1. Recent changes and Emerging trends in Inland Water Bodies in Ethiopia
A. Causes of recent changes in Rift valley
lakes as summarized by Ayenew
(2008) are:
A. Anthropogenic- Excessive water use, Mismanagement
B. Natural- Climate change- ( Global / local
),and
C. Neo-tectonics.
He also mentioned the disturbing
recent changes in Rift valley lakes as:
·
Lakes
show contrasting temporal variations in stage and size .
· Complete
drying up of lakes.(E.g. Haromaya in the
Eastern Hararghe highlands)
·
Disappearance
of springs/swamps/ connected to the lakes
B. Major emerging changes in Ethiopian Lakes
(Ayenew 2007, Ligdi 2008,2010a)
- Lakes show contrasting temporal variations in stage and size . Some are expanding eg. Lake Beseka (areal), Lake Hawassa (Rising levels)); and some are shrinking (e.g..Abiata and Ziway)
- Complete drying up of lakes.(E.g. Lange , Kersa, and recently Adele and Haromaya in the Eastern Hararghe highlands)
- Disappearance of springs/swamps/ connected to the lakes ( E.g. wetland degradation)
- Change in the biodiversity (biota degradation, decline in fishery Stocks ,etc…)
- Change in constituents and water quality deterioration (sedimentation, eutrophication, and signs of cynobacterial toxicity
C. The main
anthropogenic effects on waterbodies (Ligdi,2008, 2010a)
- Habitat & Species degradation (biota in the form of riparian vegetation, ecotones, wetlands, etc...; ,fisheries; other biodiversity);
- Water level flactuations (diversion of tributaries, excessive withdrawals, seasonal flooding, etc...)
- Sedimentation from non point source pollution ( causing rising water levels, flooding, storage loss, water quality deterioration & eutrophication) ; plus
- point source pollution from domestic waste(urban + rural), urban (and settlement+livestock) sewage,and industrial effluent leading to
- Eutrophication and Cynnobacterial toxicity due to incoming high loads of Nitrogen & Phosphorus and toxic metals [Aba Samuel, Koka, Awassa, Tana]
D. The agonizing case
of Lake Haromaya (Ligdi,2010a)
One
of the best cases in point to show the recent trends in inland water bodies in
Ethiopia is the drying up and sudden
disappearance of Lakes in the eastern Hararghe highlands in Oromia.. There were
around 3 other lakes which completely dried up in the Eastern Hararge Highlands
in Oromia in the last half century or so. Lake Haromaya is the recent
victim of the phenomenon resulting in
complete drying up since around 2004.
The Present Day Lake Haramaya : From a Water Source to a Grazing
Land.
The Haromaya lake used
to cover a surface area of 472 hectares and received a mean annual rainfall of
723 mm. The Haromaya lake was once the largest source of fresh water and
for over 40 years( 1961 – 2004) ,
it had provided fresh water for domestic(human and Livestock) water supply for the Haromaya
and Harar towns.It had also supplied water for irrigation, and reently for industrial
uses in and around the Haromaya and Harar towns and the vicinity.
Furthermore, it had supported ecological life to fishes, and other species.,
besides being a drinking source and feeding place to birds and
animals.
Some
15 years ago, the lake had a maximum and mean depth of 8 m and 3.13 m respectively
covering a surface area of 47.9 km2. Now, the lake is completely dry
since around 2004. Groundwater is on average 3 meters below the dried lake bed.
Regrettably, the present day Lake Haramaya has changed from a water source to a
grazing land.
A
multitude of factors may have contributed to the drying up of the lake which
requires serious investigations. Ecological, hydrological and Geological
(fructure) causes have been postulated since.
Nevertheless, the drying up of the lake has largely affected domestic
and industrial water uses, recreation,
fisheries and agricultural activities.
