Overall reservoir stocks

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The use of storage reservoirs helps overcome the uneven distribution of natural water resources with time (see indicator on precipitation). Run-off in the wet season can be held back and used in the dry season (seasonal regulation), while water available in wet years can be stored and used in dry years (interannual regulation). The beneficial aspects of reservoirs in safeguarding water resources and supplies have to be balanced against the significant impacts that their construction and subsequent operation have on natural landscapes and ecosystems.

The primary functions of reservoirs in Europe are for hydroelectric power production, storage for public water supply and irrigation. Water is not always available to meet demands. In particular, water for urban use must be guaranteed and irrigation demands often need to be met during the dry season, when river discharges are at their annual lowest levels. Water storage by reservoirs helps to overcome this temporal unavailability of freshwater resources. In Europe, approximately 13% of mean annual runoff is stored behind dams. It represents a significant increase in the standing stock of natural river water, with residence times for individual reservoirs spanning less than one day to several years.

The countries with the highest percentage volume of stored water in relation to their annual renewable freshwater resources (over 20%) are Turkey, Spain and Cyprus (see Figure 5.12). These countries also use the highest percentage of their resources for irrigation. This activity demands the largest water volumes in the driest seasons, requiring winter storage. Spain and Cyprus are considered to be water stressed whilst Turkey has low water stress (see indicator on the water exploitation index). In many countries (such as Austria, Finland, France, Greece, Ireland, Italy, Norway, Portugal and Sweden) the majority of major reservoirs are used for hydropower production. In particular, the primary purpose of major reservoirs in Sweden and Norway is almost exclusively for hydroelectricity (EEA, 1999).