Rain Water Harvesting in

Agricultural Production

By

B.Tandarayen

Agricultural Engineer

Ministry of Agro-Industry & FS

1

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

2

A PRELIMINARY DESIGN OF

AN INVERTED ROOF • This project is meant for agricultural fields

where there are no farm structures or

buidings

3

Outline

Introduction

Site assessment

Rain Water Harvesting System

Design calculation

Suitability – Advantages & disadvantages

Costing

Conclusions

4

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.

5

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

6

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

7

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.

8

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

10

11

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

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.

13

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

14

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

15

Storage system The rain water is ultimately stored in a

storage tank above ground level

16

Storage system

The preferred type of tank is shown below,

made up of fibre glass , which is readily

available on the local market.

17

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

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

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

20

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

21

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.

22

END WORD

Rain Water Harvesting : A Noble Goal

& a Common Responsibility

23

END of Presentation

• Thank you for your attention

• Questions and Clarifications

24