[[Image:dam icon.png|right|100px80px|]]
[[Image:Catchment dam.jpg|thumb|right|200px| Turkana men and women build a dam on the river Lokitaung in northern Kenya. Photo by AFP/Simon Maina.]]
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Water can be made available by damming a natural rainwater catchment area, such as a valley, and storing the water in the reservoir formed by the dam, or diverting it to another reservoir. Important parameters in the planning of dams are: the annual rainfall and evaporation pattern; the present use and runoff coefficient of the catchment area (e.g. bare rock catchments have high runoff coefficients, around 0.9); water demand; and the geology and geography of the catchment area and building site.
The water stored behind a dam should normally be treated before entering a distribution system.
===Suitable Conditions===
Site reservoirs where the base will be impermeable (e.g. unfissured rock or clay) in order to save costs and prevent having to find a form of lining. Alternatively cracks and fissures could be sealed up with mortar or concrete. Rock catchment dams work well mostly in hilly or mountainous regions where other water sources are scarce. These types of dams should be bare and free of vegetation/soil. When siting for rock catchment dams make sure to maximize the natural topography – to get the best volume, make dams on the lower side of existing rock pools.
===Resilience to changes in the environment===
====Drought====
'''Effects of drought''': Tend to dry up quickly, especially if unlined; Conflict over water for animals.
With climate change, some regions experience more intense rainfall events. (Several) smaller dams will receive less damage than one larger dam and be less expensive to fix. Diversify water sources so that if floods knock out one system, community has alternatives while repairs proceed. Flow paths of rivers are more likely to change, so choose dam site in most stable place along river and make dam height in consideration of past flood lines on the bank.
===Construction, operations & maintenance===
[[Image:WomenBuilding Dam.jpg|thumb|right|200px| [http://www.wfp.org/countries/uganda/media/karamoja-livelihoods Women construct surface dams in Ka’abong district.] Using food or cash incentives, WFP (World Food Programme) encourages communities to build productive physical assets and gain skills for protecting and regenerating the environment, enhancing nutrition and harvesting water.]]
* the taste of drinking-water varies between catchment areas and this can affect whether users accept the system, since they value the taste
===Costs===
This depends on local circumstances. Cost can be high – experience from Kenya shows that a 56 m3 dam cost $4,000 including labour (= $71 per m3 of storage). But then another cost of a 13 000 m3 rock catchment in Kenya was US$ 1.60 per m3 of storage volume. The corresponding cost of a 30 000 m3 earth dam in Tanzania was US$ 1.90, and only US$ 0.20 for an 80 000 m3 earth dam in Mali.
===Field experiences===
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===Manuals, videos, and links===
* [http://www.enggpedia.com/civil-engineering-encyclopedia/articles/1559-how-to-build-small-dams-small-dam-construction How to build small dams - Small Dam Construction.]
* [http://www.agu.org/pubs/crossref/2012/2011WR011155.shtml Appropriate small dam management for minimizing catchment-wide safety threats: International benchmarked guidelines and demonstrative cases studies.] Water Resources Research.
===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.
* CARE Nederland, Desk Study [[Resilient WASH systems in drought-prone areas]]. October 2010.