Water Harvesting for Groundwater Recharge

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Water Harvesting for Groundwater Recharge

• Is it effective?

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INTRODUCTIOIN

• Water scarcity is becoming an increasing problem
  worldwide.
• 35 of global land surface is semi-arid
• In 1997 it was reported that 80 of countries
  suffered from serious water shortage, which
  encompasses 40 of the worlds population

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• Main reason due to a shift in water management
  across the world.
• Individuals/communities have given over the role
  of water management to the State/Government.
• Increasing exploitation of rivers and groundwater
  these are now the key sources of water supply
  across the world.
• However, it is generally believed that enough
  precipitation falls on the worlds land surfaces
  to supply the global population.
• Use of water harvesting to capture the water that
  falls before it gets chance to be evaporated

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WATER HARVESTING

• Term was 1st used by the Australian HJ Geddes to
  denote
• the collection and storage of any farm waters,
  either runoff or creek flow, for irrigation
  use.
• Idea has been around since early civilization,
  beginning about 4000years ago in the Bronze Age
• Desert dwellers smoothed hillsides to increase
  runoff and built ditches to collect the water and
  convey it to lower lying fields
• Also included collection from rooftops and
  courtyards

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• There has been an increasing interest in the
  redevelopment and implementation of water
  harvesting systems, especially of water
  harvesting for groundwater recharge
• Groundwater is a more favourable source of water
  for several reasons
• It provides ready made storage reservoirs
• It helps reduce the problems of evaporation
• It protects against pollution due to the
  filtering action of the aquifer
• It helps keep saline waters from intruding
• It helps convert/dispose of floodwaters
• It helps to prevent subsidence of the ground
  above the depleted aquifer

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WATER HARVETSING TECHNIQUES FOR GROUNDWATER
RECHARGE

• Artificial recharge of groundwater as define by
  UNEP is
• The planned human activity of augmenting the
  amount of groundwater available through the works
  designed to increase the natural replenishment or
  percolation of surface waters into the
  groundwater system, resulting in a corresponding
  increase the amount of groundwater available for
  abstraction.

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• 2 main techniques have been described for
  increasing groundwater recharge in arid areas
• The planting of trees
• Estimates have been suggested that the planting
  of trees can increase precipitation by over 300
  and therefore more water is available for
  recharge to the aquifer
• The construction of water harvesting devices
• Include the use of both floodwater and rainfall
• Increasing surface runoff into storage/collection
  areas by decreasing infiltration
• Increasing infiltration into the aquifer along
  the route of travel of surface water
• Use of engineering structures to collect and pump
  the water into the deep aquifer

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Floodwater harvesting for groundwater recharge

• There are several methods by which floodwater
  harvesting can be carried out for aquifer
  recharge
• Check dams
• Percolation tanks
• Groundwater dams
• These are amongst the cheapest and most widely
  used methods for groundwater recharge for 2 main
  reasons
• Relatively low construction costs
• Easy to operate and maintain

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Check dams

• Temporary structures constructed from locally
  available material such as brushwood, loose rock
  or woven wire
• Aim to impede the soil and water removed from the
  catchment the impeded water collects behind the
  dam and infiltrates the soil, recharging the
  aquifer
• Cheap to construct costing approx. US 200-400
  depending on the material used, size of the gully
  and the height of the dam
• Life span of 2-5 years
• More permanent dams constructed using stones,
  brick and cement, but costs increase to US
  1000-2000

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Percolation tanks

• This is a dam built on permeable ground so that
  floodwater is held back long enough to percolate
  into the ground
• More permanent and larger than check dams
• Constructed by excavating a depression to form a
  small reservoir or by constructing an embankment
  in a natural ravine or gully
• Cost is estimated to be approx. US 5,000-10,000
• Capacity varies from 10,000-15,000m3
• 2-3 filling expected per wet season
  (30,000-45,000m3)
• Enough to irrigate 4-6 ha of irrigated dry crop
  (maize) and 2-3 ha of paddy crop

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Groundwater dams

• Structures that intercept or obstruct the natural
  flow of groundwater
• Often built within river beds to obstruct and
  detain groundwater flow so as to sustain the
  storage capacity of the aquifer and meet demands
  during high periods
• Used in many areas such as India, Africa and
  Brazil
• 2 main types
• Sub-surface dam
• Sand storage/silt trapping dams

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Sub-surface dam

• Constructed within the aquifer itself
• Dam reduces variation of the level of the
  groundwater upstream

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Sand storage dam

• Constructed above ground
• Sand and silt particles transported in floodwater
  and get deposited behind the dam
• Constructed in layers to trap the sand with each
  flood

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Important considerations

• Material used must be impermeable for the
  permanent structures
• Dam must be strong enough to withstand the build
  up of the sediment behind it
• Aquifer should be confined beneath to prevent
  seepage
• Bitumen/plastic lining

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Rainwater harvesting for groundwater recharge

• More related to the micro-catchment
• Roofs
• Artificial surfaces at ground level e.g.
  courtyards
• Slopes less than 50-150m in length
• Contour trenches
• Excavations made parallel to contours on slopes
• Rainwater ponds and infiltrate
• Helps to prevent eroding soil migrating
  down-slope
• Gully plugs
• Stone barriers built across gullies and deep
  rills
• Traps sediment eroded from higher up slope and
  impound runoff encourage encouraging infiltration

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• Rooftop harvesting
• Can be adopted by individuals
• Relatively easy to construct, operate and
  maintain
• Several techniques in use
• Abandoned dug well
• Abandoned/running hand pump
• Gravity head recharge
• Recharge pit

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• Recharge by injection well
• Direct sub-surface recharge technique conveying
  water directly into the aquifer
• Can be used in conjunction with rooftop
  harvesting
• Concerns that groundwater may get contaminated
• Potential for clogging from suspended solid,
  biological activity or chemical impurities
• Expensive to construct and harder to maintain and
  operate than the other methods

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Is water harvesting for groundwater recharge
effective?

• Important initial considerations
• Only suitable for areas where aquifers exist
• Simpler with unconfined aquifers
• Thorough survey of the area is necessary
• Climatic records (ppt, humidity, evaporation
  rates)
• Topographical maps (drainage networks, ephemeral
  streams)
• Soil data (type, distribution and thickness)
• Distribution of rock types, esp. surface features
• Definition of pore networks
• Recognition of recharge/discharge areas,
  direction of groundwater flow

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Once a suitable site has been found

• Essential to involve the local community
• Once the structure has been built
• Important that the system is managed properly
• Unnecessary consumption of water
• Groups of locals need to be put in charge of the
  system
• Ensure adequate distribution amongst the
  villagers
• Poor farmers/owners of small farms

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Potential problems

• Contamination from direct injection
• Contaminate water already stored in the aquifer
• Lower the value of the water (drinking water)
• Water quality problems from rooftop harvesting
• Dust/faeces washed off the roof
• Large evaporative losses
• Silting
• politics

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SUMMARY

• Many potential problems
• If planned and managed properly can be a great
  success
• More effective for shallow aquifers
• More accessible for farmers
• Cheaper to extract the water
• Recharged more effectively with little high-tech
  equipment