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[[Image:ground ctch icon.png|right]]
[[Image:MasonryTank.jpg|thumb|right|300px200px|Stone masonry round berked under construction showing wires for roofing materials. Somaliland. <br>Photo: Eric Fewster, BushProof / Caritas.]]
These are natural, artificial or modified catchments that have low to relatively high runoff coefficients. Water from these catchments is captured and stored in lined sub-surface reservoirs excavated below ground level. The reservoirs are known by different names (''berkeds'' in Somaliland, ''taankas'' in India, hemispherical sub-surface tanks in Kenya – also included in this category are excavated water cellars such as the ''shuijiao'' in China) and have been lined with many different materials. These tanks normally have a larger depth to surface ratio compared to open ponds and their scale means a roof of some description is a possibility. When the lining is constructed well, there will be no leakage, and water will either evaporate or be abstracted. These tanks are often privately-owned by one or more families, but can be communal.
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! width="50%" style="background:#efefef;" | Advantages
! style="background:#ffdeadf0f8ff;" | Disadvantages
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| valign="top" | - Less evaporation than natural ponds due to less surface area to depth <br>
====Limiting evaporation====
[[Image:SandFilledTanks.JPG|thumb|right|400px200px|Sand-filled sub-surface tank, Botswana Image courtesy of WEDC. © Ken Chatterton. In: Hussey, S.W. (2007) ''Water from sand rivers: guidelines for abstraction.'' WEDC, Loughborough University, UK.]]
Sub-surface tanks are usually small enough that it is viable to have a roof to limit evaporation (and improve water quality if possible, which means less algae build-up). Shading can reduce evaporation by around 30%. Placing local bush or grass materials on a frame of wires doesn’t seem to work well because they get blown off, and also still let light in, which creates algae growth. Corrugated iron roof on wooden frame works well but is expensive (about $20 per m2 in Somaliland). In addition, if the tank is not fenced, animals walking on the roof can damage it. The challenge is to make a roof that is cost-effective for small-scale farmers – one idea is to investigate income-generating roofs since that can help pay for the structure (e.g. passion fruit). Excavated water cellars by their nature have small area roofs.
==Manuals, videos, and links==
* [http://www.unep.or.jp/ietc/publications/techpublications/techpub-8a/sand.asp Sand Abstraction]. Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa. United Nations Environment Programme.
* [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDcQFjAB&url=http%3A%2F%2Fwww.ijest.info%2Fdocs%2FIJEST11/docs/IJEST11-03-01-093.pdf&ei=eAB_T-bNMeTfiAKZtoH0Ag&usg=AFQjCNE5fvsBiHjyMK9xwYxL9qwsE094kw&sig2=vg0VXVBOW94zSxFkv-LoFQ Evaluation of the Sand Abstraction Systems for Rural Water Supply: the case of Lupane District, Zimbabwe]. Vol 3, No. 1, January 2011.
==Acknowledgements==
* CARE Nederland, Desk Study: [[Resilient WASH systems in drought-prone areas]]. October 2010.
