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[[Image:subsurface harvesting systems.png|right|80px]]
[[Image:SubsurfaceDamSite.JPG|thumb|right|200px|Subsurface dam being built out of clay soil across a sandy riverbed in Kitui, Kenya, 2007. They are working in the dry season; note the riverbank is much higher than they are working. Photo: [http://www.waterforaridland.com/publications/Book8/book8chapter6.asp Water for Arid Land.]]]
The use of subsurface dams in naturally occurring alluvium creates groundwater storage upstream of the dam, raising the water table and preventing evaporation losses. These are structures that reduce or stop the flow of shallow groundwater. A subsurface dam is constructed across a valley in a seasonally dry, sandy riverbed, by digging a trench down to the bedrock or other impervious layer (clay) at the base of the river. The dam, which is placed in the trench, may consist of a wall or screen and covered with excavated material until it is completely concealed. The refill material must be properly compacted. There are two types of groundwater dams: a subsurface dam (which is explained here) and a [[sand dam]]. A subsurface dam is the easier one to build of the two. The structure is built inside an existing riverbed, so that after a flooding event, for example, water is increased due to a new and higher water level within the sand.
===Suitable conditions===
* Siting requirements are similar to the [[sand dam]].<br>
* Subsurface dams should be built where sand volume is already sufficient to store water.<br>
===Resilience to changes in the environment===
====Drought====
'''Effects of drought''': Can dry up. <br>
More information on managing drought: [[Resilient WASH systems in drought-prone areas]].
===Construction, operations and maintenance===
'''General advice on cement''': A common cause of cracks in structures and linings (e.g. in tanks, dams, waterways, wells) is errors in mixing and applying the cement. First of all, it is important that only pure ingredients are used: clean water, clean sand, clean rocks. The materials have to be mixed very thoroughly. Secondly, the amount of water during mixing needs to minimal: the concrete or cement needs to be just workable, on the dry side even, and not fluid. Thirdly, it is essential that during curing the cement or concrete is kept moist at all times, for at least a week. Structures should be covered with plastic, large leaves or other materials during the curing period, and kept wet regularly.
Users may need to establish a local committee to manage issues, such as controlling or supervising water use, preventing water contamination, carrying out O&M activities, financing O&M, and monitoring how much stored water is still available (a piezometer or auger hole may be installed to allow a caretaker or watchman to estimate how much water is left and decide if rationing has to be introduced). Proper management may also help to prevent social conflict. For O&M tasks at the dam site, a person who lives or farms near this site could be appointed. This person could also be responsible for water alloca- tion and be involved in monitoring activities, if users obtain the water near to, or at, the site. His or her authority should be clear and accepted by all users.
===Costs===
'''Initial costs''': A 3500 m3 dam costs US$2.40/m3 in Kenya and US$3.90/m3 in Tanzania.
* Operation and maintenance: low
===Field experiences===
* A subsurface reservoir in Pernambuco, Brazil has an average depth of 4m, is 50m wide, and 500m long, storing about 4,000m3 of water.
* [http://www.waterforaridland.com/publications/Book8/book8chapter6.asp Chapter 6: Community Water Projects]. Scroll down to the section, 6.2 Subsurface Dams. Waterforaridland.com.
===Acknowledgements===
* Brikke, François, and Bredero, Maarten. [http://www.washdoc.info/docsearch/title/117705 Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff] or ([http://www.who.int/water_sanitation_health/hygiene/om/wsh9241562153/en/ alternative link]). World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.