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Rainwater HarvestingIn Agriculture

Induction Training for EU - SDDP staff November 2013

Dr. P.B. Dharmasena, National Consultant/ Agriculture and Water Management

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Proud of being a

Sri Lankan

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What is Rainwater Harvesting?1. RWH technology consists of simple

systems to collect, convey, and store rainwater. Rainwater capture is accomplished primarily from roof-top, surface runoff, and other surfaces.

2. RWH either captures stored rainwater for direct use (irrigation, production, washing, drinking water, etc.) or is recharged into the local groundwater and is called artificial recharge.

3. In many cases, RWH systems are used in conjunction with Aquifer Storage and Recovery (ASR). ASR is the introduction of RWH collected rainwater to the groundwater / aquifer through various structures in excess of what would naturally infiltrate then recovered for use

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Why Rainwater Harvesting?1. Conserve and supplement existing water

resources 2. Available for capture and storage in most global

locations3. Potentially provide improved quality of water4. Supply water at one of the lowest costs possible

for a supplemental supply source. 5. Capturing and directing storm water (run-off)

and beneficially use it6. Commitment as a corporate citizen - showcasing

environmental concerns7. Public Mandate 8. Replenishing local groundwater aquifers .where

lowering of water tables has occured

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Why Not RWH?Not applicable in all climate conditions over the worldPerformance seriously affected by climate fluctuations that

sometimes are hard to predictIncreasingly sophisticated RWH systems (ASR) necessarily

increases complexities in cost, design, operation, maintenance, size and regulatory permitting

Collected rainwater can be degraded with the inclusion of storm water runoff

Collected water quality might be affected by external factorsCollection systems require monitoring and continuous

maintenance and improvement to maintain desired water quality characteristics for water end-use

Certain areas will have high initial capital cost with low ROI

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Condensation

Precipitation

Evaporation

Surface Water

Infiltration/ percolation

Evapotranspiration

The Water Cycle

Consumption

Surface Runoff

Groundwater

Sea water intrusion

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Condensation

Precipitation

Surface Water

Groundwater

Consumption

Rainfall Definitions

Intensity – Quantity per time of the rainfall event (mm/hour)

Duration – period of time for the rainfall event

Average Annual and Monthly Rainfall – Average rainfall over one year period and monthly intervals and usually based on 30 or more years of data

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Design and Feasibility Criteria• Collection Area (catchment) • Rainfall• Demand• Primary Use - Direct Use, Artificial Recharge (AR)

or Aquifer Storage and Recovery (ASR)• Storage capacity • Level of Security - risk of the storage tank running

dry

Harvesting potential(m3) = Area (m2) X Rainfall (m) X Collection Efficiency

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Collection Area and Characteristics

Measure Area Runoff Characteristics

– Roof top 0.75 – 0.95– Paved area 0.50 – 0.85 – Bare ground 0.10 – 0.20 – “Green area” 0.05 – 0.10

Water harvesting potential(m3) = Area (m2) X Rainfall (m) X Collection Efficiency

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Rainfall data source

Mullaitivu District

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Kilinochchi District

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0

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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

DR

F m

m

Month

DL1d

Annual DRF = 900 mm

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TotalRainfall 44 11 3 16 18 0 5 24 45 103 166 178 613

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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

DR

F m

m

Month

DL1e

Annual DRF = 900 mm

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TotalRainfall 4 11 5 38 24 0 7 16 52 125 188 167 686

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0

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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

DR

F m

m

Month

DL1f

Annual DRF = 800 mm

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec TotalRainfall 9.4 2.0 12.3 72.3 27.5 0.4 0.3 2.8 17.8 129.5 157.2 99.5 531.0

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Feasibility Analysis

Example :Roof area = 600 sq meters Collection Coefficient = 0.90Collection = 600 sq meters * RF (m) * 0.90Possible months for collection: January, March, April, May, September, October, November, DecemberCapacity of the tank: 5 m3

Family consumption: 1.5 m3/ month

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecRainfall (mm) 9.4 2.0 12.3 72.3 27.5 0.4 0.3 2.8 17.8 129.5 157.2 99.5RWH (m3) 5.1 1.1 6.6 39.0 14.9 0.2 0.2 1.5 9.6 69.9 84.9 53.7Collection (m3) 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0Consumption (m3) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

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1 Roof2 Screen3 Discharge of water

4 Pre-filter5 Storage tank6 Flow meter7 Storm water discharge

Raw water tank or Aquifer

1

2

3 4

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Rain Water as Source WaterDesign Considerations

Typical Diagram Recomendation

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Quality IssuesRoofs contain: bird droppings, atmospheric dust, industrial and urban air pollution

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Aquifer Storage and Recovery or Artificial Aquifer Recharge?

Require complete hydrogeological analysis, stakeholder engagement and potentially regulatory

approval

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Ground Water Recharge

Under natural conditions it may take days to centuries to recharge ground water by rain water. As we need to replenish the pumped water, Artificial Recharge of Ground water is required at some locations.

Eye-brow bund and pitcher system Pathaha system

Rainwater collecting wells Micro-tank system

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