rainwater harvesting in agriculture
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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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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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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
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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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