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Water Portal / Rainwater Harvesting / Groundwater recharge / Controlled flooding and Spreading basins

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[[Image:controlled flooding icon.png|right|80px]]

[[Image:ControlledFlooding.jpg|thumb|right|200px|Groundwater recharge in spreading basins. Arizona, USA. Photo: CAP (2002)]]

This is a floodwater harvesting technique called '''controlled flooding''', where the water diverted from a river, with the help of diversion structures and canals, is spread evenly over a large surface area , known as '''spreading basins''', where it is used for recharging groundwater, irrigation, filling ponds, and watering grazing land. The concept is that a thin sheet of water flows over the land but at minimum velocity in order to avoid disturbing the soil cover. This includes [[Irrigation_-_Spate_irrigation|spate irrigation]], but also standard channel irrigation which takes river water via channels to fields.

===Suitable conditions===

Site in areas of high volume and intensity river flows where conventional irrigation structures are not feasible (spate systems should probably be promoted in areas where the practice is already in use).

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! width="50%" style="background:#efefef;" | Advantages

! style="background:#ffdeadf0f8ff;" | Disadvantages

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| valign="top" | - Minimum land preparation is needed, so is very cost-effective compared to other infiltration methods <br>

===Resilience to changes in the environment===

====Drought====

'''Effects of drought''': Lower crop yields.<br>

Spreading basins are perfect for areas with floods. They are built to transfer excess water away from rivers, so that uncontrollable floods don't happen in unwanted places. However, in the event of too much flooding, the spreading basin must be large enough to handle the increased volumes of rain. Plus, with intense rain events, the higher velocities might be too much for the soil of the basin and create a washed out effect or likely just runoff before infiltrating. In this case, additional vegetation enhancement (to help the soils absorb more water), wetland enhancement, levee protection, and/or additional water storage will be helpful.

===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.

* Provision of bulldozers has been very popular and has enabled spate farmers to build or restore damaged structures more easily. Problems with that is that sometimes downstream effects become too great since farmers upstream can build much larger structures, and also that bulldozers cannot easily be run and maintained in a self-sustaining fashion due to high costs. Support is therefore too large for small farmer groups and is best organized on a regional basis through local government, or with subsidies to allow participation of the private sector.

==Costs=Field experiences===[[Image:FloodwaterSpreading.jpg|thumb|right|200px|Plan of how floodwaters will spread. Click image to zoom. <br> Drawing: [http://unesdoc.unesco.org/images/0014/001438/143819e.pdf ''Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.''] UNESCO.]]

==Field experiences==

[[Image:FloodwaterSpreading.jpg|thumb|right|200px|Plan of how floodwaters will spread. Click image to zoom. <br> Drawing: [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDgQFjAB&url=http%3A%2F%2Funesdoc.unesco.org%2Fimages%2F0014%2F001438%2F143819e.pdf&ei=uqt8T9_sL6ibiQKTqdzFDQ&usg=AFQjCNFjJrN8OJHYuNSZrDZIrDtZxc_LRg&sig2=J7p3YnDub8Ixn2n4DU-uBw ''Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.''] UNESCO.]]

====Iran====

Overexploitation of groundwater has caused significant drawdown of the water table (1.5m/year) and deterioration of groundwater quality in the Dorz Sayban Plain, which is located 115 km to the southeast of Larestan, Iran. 3500 hectares of land are irrigated using groundwater in this plain. To decrease the rate of the drawdown of the water table, five floodwater spreading systems for recharge of groundwater were designed and constructed in the region 1983 and 2001.

More than 70% of the suspended load has settled in the system. This will inevitably lead to clogging and reduction in efficiency in the system, but also an improvement of the soil for agriculture purposes. Additionally, the managed aquifer recharge improves the quality of groundwater, as the EC of floodwater is much lower than that of the groundwater (0.3-0.4 versus 2.0-9.0 dS/m).

===Manuals, operations and maintenance===* [http://web.archive.org/web/20151106164515/https://www.iaea.org/technicalcooperation/documents/Brochures/sust-groundwater.pdf Sustainable Development of Groundwater Resources in Southern and Eastern Africa]. International Atomic Energy Agency.

===Acknowledgements===

* CARE Nederland, Desk Study: [[Resilient WASH systems in drought-prone areas]]. October 2010.

* Gale, Ian, [http://unesdoc.unesco.org/images/0014/001438/143819e.pdf Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.] UNESCO's International Hydrological Programme (IHP), 2005.

* [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDMQFjAA&url=http%3A%2F%2Fwww.colusarcd.org%2Fnodes%2Fprojects%2Fdocuments%2FGoals_Objectives_Actions_Climate.doc&ei=jVGQT9yCH8SoiAKYyJCjAw&usg=AFQjCNFPToFx9KEpNSM7gTbs2dn_Dz8zqQ&sig2=eNOQzbhrGc7kySDzZdno-w / Address unknown future effects of climate change.] Colusa County Resource Conservation District, USA.

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