Chapter 6

CHAPTER SIX
WATER HARVESTING
Mengistu Zantet (MSc.)
Lecturer @ Hydraulic and Water Resources Engineering department
Mizan Tepi university
Email: mengistu.zantet@gmail.com
P.O.Box: 260
Tepi, Ethiopia

Outline
6.1. Principles of Water Harvesting
6.2. Water Harvesting Techniques
mengistu.zantet@gmail.com .
lecturer@ Hydraulic and water
resources Engineering Department

What is Water Harvesting?
is “collection of runoff for its productive use”.
 is an umbrella term for techniques which
collect temporary surface and sub-surface
flows and store them for later use.
 The sources of water that are to be harvested
are the roof water, sheet flow or intermittent
and even perennial water.

Why harvest water?
As land pressure rises, more and more marginal areas in
arid or semi-arid areas are being used for agriculture.
On an area where rainfalls irregularly and much of the
precious water is soon lost as surface runoff.
Irrigation has proved costly and can only benefit a
fortunate few.
Harvesting benefit runoff from causing erosion.
Can improve reliability and yields of rainfed agriculture.

Catchments types for harvesting
Roofs
Surface catchments
Sub-surface

Water harvesting restrictions
1) Slope: Not recommended for area having slope > 5% this is
because
 uneven distribution of runoff
 Large quantities of earthwork required which is not economical
2) Soil
– Should be deep >1m, 2m is ideal and underlain by
impermeable layer
– Should not be saline/Sodic and ideally possess inherent
fertility

6.1 Principles of water harvesting
Similar to other hydraulic structures,
water harvesting structure require a wide
range of input.
In many regions local thumb rules are
used for designing water harvesting
structures mengistu.zantet@gmail.com .

The following procedure can provide the help
for driving the thumb rule ratio.
{Cultivated area (ClA) * [Crop water requirement (CWR)
- Design rainfall (DRF)]}= {Catchment area
(CtA)*DRF*Efficiency (EF)*Runoff coefficient(C)}
• CtA + ClA = Total area
Design rainfall (DRF)
• It is the rainfall amount which may be expected to fall
during one rainy season with a reliable probability of
occurrence

Efficiency factor (EF)
This factor takes into account the inefficiency
of uneven distribution of the water within the
field as well as losses due to evaporation and
deep percolation.
Where the cultivated area is leveled and
smooth the efficiency is higher.
Micro catchment systems have higher
efficiencies as water is usually less deeply
ponded.
Selection of the factor is left to the discretion
of the designer based on his experience and of
the actual technique selected.

Example: In arid climate a portion of the total land is required to be grown with Millet crop
usingthe harvested water from the left portion. Based on the followinginformation estimate the
percentage or portion of cultivated land out of the total land. CWR for Millet (total growing
season)=475mm,Designrainfall/DRF/(seasonal)=250mm(ataprobabilitylevelofP=67%),
Runoff coefficient/RC/(seasonal) = 0.25( low due to relatively long catchment and low slope),
Efficiencyfactor/EF/=0.5
Solutions:
EF
RC
DRF
DRF
CWR
ClA
CtA
*
*

 = ; 2
.
7
5
.
0
*
25
.
0
*
250
250
475



ClA
CtA
7.2ClA+ClA=TotalArea ; %
2
.
12
%
2
.
8
100


TotalArea
ClA

Rain water harvesting has the following four main
components
1) Catchment area;
2) Diversion channel;
3) Sediment pond/Silt trap/ Chamber box,
4) Storage;
I) Open reservoir
II) Sub-surface reservoirs
III) Tank

Home Assignment
1) What are the design criteria considered in the
construction of rainwater harvesting
structures in Ethiopia.
2) Discuss the performance of the structures in
the field, nowadays.
3) Discuss and compare the benefit and ill-effect
of open reservoirs and sub-surface reservoirs

6.2 Water harvesting techniques
Water harvesting techniques include a wide
range of methods, which are based on the
following three basic points:
sources of water available
required storage duration, and
Intended use of harvested water

On the bases of above listed points, the harvesting
techniques are classified as:
1) Roof harvesting
2) Runoff harvesting (for short term storage
and long term storage)
3) Flood water harvesting

Generally,
 In general, water harvesting techniques
which harvest runoff from roofs or ground
surfaces fall under the term: RAINWATER
HARVESTING
 While all systems which collect discharges
from watercourses are grouped under the
term:
FLOODWATER HARVESTING

Basic categories of water harvesting systems for
plant production
1. Micro catchments (rainwater harvesting)
 (Sometimes referred to as "Within-Field Catchment
System")
 Main characteristics:
 Overland flow harvested from short catchment length
 Catchment length usually between 1 and 30 meters
 Runoff stored in soil profile
 Ratio catchment: cultivated area usually 1:1 to 3:1
 Normally no provision for overflow
 Plant growth is even
 Typical Examples:
 Negarim Micro catchments (for trees)
 Contour Bunds (for trees)
 Contour Ridges (for crops)
 Semi-Circular Bunds (for range and fodder)

• 2 External catchment systems (rainwater
harvesting)
(Long Slope Catchment Technique)
Main Characteristics:
 Overland flow or rill flow harvested
Runoff stored in soil profile
 Catchment usually 30 - 200 meters in length
Ratio catchment: cultivated area usually 2:1 to
10:1
Provision for overflow of excess water
 Uneven plant growth unless land leveled
Typical Examples:
Trapezoidal Bunds (for crops)
Contour Stone Bunds (for crops)

3 Floodwater farming (floodwater harvesting)
(Often referred to as "Water Spreading" and
sometimes "Spate Irrigation")
Main Characteristics:
Turbulent channel flow harvested either (a) by
diversion or (b) by spreading within channel
 bed/valley floor
Runoff stored in soil profile
Catchment long (may be several kilometers)
Ratio catchment: cultivated area above 10:1
Provision for overflow of excess water
Typical Examples:
Permeable Rock Dams (for crops)

7/15/2021
Hydraulic structure I ,Hydraulic and
water resources engineering,MTU 19

Chapter 6

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