe Tetzlaff
Department
(Dept. 1) Ecohydrology and Biogeochemistry
Start
10/2013
End
03/2019
Topic

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