Expertise
Rain in July balances water levels in the forest
Tanja Sanders, Marco Natkhin, Björn Christen | 01.08.2025
This July the weather, while not particularly suitable for outdoor swimming, was good news for the forests. After the long dry spell in spring, plenty of rainfall on the Thünen experimental site in Britz balanced the deficit out. Other research plots showed that the moors are also well filled.
July is the wettest month of the year so far, with an average of 76 millimeters of rain (average for the years 1993-2022) per square meter. This is shown by long-term measurements taken at the Thünen Institute of Forest Ecosystems' experimental site in Britz, Brandenburg. This year, July saw significantly more precipitation, with 190 mm per square meter. The deficits which had built up due to the very dry spring have been summarily offset (Figure 1, orange line). Overall, the total amount of precipitation reached the long-term average (blue line). The black line clearly shows the difference compared to 2024. At that time, there was above-average rainfall in winter and spring
Last week's rainfall is also reflected in the water levels of the moors along the power line in Spechthausen (Brandenburg) and at Lake Drewensee (Mecklenburg-Western Pomerania). The area along the power line is an undrained bog, while the area at Lake Drewensee is a drained bog. In both bog areas, the bog water level have risen by 40 centimeters in recent days. When it rains, bogs absorb a lot of water very quickly and then release it continuously over a longer period of time.
Figure 1: Cumulative precipitation in the open space in Britz – The high rainfall in July compensated for the drought that began in February in the annual balance.
Precipitation amounts vary greatly from place to place
Summer precipitation often occurs in the form of convective precipitation, better known as showers or thunderstorms. These are usually regional and lead to large differences in rainfall amounts over relatively small areas. This year, however, bands of rain often stretched across the whole of northern Germany. It rained frequently from Dresden to Hamburg. And although it feels like this was the wettest July ever, it rained much more in July 2011 (Fig. 2). At that time, there was flooding in the northeast of the country, for example in Halle/Saale and along the Oder River.
Figure 2: Total precipitation in July for the years 1993 to 2025. July 2011 ranks first with 230 mm per square meter, followed by July 2025 in second place. The driest Julys were in 2006 and 2013 with less than 20 mm.
Moist soil under the Britz pine trees
On the test site in Britz, scientists have observed rainwater seeping into the soil — a rare occurrence, especially under the ever-dry pine trees. Currently, this “water front” reached three meters under the pine and oak trees.
One reason for the infiltration was heavy rainfall, which leads to infiltration especially under the pine trees. Most of the water came from heavy rainfall on July 21. Almost 59 millimeters fell within six hours.
