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[[Image:Natural_or_artificial_ground_catchment ... icon.png|right]]
[[Image:MasonryTank.jpg|thumb|right|300px|Stone masonry round berked under construction showing wires for roofing materials. Somaliland. 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.
==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.