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Water Harvesting for Groundwater Recharge

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Water Harvesting for Groundwater Recharge Is it effective? INTRODUCTIOIN Water scarcity is becoming an increasing problem worldwide. 35% of global land surface is ... – PowerPoint PPT presentation

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Title: Water Harvesting for Groundwater Recharge


1
Water Harvesting for Groundwater Recharge
  • Is it effective?

2
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

3
  • 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

4
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

7
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.

8
  • 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

9
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

10
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

11
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

14
Sub-surface dam
  • Constructed within the aquifer itself
  • Dam reduces variation of the level of the
    groundwater upstream

15
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

16
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

17
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

18
  • 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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23
  • 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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25
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

26
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

27
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

28
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
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