Roof Top Rainwater Harvesting: Who Gains and Who Loses

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Roof Top Rainwater Harvesting Who Gains and Who
Loses?

• M. Dinesh Kumar
• IWMI, VV Nagar
• Email d.kumar_at_cgiar.org

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Purpose of the Paper

• In the wake of growing urban, RWHS are widely
  promoted as a measure to reduce stress on urban
  water utilities and provide domestic water
  security
• In rural areas, they are promoted as a tool for
  domestic water security
• No scientific assessment or evaluation of the
  systems from technical and economic angle

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Hydrological Opportunities for Roof Water
Harvesting

• Urban areas
• Hydro-opportunity depends on per capita roof area
  and rainfall
• Per capita roof area varies across classes
• Rainfall varies from region to region and year to
  year
• High variability in rainfall in regions of low
  mean annual rainfall

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The City of Ahmedabad

• In Ahmedabad
• Maximum rainfall 1281 mm Minimum 392 mm
• For LIG living in 3-storey apartments, and MIG in
  multi-storey apartments, water is sufficient for
  a week in bad year
• I a good rainfall year, it will be sufficient for
  4 weeks
• For HIG, in small bungalows, water is sufficient
  for 23 days in bad year, 80 days in good year

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Hydrological Opportunities for Roof Water
Harvesting

• Rural Areas Radhanpur
• In a rural household in Radhanpur, water
  collected from roof will be sufficient for just
  11 days in a bad year 32 days in a good year
• In a high rainfall area Like Dangs, with a roof
  area of 20 m2, water will be sufficient for 144
  days

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Physical Feasibility of Harvesting

• Volume of water to be captured from a roof area
  of 320 sq. m would be 416 m3 and 112 m3
• Capacity of existing storage tanks would be 30-40
  m3.
• Urban areas wont provide the land area required
  for such large storages
• The rate of inflow into the tank would be very
  high in relation to the withdrawal rate given few
  rainy days
• Existing storages wont work

Technical feasibility poor for large housing
stocks good for bungalows
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Costs and Economic Viability of Roof Water
Harvesting System

• Two types of systems
• Rooftop water harvesting
• Rooftop rainwater recharging
• Rooftop water harvesting costs
• Rs.50/m3 for 20-year life
• Rs.25 for a 40-year life
• RWHS wont replace public systems in most
  physical and climatic conditions.

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Economics of Roof Water Harvesting System

• It can only supplement a system which already
  exists, especially in low rainfall areas where it
  is marginal
• Evaluation of RWHS should involve comparison of
  cost of production of water through RWHS and the
  marginal cost of supplying water through public
  systems.
• RWHS it can be justified only if the marginal
  cost of production of water is less than the unit
  cost of water through RWHS

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Economics of Roof Water Harvesting System

• RWHS will not be economically viable in regions
  with concentrated populations but could be
  viable in areas of scattered populations,
  especially hilly regions
• Desalination costs Rs. 45/m3 for a plant having
  capacity of 1000 m3/day Cost lower (Rs.17/m3) if
  salinity is below 6000 ppm
• In coastal areas of Saurashtra and Kachchh,
  desalination would be economically viable if made
  decentralized

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When Does RWHS Work?
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Policy Implications

• For majority of urban dwellers and rural
  households, RWHS offer little scope in augmenting
  domestic water supplies
• Even if rich families invest in RWHS, they would
  continue to access water from public utilities,
  and corner lions share of the subsidies provided
  in public water supply
• In low rainfall regions with scattered
  populations, public investment in RWHS can be
  justified from a macro economic point of view.

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Policy Implications

• At micro level, it would not only fail to ensure
  desired physical results, but would fail to yield
  equitable social benefits.
• In high rainfall, hilly areas such as the Dangs
  and other parts of Western Ghat, government
  subsidies for RWHS would not only be economically
  viable, but also bring about larger social
  benefits