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Drought is a natural disaster that puts water resources and ecosystems under great pressure. It is a sustained occurrence of below average natural water availability over a large region, reflecting a deviation from normal conditions of variables such as precipitation (meteorological drought), soil moisture (agricultural drought) and groundwater and streamflow (hydrological drought). Drought is not the same as aridity, which is a permanent feature of climate. Drought, in contrast with aridity, can occur in most climates, including Norway. One example is the drought in 2002/2003 that caused electricity prices to reach unprecedented levels and wells to dry up. A thorough understanding of drought and low flows and their effects on the environment, society and economy is therefore important. In a drought situation, streamflows are low or even zero. However, low flows occur as a natural part of the hydrological regime in a river, and low flows do therefore not automatically imply that there is a drought.
Norway is highly diverse with respect to physiography, climate and hydrology. As a result the low flow season varies. In the southernmost part of the country and along the coast, low flows occur in the summer caused by a lack of precipitation and high evaporation losses. At higher altitudes and latitudes, winter low flows are dominant due to snow storage and frost. In periods of low flow more than 80 % of the water in the stream comes from groundwater (i.e. baseflow)
Drought and low flow analyses and R&DInformation about low flow characteristics is important for various aspects of water resources management and engineering such as hydropower planning, determining allowable water transfers and withdrawals, and decisions regarding environmental flows. Applications of the European Water Framework Directive to assess the state of water bodies may also require information about low flows. In Norway a regional regression procedure is used to estimate various low flow characteristics at ungauged sites.
Knowledge about climate change effects on low flows and drought is important for water resources management and climate change adaptation. NVE is monitoring and analyzing changes in low flows and drought and is also studying climate change effects on droughts and other factors through participation in the EU-funded WATCH project.
More information about droughts and low flows can be found at http://www.geo.uio.no/edc