Low flow or drought refers to water levels and discharge conditions that are significantly below the normal values. Severe low-flow events, which can occur as a result of lengthy periods of low precipitation, have major impacts on aquatic ecosystems. For example, fish fauna can be endangered by falling water levels. Low flows can also affect water use: for example, water shortages can interfere with agricultural irrigation or hydropower production. If the frequency of low-flow events increases, this intensifies competition between the water requirements of ecosystems and consumers.
At the discharge measurement stations in the Alps, the frequency of low-flow events has tended to fall since the 1970s; previously, there were significantly more years with an above-average number of low-flow events. This development is attributable to the increasingly warmer temperatures in winter, as a result of which less precipitation is retained in the form of snow (i.e. more discharge, less retention), and melting processes occur more frequently. There are no indications of a similar trend at the discharge measurement stations outside the Alps, though a particularly high number of low-water events occurred in 1947, 1949, 1962 and 2003. In lower-lying catchment areas, low water levels are caused by prolonged dry periods and are normally observed during summer or autumn. It is likely that lengthy dry periods will become more frequent as a result of climate warming.
Within the European indicator system maintained by the EEA, NM7Q projections are calculated for the period from 2071 to 2100. Furthermore, trends are reported for the number and severity of river flow drought events in Europe over the period from 1962 to 1990.
The annual minimum discharge levels recorded during 7 consecutive days (NM7Q) were evaluated for 41 permanent FOEN discharge measurement stations where there are no major anthropogenic influences (i.e. no power plants, major withdrawals or releases in the catchment area). An annual minimum discharge is defined as a low-water event if its flow rate is lower than the NM7Q10 flow calculated for the station (NM7Q10 flow = level below which discharge only falls statistically every 10 years). Data are evaluated separately for discharge measurement stations in and outside the Alps.