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[[Image:Underground_tank_icon.png|right|80px]]
[[Image:Underground_tank.PNG|thumb|right|200px|Underground storage of rainwater in Bolivia. Photo: Netherlands Water Project.]]
Where access to groundwater is limited, [[Rainwater harvesting - general Harvesting | rainwater harvesting]] in underground tanks can be an effective and low-cost alternative. Water stored in the wet season can be used in the dry season and lifted from the tank with a [[rope pump]] or with a [[Deep well pump|deep well pump]], which can elevate water up to 30 m.
These tanks can be constructed of concrete blocks or with other materials. The tanks are partly (1.5 m) built below the soil surface. These tanks can store up to 20,000 liters of water, used for domestic use and crop cultivation.
===Suitable conditions ===
The area should be suitable for [[Rainwater harvesting - general Harvesting | rainwater harvesting]]. Depending on the design the capacity of the tank varies from 3,500 to 20,000 liters.
====Build the tank====
[[Image:TankLiningOptions.jpg|thumb|none|500px|Recommended options for the tank lining when plastering the walls.]]
[[Image:RootZoneDepth.jpg|thumb|right|350px200px| Preparing to ram the bottom of the underground tank <br> ©Practical Action/Douglas Gumbo.]][[Image:AddTankLining.jpg|thumb|right|350px200px| Tank lining / Plastering <br> ©Practical Action/Douglas Gumbo.]]
# Along any dug dead level contour (which is your collection area) choose where you can construct your storage tank. If you would like to consider nearby crops, the storage tanks should be near all soils that allow rapid percolation. This is because your soil quickly loses water and that's where your plants are likely to show signs of moisture stress during the dry spell.
# Measure along the channel the length of the tank you want and mark using pegs. Dig the marked area up to a further depth of 500cm or more. The deeper it is the more water will be harvested.
===Costs===
* Material costs for [[EMAS Cisterncistern]] tanks consist of sand and four 50 kg bags of cement for each tank.
* Subsurface ferrocement tanks of 60 m3 costs about US$ 1,900.
* Underground tanks can be high and variable in cost per m3 of storage (averages around US$30-40 per m3 of storage, sometimes a lot more depending on various factors). Sub-surface hemispherical tanks made from stone masonry and bricks/cement in Ethiopia cost in the range of 113 - 219 Euro per m3 of storage including all costs such as trucked water for construction and solidity of construction (more solidly built tanks in clay areas cost more). Therefore costs currently limit the replicability of the technology for poorer families and potential to scale things up.
===Manuals, videos and links===
* MANUAL: [http://www.enviro-friendlyspacepac.com.au/Brochures/WaterTanks/TankMasta/Tankmasta-Underground%20Tanks-InstallationInstallation_8pg_np.pdf Underground tank installation manual] by Tankmasta explains all the steps needed to build an underground tank.* VIDEO: [httphttps://emasvimeo.blip.tvcom/ emas Large collection of EMAS movies on different subjects.]
* [http://www.rainwaterharvesting.org/Rural/traditional2.htm Various tanks and irrigation channels in India.] RainwaterHarvesting.org.
*[http://www.rainfoundation.org Underground tanks] as part of a project in Burkina Faso.