This document discusses rainwater harvesting for agriculture. It defines rainwater harvesting as systems that collect, convey, and store rainwater, primarily from rooftops and surfaces, for direct use or artificial groundwater recharge. The benefits of rainwater harvesting include conserving water resources, providing improved water quality, and replenishing local groundwater aquifers. However, rainwater harvesting performance depends on climate and water quality can be affected if stormwater runoff is included. The document provides information on designing rainwater harvesting systems, including calculating harvesting potential based on collection area and rainfall, and conducting feasibility analyses.

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Rainwater Harvesting
In 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
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
locations
3. Potentially provide improved quality of water
4. 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 it
6. Commitment as a corporate citizen - showcasing
environmental concerns
7. 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 world
Performance seriously affected by climate fluctuations that
sometimes are hard to predict
Increasingly 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 factors
Collection 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

11. Mullaitivu District
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12. Kilinochchi District
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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
DRFmm
Month
DL1d
Annual DRF = 900 mm
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Rainfall 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
DRFmm
Month
DL1e
Annual DRF = 900 mm
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Rainfall 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
DRFmm
Month
DL1f
Annual DRF = 800 mm
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Rainfall 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 = 0.90
Collection = 600 sq meters * RF (m) * 0.90
Possible months for collection: January, March, April,
May, September, October, November, December
Capacity of the tank: 5 m3
Family consumption: 1.5 m3
/ month
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rainfall (mm) 9.4 2.0 12.3 72.3 27.5 0.4 0.3 2.8 17.8 129.5 157.2 99.5
RWH (m3
) 5.1 1.1 6.6 39.0 14.9 0.2 0.2 1.5 9.6 69.9 84.9 53.7
Collection (m3
) 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
Consumption (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 Roof
2 Screen
3 Discharge of water
4 Pre-filter
5 Storage tank
6 Flow meter
7 Storm water discharge
Raw water
tank or
Aquifer
1
2
3
4
5
6
7
Rain Water as Source Water
Design Considerations
Typical Diagram Recomendation

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Quality Issues
Roofs 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.

25. Eye-brow bund and pitcher system Pathaha system
Rainwater collecting wells Micro-tank system

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