Rainwater harvesting is an indispensable solution when it comes to addressing the challenges of climate change and variability

1. A
Haramaya University, Ethiopia
Water management and water harvesting
Tasisa Temesgen Tolossa (Assistant Professor in
Irrigation Agronomy)

2. Why Water harvesting is needed?
o A growing global population and
changing diets are driving up the
demand for food ( World Bank, 2020
o Yield decreased
o The problem is very serious over the
glob
o Food insecurity.

3. Con…
 Globally in 2021 estimated that
between 702 and 828 million
people were affected by hunger
(FAO; IFAD et al. 2022).
 However, the global population
could grow to around 8.5 billion in
2030, and add 1.18 billion in the
following two decades, reaching 9.7
billion in 2050 (UN, 2022).
 The food security challenge will
become more difficult, as the global
food demand which is expected to
rise by up to 110% by 2050 to feed
an estimated population (van Dijk
et al. 2021).
2000’s = 6.144 billion
29.26% growth rate
2.05 billion mt
3.00 billion mt
Source: World Bank 2022. https://data.worldbank.org/

4.  The earth’s population is
increasing
 Demand for food is also
increasing
 This force increases
agriculture
 Agriculture uses about
70% of the world’s
freshwater
 To a great extent,
agriculture threatens
global water resources in
terms of both quantity
and quality.
Con…

5.  Today, the global community is facing two
interconnected challenges of climate
change and food insecurity
 Due to limited availability of agricultural
land, freshwater for agriculture use, and
impacts of climate change, the agriculture
sector has to feed 9 billion people that are
going to inhabit on Earth by 2050.

6. Con…
 CSA as research and policy links between
climate change and agriculture have
advanced, ‘climate-smart agriculture’ has
emerged as a framework to capture the
concept that agricultural systems that can
triple wins
 Agricultural development communities
and climate-smart agriculture includes
many of the field-based and farm-based
sustainable agricultural land management
practices.

7. Con…
Mitigation
Productivity
(Food Security)
Adaptation
CSA
CSA aims to achieve three outcomes
Increase productivity
Enhanced resilience
Reduced emission
Figure 1. Climate-smart agriculture (CSA)

8. Rainwater harvesting (RWH)
o Rainwater harvesting (RWH)
technology can be regarded as
a guarantee to improve
livelihoods by making water
available in dry area (Senay
and Verdin 2004).

9. Roof water harvesting
o Rainwater harvesting is among the
prominent strategies accepted by
Ethiopian Government to cope with
the challenges of drought and food
insecurity (Getachew, 1999).
o Water shortage is becoming the major
cause for conflicts in pastoral parts of
the country.

10. RWH Cont’d------
o Mostly affected people who are
forced by water shortages to move
their livestock elsewhere, creating
tension between villagers to share the
same land and water points.
o Rainwater harvesting can be
measure to reverse the situation if
implemented on the massive scale

11. o Most of the districts in Eastern
Hararghe zone have critical
problem of water shortage even
though there is huge potential for
water harvesting.
o During the last few years, climate
change is emerging as foremost
challenge causing erratic rainfall
distribution.
Roof water harvesting------------

12. Cont’d----
o Collecting water improves the
accessibility and convenience of water
supplies and has a positive impact on
livelihood and health.
RWH costs less to collect rainwater
than to exploit groundwater.
Arada Baarkale, ganda Rarree Sirb

13. Installation of Roof Top Rainwater Harvesting
System
o Selection will be based on the type
of the roof (corrugated iron sheet)
and the size (area of the roof).
o Total area of house rooftop will be
calculated.
o Appropriate location for
constructing water storage
structures will be identified near
the house and georeferenced.

