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[[Image:subsurface harvesting systems.png|right]]
[[Image:SubsurfaceDamSite.JPG|thumb|right|300px200px|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.
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! width="50%" style="background:#efefef;" | Advantages
! style="background:#ffdeadf0f8ff;" | Disadvantages
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| valign="top" | - Subsurface dams are not likely to deteriorate, with little danger of breaching <br>
==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.
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|[[Image:rsr 393.jpg|thumb|none|200px|<font size="2"><center>Project 393</center></font>|link=http://www.akvo.org/rsr/project/393/]]
|'''AKVO RSR Project:''' [http://www.akvo.org/rsr/project/393/ Dawa Eresa Subsurface and Sand Dam project]
This project aims to provide the people in Konda and Kora with access to water. Currently the water source they use is of poor quality without year-round access.
By constructing a sand dam and subsurface dam that harvest rainwater, the people will gain year-round access to water of better quality. This will reduce the time spent on collecting water and thus free time for school enrollment for children and other activities for women.
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==Manuals, videos, and links==
