low water | The drought extends into the new year

Stranded boats on Lake Constance: The low water levels were particularly noticeable in eastern and southern Germany this year. Photo: IMAGO/imagebroker A new year begins next Sunday. Although the Gregorian calendar has to wait another two months, the new hydrological year begins on November 1st. What falls as snow in winter often only flows into the rivers months later when the snow melts. The hydrological year therefore covers the period from November 1st to October 31st. It makes it possible to balance precipitation and runoff over a continuous period and thus to better classify the development of the water balance.

"With the end of the discharge year 2025/2026, the NNQ - the lowest discharge ever observed - must be redefined at many gauges," says Enno Nilson from the Federal Institute of Hydrology (BfG) in Koblenz. Discharge is the amount of water that passes through a river cross section per second. Unlike the water level, it directly describes the amount of water flowing through and allows low water levels to be compared even if the river bed has changed.

As early as January, the BfG's new low water information system registered unusually low discharges in nine large river basins in Germany. These include the main river – such as the Danube, Elbe or Maas – and all tributaries. The winter rainfall was apparently not enough to compensate for the water deficit from the previous year.

The situation worsened as the year progressed. Images of dry riverbeds and stranded ships dominated the reporting on the dry summer. Cargo ships on the Rhine had to drastically reduce their loads in August. Freight traffic came to a complete standstill in some sections. In Austria, where hydropower is by far the most important source of electricity, electricity production from hydroelectric power plants was at times more than 40 percent below normal levels.

While at the beginning of the year the BfG classified around a fifth of the outflows as very low or extremely low, in September it was already 35 percent. The outflows from 1991 to 2020 serve as a benchmark: the lowest five percent are considered very low, the lowest percent is considered extremely low.

According to forecasts, the extreme low water phase is now coming to an end, says Nilson. Normally the water reservoirs fill up again in the coming months. However, since they have recently emptied significantly, says Nilson, this requires “above-average long and extensive rainfall, so-called land rain.”

But even in the past 30 days there was unusually little rainfall. In large parts of the Rhine and Danube catchment areas it was only about half as much as usual.

There was also severe soil drought in the summer, particularly in southern Germany. Rain at the end of August moistened the upper layers of the soil, but the deeper layers remained dry. In the warm, low-rainfall September, the soil dried out further in many places. Trees dropped their leaves prematurely, rapeseed could not be sown in many places in the southwest, and the sowing of winter barley had to be postponed in some cases.

As extreme as the dry year 2026 was, it fits into a trend. Satellite data from the Helmholtz Center for Georesearch (GFZ) in Potsdam show: The water deficit has worsened since 2015. Eastern and southern Germany are particularly affected.

This is not simply because there is less precipitation. In the long-term average, no downward trend can be seen for Germany. However, as temperatures rise, evaporation increases. So not necessarily less water is added, but more of it is lost to the atmosphere.

At the same time, the temporal distribution of precipitation is changing. Dry phases last longer, while rain falls more frequently in the form of heavy rain. This development is typical of a warmer climate, explains Thorsten Wagener, Professor of Hydrological Systems at the University of Potsdam, when asked. If heavy rain hits dry soil, the water often cannot seep away quickly enough and flows off above ground. This means it quickly reaches streams and rivers instead of remaining stored in the ground for a longer period of time.

Many rivers, especially the Rhine, are fed by melting glaciers and snow masses in spring. “Due to the increasing influence of climate change, we are losing the natural snow and glacier storage in mountain regions,” says Wagener.

Less snow falls and it melts earlier in the year. As a result, the meltwater flows away earlier and is missing later in the summer when heat and evaporation increase water requirements. The flows are increasingly shifting from summer to spring. Wagner explains: "If this problem overlaps with dry winter and spring as well as high evaporation in the hot summer, then we have a 'perfect storm' - or rather a 'perfect drought'."

However, looking for the cause solely in man-made climate change would be too short-sighted. “Since the 19th century, many German rivers have been straightened, shortened, fortified and expanded as waterways,” explains Christian Wolter, head of the river revitalization research group at the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) in Berlin-Friedrichshagen.

Due to this scarcity of paths, the water flows away much more quickly. On the fortified banks, the river can no longer spread laterally and instead digs deeper into the river bed. If the river bottom is deeper than before, the groundwater level in the area can also drop.

You can imagine the river and groundwater as two connected water basins in permeable soil: If the river bottom is deeper, a greater gradient is created between the groundwater and the river. The water then flows from the surrounding soil towards the river, and the groundwater level drops.

Moor rewetting, floodplain renaturation and forest conversion – all of this can contribute to water retention in the landscape and groundwater recharge, says Wolter. But it takes time for such measures to have an effect. A comparatively quick-acting measure to retain water would be to close drainages in the winter months. Drains are underground pipes that collect excess water from agricultural land and direct it into ditches or streams. But there is no panacea for low water, warns Wagener. The “critical measure” is climate protection.

It is also uncertain whether the coming winter will bring relaxation. At the moment, everyone is watching the development of El Niño, said the hydrologist. The climate phenomenon could lead to a very warm winter in Central Europe - with correspondingly little snow. Then the Gregorian New Year would also have the worst possible hydrological auspices.

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