[[Image:MasonryTank.jpg{{Language-box|thumb|right|300px|Stone masonry round berked under construction showing wires for roofing materials. Somaliland. Eric Fewster, BushProof / Caritas.]]These are natural, english_link= Natural or artificial or modified catchments that have low to relatively high runoff coefficients. Water from these catchments is captured ground catchment 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 Lined 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.| french_link= Coming soon | spanish_link= Coming soon | hindi_link= Coming soon | malayalam_link= Coming soon | tamil_link= Coming soon | swahili_link=coming soon | korean_link= Coming soon | chinese_link=天然或人工地表集水区和衬砌式地下贮水池 | indonesian_link= Coming soon | japanese_link= Coming soon }}
==Suitable conditions=={{Climate_change[[Image:ground ctch icon.png|right|heading1=Drought 80px]]|text1= '''Effects of drought'''[[Image: Water storage used upMasonryTank. <br>'''Underlying causes of effects''': Lack of rainfall; High evaporation rates; Leaking linings due to bad jpg|thumb|right|200px|Stone masonry round berked under construction; Storage not sufficient for demand – tanks are too expensive showing wires for volumes of water to outlast extended dry periods. <br>'''To increase resiliency of WASH system''': Build smaller tank structures but more of them over longer time, which means less reinforcement per tank, more manageable to construct and cover, and more affordable; Reduce evaporation & seepage due to poor construction & siting; Follow proper concreting guidelines; Make tanks from cheaper roofing materials and repair more often; Improve access to micro-finance; Support the capacity of the government or private sector to be able to provide (for payment) a tankering scheme.|heading2=Drought effects on cement|text2='''Effects of drought''': Badly made concrete or cracked linings (e.g. in tanks, dams, waterways, wells, and other structures). <br>'''Underlying causes of effects''': Less water used for curing; Impure water used for mixingSomaliland. <br>'''To increase resiliency of WASH system'''Photo: Ensure adequate mixingEric Fewster, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curingBushProof / 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.
===Suitable conditions===
* Site in a place where run-off is seen to flow after rains.
* Artificial catchments are created where infiltration of runoff zone is high.
* Do not site sub-surface tanks in areas of high water tables to reduce risk of flotation.
{{procontable | proborder="1" cellpadding="5" cellspacing="0" align="center"|-! width="50%" style="background:#efefef;" | Advantages! style="background:#f0f8ff;" | Disadvantages|-| valign="top" | - Less evaporation than natural ponds due to less surface area to depth <br>
- Good for areas where ground would otherwise be permeable <br>
- They work well when privately owned and maintained <br>
| convalign= "top" | - Sub-surface tanks often cannot hold enough water for the whole dry season; making bigger berkeds is possible but is more difficult and expensive; if not affordable it is not replicable.<br>
- Costs currently limit the replicability of the technology for poorer families and the potential to scale things up <br>
- Considerable amounts of silt accumulate in tanks, exactly how much will depend on the area <br>
- When built in a remote area, construction is difficult due to lack of water and large distances to transport materials <br>
- Microbiological and chemical water quality is likely to not be acceptable for direct consumption
|}} ===Resilience to changes in the environment=======Drought===='''Effects of drought''': Water storage used up.<br>'''Underlying causes of effects''': Lack of rainfall; High evaporation rates; Leaking linings due to bad construction; Storage not sufficient for demand – tanks are too expensive for volumes of water to outlast extended dry periods.<br>'''To increase resiliency of WASH system''': Build smaller tank structures but more of them over longer time, which means less reinforcement per tank, more manageable to construct and cover, and more affordable; Reduce evaporation & seepage due to poor construction & siting; Follow proper concreting guidelines; Make tanks from cheaper materials and repair more often; Improve access to micro-finance; Support the capacity of the government or private sector to be able to provide (for payment) a tankering scheme. ====Drought effects on cement==== '''Effects of drought''': Badly made concrete and cracked linings (e.g. in tanks, dams, waterways, wells, and other structures). <br>'''Underlying causes of effects''': Less water used for curing; Impure water used for mixing. <br>'''To increase resiliency of WASH system''': Ensure adequate mixing, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curing. 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. '''Specific advice''':
==Construction, operations and maintenance==
The reason for constructing a sub-surface tank is to store the water. Therefore one of the most important aspects is that seepage and cracks must be avoided. Therefore good quality construction work with adequate supervision is vital to create a sound structure – this is especially important in areas with swelling soils that can affect the integrity of the lining. While ownership and management of tanks is important, such privately-owned tanks have often failed due just to the technical construction component. Construction materials vary and include the natural soil formation itself, clay, stone masonry, bricks/cement, ferrocement, anthill/lime/cement and plastic/rubber lining. Material may affect cost but choice may also depend on what is available and the type of surrounding soil. To generally prevent cracking/seepage:
* Round tanks are inherently stronger than rectangular ones. Hemispherical and cylindrical designs are commonly used.
====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.
* Support the capacity of the government or private sector to be able to provide (for payment) a tankering scheme to fill tanks during the driest parts of the year.
===Costs===
Access to finance is a main obstacle to promotion of rainwater harvesting for households, and is important so that users can replicate the technology – there are few examples on a global level with micro-credit for rainwater harvesting. The cost of underground tanks can be high and variable in cost per m3 of storage (averages around US$30-40 per m3 of storage or more depending). Sub-surface hemispherical tanks made from stone masonry and bricks/cement in Ethiopia costs 113 - 219 Euro per m3 of storage including all costs. May need to pay for trucked water for construction and for a higher solidity of tanks (in clay areas). However there are several ways to save money.
* However, care needs to be taken with cheap linings – in some areas with swelling clay and differential settlement, linings can easily crack, as has often been observed in some areas. The areas where cheap linings might work therefore may be site-specific, and depend on the clay content of the soil. For plastic linings, experience from India shows that these can be punctured by rodents, crabs or insects if there is no rodent/insect-proof layer before the plastic.
==Reference manuals=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.