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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.

'''Dams ''' can consist of raised banks of compacted earth (usually with an impermeable clay core, stone aprons and a spillway to discharge excess runoff), open rock reservoir catchments, and masonry or concrete (reinforced or not).

* 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.