Climate-induced changes in the precipitation regime have been observed in Germany for several years impacting on water availability. Dams are affected but also sectors such as navigation, agriculture, forestry, hydropower, energy production, industry and industrial water supply, and of course drinking water supply. The need for long-term and adaptive planning for dry periods has become apparent since the summer of 2018 at the latest.
According to the IPCC (Intergovernmental Panel on Climate Change), such dry periods will occur more frequently and increase in intensity. Especially challenging are coincidences of unfavourable consecutive meteorological conditions, e.g., a dry summer following a dry winter. Current projections show a sharp increase in drought intensity and duration for Germany (Samaniego et al. Anthropogenic warming exacerbates European soil moisture droughts, Nature Climate Change 8(5): 421-426. 2018). Drought conditions currently defined as extreme will become normal with 3°C warming. The resulting impacts on soil moisture, groundwater, runoff, and concomitantly on all water-dependent uses will be consistently negative. Water storage will therefore become more important in the future and meeting existing and competing uses will become more difficult. In times of dry spells, prioritization is essential and lower prioritized uses may need to be adjusted in favor of higher prioritized uses. Therefore, affected stakeholders, especially dam operators and water management administrations, signal the need for additional water management and assessment procedures. Thereby, a transparent and comprehensible method for the early detection of dry periods is missing.
The initial situation at the dams of the Thüringer Fernwasserversorgung (TFW) is that already in the last 30 years a significant shift of the flow regime has taken place towards a greater seasonality. Whereas in previous decades an average of 7 to 8 months per year were available for reservoir refilling, this has now been reduced to only 3 to 5 months. As a result, water storage to counter the increased seasonality is more important than ever before. At the same time, water demand is increases due to higher water stress among various water user groups (e.g. for irrigation water from dams). Often, time series of the last 80 to 100 years provide the basis for allocating the available water quantity and consistent supply. However, as mentioned above, the last 30 years differ significantly. This creates an urgent need to shift from static operation rules to dynamic and adaptive operation rules for reservoir management.
The dry year 2018 revealed the deficit situation also in the urban area of Erfurt. The water shortage in combination with extreme low flow conditions in rivers were responded to by the local water authorities with an area-wide general order limiting water uses. With the commissioning of the so-called “Westring cascade” in 2020, water from the Thuringian Forest 45 km away will be diverted through a pipeline where it is used to generate hydropower before it will be fed into the drinking water supply network north of the city (site of the BUGA 2021) The operation of this new service water pipeline is closely linked with the operation of the surrounding water infrastructure and constitutes an important part of this project.