14. Determination of rainwater endowment of roof
catchment
o Rainwater endowment of an area is the total amount of water that
could be received in the form of rainfall for water harvesting.
o Out of the total rainwater endowment of an area, the amount that
can be effectively harvested is water harvesting potential.
 Therefore, Water harvesting potential = Rainfall (mm) x Runoff coefficient
o The amount of rainwater that can be collected depends on three
things: Amount of precipitation in an area (W*L), Size of roof
catchment area and Proportion of the rainfall

15. Proportion of Total Rainfall (Collection Efficiency)
o Collection efficiency is the % of rainfall landing on the roof
catchment that will find its way to the storage tank.
o It depends on several site specific and seasonal factors.
o As a general rule of thumb, an average of 75–80 percent is
expected to be collected out of the actual precipitation.
o Common reasons for water loss include:
 Sit specific water losses that are affected by features of the
catchment.
 Collection system losses due to overflow and spillage.
 Seasonal factors that decrease the proportion of the rain

16. Calculating actual annual rainwater collection:
Step 1: Potential Annual Rainwater Collection
Potential water collection of roof catchment is based on the following
formula:
Catchment area multiplied by millimeter of rain
Example: 1m2 of catchment area receiving 1mm of rain could
produce 1 liter of water
(1 m2 x 1 mm = 1 liter)
Therefore, Catchment Area (in m2) multiplied by Annual
Precipitation (in millimeters) equals potential water collection (in
liters)

17. Actual Annual Rainwater Collection
o Actual Rainwater Collection equals Potential
Rainwater Collection multiplied by the Collection
Efficiency.
For above example, if the collection efficiency is
75%, the actual water to be collected annually would
be:
110 m3 x 75% = 82.5 m3, or 82,500 liters.
o Therefore, the monthly water supply and demand
will be estimated for the rainwater harvesting
system.

18. Demand (How Much Water, and for Which
Purposes)
o Decisions about water usage will affect how much water to be
collected and stored, and how to design the entire RWH system,
from a roof catchment to tap.
o These choices are best made at the outset of planning.
 Storage:
o How Much Rainwater to be stored can be determined on the basis
of demand and usage of the water.
o Amount of water to be stored will be estimated based on water
demand and for what purposes.

19. Estimation of water demand and storage size
o Estimation of water demand for domestic purpose will be
estimated based on the member of the households and expected
uses:
o Suppose the water requirement of a five member family living in a
house of 100 m2 roof is 10 liters per person, average annual rainfall
is 800 mm:
o Annual water harvesting potential will:
 (Area of the roof) x (annual rainfall in meter) x (runoff
coefficient) x (rainwater harvesting efficiency)
= (100 m2) x 0.80m x 0.85 x 0.80
= 54.4 m3
= 54,400 liters
10m
10m

20. Cont’d’-------
o The size of water storage tank will be designed for the dry period between two
consecutive rain seasons.
o For the Haramaya short rainy season starts from March to May and the long
rainy season from July to September.
o Long dry season is from October to March i.e. 6 months (180 days).
o Drinking & cooking water requirement for a 5 member family is:
 180 days x 10 liters/day x 5 = 9000 liters
o For safety factor the storage tank should be built 1.2% larger than the actual
size.
 Final size of the storage tank = (1.2 x 9000 liters)
= 10,800 liter (10.8 m3)

21. Materials for installation of roof top rainwater
harvesting
o Gutter for collection of the water from the roof will be PVC or
galvanized iron sheet.
o Depth of the collection gutter will be adjusted to the depth of the
highest peak of rainfall of the area to avoid overflows from the
gutter system.
o A PVC pipe will be used to deliver water from the gutter system to
the storage tank.
o This deliver system will be installed with 1st flush diverter.
o In between the storage tank and the delivery system leave screen
and filtration system will be installed to clean leave and suspended

22. Storage Tank
Water inlet
Water outlet
Entrance for
cleaning and
maintenance
3
m
0.5m
2
m
Overflow
Figure . Water storage tank

23. Storage Tank cont’d----
Gutter
system Leaf
screen
1st
flush
diverter
Storage tank
Water outlet
Overflow
Entrance for cleaning
and maintenance
Figure . Typical roof top above ground rainwater harvesting system

24. Gutter
system
Leaf
screen
1st
flush
diverter
Overflow
Entrance for cleaning
and maintenance
Water outlet
Figure . Typical roof top partially underground rainwater harvesting system
Storage Tank cont’d----

25. Gutter
system
1st
flush diverter
Settlement
tank
Storage Pit in ground
Opening for
fetching water
Figure . Typical roof top rainwater harvesting system in pit
Storage Tank cont’d----

26. Thank you for your Attention