Runoff year 2025: It was far too dry, especially from February to September

In contrast to the two previous, very wet discharge years, the 2025 discharge year in the Ruhre catchment area was again too dry at 885 millimetres (minus 16 % compared to the long-term average, preliminary calculation). This below-average value was primarily due to the eight months from February to September, in which less than two thirds of the usual precipitation fell during this period (minus 36 %). As a reminder: from 2009 to 2022, the Ruhre catchment area had experienced the longest consecutive phase of excessively dry discharge years before this was broken two years ago.

February (minus 74 % precipitation) and March (minus 82 % precipitation) posed a particular challenge for the Ruhrverband's reservoir system in the current runoff year 2025, because while these two months can usually be used to build up water reserves, in the 2025 runoff year the Villigst gauging station was already under an obligation to make additions from the reservoirs from March onwards. As a result, the total reservoir content fell sharply until the end of April, which is not typical for the season. A few rainy days at the beginning of May brought a further increase to the highest level in the discharge year on 8 May (88 % of full storage), but this was followed by a continuous build-up until the last third of October due to the often low precipitation. The lowest level of the discharge year was reached on 23 October at 64 % of full water level.

The change in the law, which came into effect for the first time this year, had a positive effect, allowing the Ruhrverband to comply with lower limits for the minimum water flow in the legally defined control cross-sections from July. Compared to the old, higher limit values, this enabled almost 29 million cubic metres of water to be saved in the Ruhrverband's reservoirs. This corresponds to 2.3 times the capacity of the Ennepetal dam! Nevertheless, the Ruhrverband recorded the third-highest number of days requiring a contribution in 35 years at the Villigst gauge, traditionally the most heavily loaded of the two control cross-sections.

Another look at the temperatures: although the average annual temperature in the Ruhr catchment area as a whole was below the magic 10-degree mark for the first time in three years, at 9.6 degrees it was still 0.7 degrees too warm compared to the long-term average of the 1991 to 2020 time series. November, May and July were almost average (deviation from the long-term average 0.1 degrees each), while the other nine months were each warmer than usual. April (10th place with 9.9 degrees) and June (8th place with 17.3 degrees) made it into the top ten warmest monthly average temperatures since records began over 140 years ago. The discharge year 2025 also confirms the realisation that the challenges for public services caused by global warming and the associated extreme weather phases are the "new normal".

Background: Discharge years (also known as hydrological years or water management years) differ from calendar years so that precipitation in the form of snow and ice, which falls in early winter, can also be recorded in the annual balance. This is because they only affect runoff as meltwater in the following calendar year. In Germany, a DIN standard defines the runoff year from 1 November to 31 October, because experience shows that water reserves are at their lowest at the end of October.