1. Rain Water Harvesting in
Agricultural Production
By
B.Tandarayen
Agricultural Engineer
Ministry of Agro-Industry & FS
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2. Water & Food Security
• Today the classical sources of irrigation
water are at the out break of overuse and
there is the urgent need to tap the new
sources of water to sustain agricultural
production in Mauritius
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3. A PRELIMINARY DESIGN OF
AN INVERTED ROOF
• This project is meant for agricultural fields
where there are no farm structures or
buidings
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4. Outline
 Introduction
 Site assessment
 Rain Water Harvesting System
 Design calculation
 Suitability – Advantages &
disadvantages
 Costing
 Conclusions
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5. Introduction
Rainwater harvesting is a method for
inducing, collecting, storing and
conserving rain water for use.
There are various ways to collect water
in agriculture.
Rain water harvesting dates back to
some 5000 years bc.
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6. Site assessment
 Assessing the site conditions together
with the future system owner is the
first step towards a sound system
design.
The site conditions that need to be
assessed:
1. Foundation characteristics of soil near the
site
2. Estimated rain to be captured
3. Location of trees
4. Availability and location of construction
materials
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7. Rain Water Harvesting System
 A typical system will consist of three
basic elements:
1. A collection area
2. A conveyance system
3. A storage system
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8. 1. The collection Area
• For this project, the collection area will be
an inverted roof made from galvanised
iron sheets and supported with galvanised
pipes, properly anchored in the ground. A
preliminary drawing is shown in Figure 1.
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10. The collection Area
The collection area of the model is about 20
m2. and the height is about 3.5 m above the
ground. Figure 2, shows the plan view of the
construction.
The effective catchment area of a freely
exposed roof draining to a gutter is equal to
the plan area of the roof
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11. 11

12. Calculation of roof run-off
• Run-off from the inverted roof will be
calculated assuming that the surfaces are
impermeable. The rate of run-off is given
by the equation :
• Q = A Lµ
• Where Q is in l/s
• Ae is the effective catchment area in m2
• L is the design rate of rainfall in mm
• µ : run off coefficient 12

13. 2. The conveyance system
• The conveyance system from the roof to
the tank will consists of UPVC gutters and
pipes. The conveyance system to include
a coarse mesh for retention of leave, dust
etc.
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14. Sizing of gutters and rainwater
pipes
• Factors to consider:
• Roof area
• Depression of roof
• Among others
• The rain water pipes should be appropriate to
the gutter size
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15. Materials for gutters, rainwater
pipes and joints
• The material preferred is UPVC to BS 4576
which is a very light material , easy to
assemble, free from rust , rot or corrosion
and virtually eliminates maintenance. It is
very aesthetic. UPVC has a relatively high
co-efficient of thermal expansion and must
be positively allowed for
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16. Storage system
The rain water is ultimately stored in a
storage tank above ground level
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17. Storage system
The preferred type of tank is shown below,
made up of fibre glass , which is readily
available on the local market.
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18. Storage system
A rain water tank should include:
• A solid secure cover
• A coarse inlet filter
• An overflow pipe
• A drain for cleaning of tank
• An extraction system; Tap etc
• A soakaway for evacuation of spilled water
near the tank 18

19. Suitability of the system
Advantages:
• Rainwater provides a source of water at
the point in the fields where it is needed
• The construction of this inverted rooftop
rainwater catchment is very simple
• The construction is flexible. The system
can be build to meet any requirements
• Running cost very low
• The construction, operation and
maintenance are not labour intensive 19

20. Disadvantages
• The success of the system depends upon
frequency and amount of rainfall
• Low storage capacities will limit rainwater
harvesting, while increase storage
capacities will add to construction cost
• The factor of cyclone needs to be taken
into account
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21. Cost
• Materials and Labour: RS 25, 000
• Water tank: RS 20 000 –Rs 30 000
• There are several factors that may be
considered so as to decrease the cost
• At the same time, depending on the funds
available, the storage capacity may be
increased accordingly
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22. Conclusions
• Rainwater harvesting technology has
proved to be very effective in Latin
American countries, Caribbean islands,
China, India and much more.
• And definitely, if properly designed, it will
prove to be very effective and efficient in
Mauritius too.
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23. END WORD
Rain Water Harvesting : A Noble Goal
& a Common Responsibility
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24. END of Presentation
• Thank you for your attention
• Questions and Clarifications
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