Man-made Reservoirs

A reservoir is an artificial lake called man-made reservoir. It can be formed by building a dam across a valley, by excavating the land or by surrounding a piece of land with dykes and diverting a part of the river flow into the reservoir. The water is stored in the reservoir and can be used for irrigation, hydro-power or as a water source for domestic or industry use. Man-made reservoirs are also very effective constructions to control unexpected floods (see also stormwater management). A reservoir is fed by precipitation, rainwater runoff or from a constant flow of a river. Water loss can occur due to evaporation (especially in arid regions) and depending on the reservoir bottom due to percolation (small reservoirs are often lined). Sediments from rivers or surface runoff can reduce the storage volume of a man-made reservoir significantly (FAO 1992).

Water stored in a valley usually has a higher level than the valley bottom downstream of the dam. Because of this difference in level, the valley can be irrigated by a gravity system or other distribution systems. Water can be taken from the reservoir via a concrete or steel pipe. This pipe connects the reservoir to an irrigation canal downstream. A valve is usually located on the upstream end of the pipe to control the discharge of water into the canal (FAO 1992). The kinetic energy of reservoirs is often used to produce electricity (see also hydropower small-scale and hydropower large-scale).

Ecological Impacts

Where no such water-body previously existed the presence of a reservoir in a drainage basin and the abstraction of significant water amounts for storage upstream significantly impacts the watercourse, the flora and fauna, and the human inhabitants in the drainage basin. These potential impacts should be identified and thoroughly examined prior to reservoir construction, in order to comprehensively assess the total value of the reservoir project. Procedures to identify and properly evaluate potential environmental, social and economic consequences of reservoir construction involve so-called ‘Environmental Impact Assessment’ (EIA). Such an assessment is now obligatory by law in many countries for all new dam constructions (UNEP 2000).

Ecological impacts of reservoir dams have been reported from various aspects such as barrier for migratory animals like anadromous fish, eutrophication of reservoirs by plankton blooming, decreasing flow volumes in tail waters, stabilisation of flow regimes by flood peak cut, changes in thermal regimes of river water, river bed degradation and increase in substrate grain size by sediment trapping, etc. (TAKEMON 2006). Furthermore big dams and extraction of water (e.g. for spate irrigation) can create riparian conflicts (see water conflicts). Also read the paragraph “Impact on Environment” in the rivers factsheet.

Adapted from UNEP (2000)

Like lakes, reservoirs range in size from pond-like to very large water-bodies (e.g. Lake Powell, U.S.A.). The variations in type and shape, however, are much greater than for lakes. The term ‘reservoir’ includes several types of constructed water-bodies and/or water storage facilities:

• Valley reservoirs – created by constructing a barrier (dam) perpendicular to a flowing river.
• Off-river storage reservoirs – created by constructing an enclosure parallel to a river, and subsequently supplying it with water either by gravity or by pumping from the river.

The latter reservoirs are sometimes called embankment or bounded reservoirs, and have controlled inflows and outflows to and from one or more rivers. In addition to single reservoirs, reservoir systems also exist, and include cascade reservoirs - consisting of a series of reservoirs constructed along a single river, and inter-basin transfer schemes – designed to move water through a series of reservoirs, tunnels and/or canals from one drainage basin to another.

Pumping from a Reservoir for Irrigation

Adapted from FAO (1992)

The fields located around the reservoir upstream of a dam or surrounding a natural lake are higher than the reservoir or lake's water table. Here irrigation is only possible with the help of pumping stations, manual or motorised pumping.

The water level in the reservoir is usually highest at the end of the rainy season, and lowest at the end of the dry season or the irrigation season. Pumps installed at reservoirs and lakes must be able to handle these fluctuations, which are not only vertical, but even more pronounced horizontally, because the water recedes back to the lowest parts of the reservoir. A dead branch of a river can also be made to function as a reservoir. The branch is filled with water during the wet season and closed off during the dry season so that the stored water may be used. Due to the low water level, pumps are normally needed to irrigate fields from such a reservoir.

Especially for large reservoirs costs are high. It must be designed by engineers and heavy machinery is necessary. Furthermore, costs for material as well as O&M should be considered. If it is constructed for a hydropower water source costs are even higher because much more technical equipment as well as operation and maintenance is needed.

Because reservoirs are man-made water-bodies, they are more amenable to artificial operation and regulation than lakes. As previously noted, operational possibilities unique to reservoirs include the ability to discharge known volumes of water at predetermined times, and selective discharge of water from different water layers within the reservoir. This must be planned carefully as it directly impacts the environment as described above. Also read the document “Reservoir Operations and Managed Flows” (WMO and GWP 2008).

Dams, especially the very large ones, must be checked regularly to ensure their stability and security. Furthermore, many man-made water reservoirs are affected by high sedimentation rates. The accumulation of sediments in the reservoir reduces the main reservoir asset i.e. its volume capacity. Moreover sediments can negatively affect pumping and hydropower equipment.Therefore the designers should consider the soil erosion and sediment transport (CHANSON and JAMES 1998). There are several approaches to minimise or deal with sedimentation.

When a reservoir serves different functions it is nearly impossible to operate each function at its maximum level. For example, a reservoir that provides irrigation, power generation (see small scale and large scale hydropower), flood control, and recreational use may cause conflicting demands by its users (WATERENCYCLOPEDIA 2011).

Faecal pollution and other contamination of reservoirs has to be prevented by wastewater treatment and buffer zones in case of non-point sources of pollution (see also the factsheets on lakes or water source and well protection). If the reservoir is also used as a source of drinking water, please also check water purification as a measure to protect human health.

It should also be considered, that surface water sources can lead to mosquito breeding.