2. Characteristics
of some Reservoirs in Ethiopia: Recent Status of Representative old Dams (Ligdi,
2010a)
The Aba Samuel Dam
One of
the oldest and abandoned storage reservoir constructed on the Akaki River at
Akaki-South of Addis Ababa, is the Aba Samuel Dam. It was constructed in 1939
for hydro-power production. Reservoir sedimentation survey was carried out in
1983 after 44 years of service & the average specific sediment yield was
estimated at 445 t'km2/yr.). Besides the
non point source pollutants from agricultural sediments, the Aba Samuel
reservoir has long been entertaining the sediment yields from construction and
mining sites in its watershed in and
around the city. Furthermore, the dam receives point source pollutants
in the form of urban waste, domestic (raw) sewage and factory effluents and
contaminants from the tributaries of Akaki River which drain the city of Addis
Ababa. As a result, it is polluted with eutrophication and signs of
cynobacterial toxicity and abandoned long ago.
The
Koka Dam
The
Koka reservoir has been created as a result of the construction of the Koka Dam
in 1959 for developing hydroelectric power. Awash and Modjo rivers which are
heavily laden with sediments during the flood season are the two main rivers
which flow to the reservoir. According to a recent bathymetric survey, the
capacity of Koka reservoir has been reduced from 1667Mm3 in 1959 to 1186 Mm3 in
1998 .The loss on total capacity over 39 years is 481 Mm3 i.e. 28.8% of the
total storage volume. The average annual loss of capacity is 0.74% and annual
silting rate is 12.32 Mm3. It was found that about 397.1 Mm3 (i.e. .82.6%) has
been deposited in the active storage. The displacement of 481 Mm3 of
water per year by sediments translated to energy loss and the
subsequent output lost (by manufacturing sector) will be tremendous. The sediment deposits in the
reservoir including the year 2000 have grown to be a serious threat to the
intakes at the dam and have already reduced the useful storage volume by
30.3%.(Haile,2001) .In addition
to sedimentation the lake is affected with incoming factory effluents and
contaminants from the tributaries leading to eutrophication and signs of
cynobacterial toxicity.
A recent study revealed that the pollution level from one of the factories effuluents before entering storage ponds in particular; and toxicity in water bodies(reservoirs) in general was shown to be even worse than dams fed by rivers draining the 18th century built factories in central Europe.Besides, the Koka dam has been reported as the "green lake'' in reference to the algael blooms causing toxicity and endangering the health of the society
A recent study revealed that the pollution level from one of the factories effuluents before entering storage ponds in particular; and toxicity in water bodies(reservoirs) in general was shown to be even worse than dams fed by rivers draining the 18th century built factories in central Europe.Besides, the Koka dam has been reported as the "green lake'' in reference to the algael blooms causing toxicity and endangering the health of the society
The Gefersa Dam
The
Geffersa dam with a catchment area is 55 km2 was constructed in 1955
at some 15 kms. west of the city, as a source of Addis Ababa city water supply.
Over and above lose of capacity, the increasing sediment loads and siltation is
causing water quality deteriorations and initiating costly additional water
purification requirements further exacerbating the task to satisfy the
skyrocketing demands of the city. To this end, it has gone through various improvements
and upgrading including construction of series of additional dams built upslope
,in addition to raising the dam height during its recent
renovation. Currently, Geffersa III (since 1986) dam acts a sediment
trap for Geffersa
I and II.
3.` Strategies for Dealing with Reservoir Sedimentation
The following four main strategies are
recommended to reduce sedimentation problem in storage reservoirs (Basson &
Rooseboom. (1996) mentioned in Haile,(2001))
1. Minimize sediment loads entering the reservoir
through;
Soil
and water conservation programmes (Integrated Watershed Management,( IWRM) ;
Upstream trapping of sediment (debris dams or vegetation screens); and Bypassing of
high sediment loads (bypass tunnel or channel, or off-stream storage that allows floods
Upstream trapping of sediment (debris dams or vegetation screens); and Bypassing of
high sediment loads (bypass tunnel or channel, or off-stream storage that allows floods
to be passed in the river).
2.
Minimize deposition in reservoirs through;
Drawdown
and sluicing: passing sediment-laden flows through the reservoir by means
of drawing down the water level; and venting density current
3. Removal
of accumulated sediment deposits through;
Flushing by drawing down the water
level , in many cases, emptying the reservoir
during floods or in the rainy season; Mechanical excavation or dredging; Conventional hydraulic dredging; and Hydraulic dredging by use of gravity (Transport of sediment in pipeline or by free surface flow in channels or tunnels).
during floods or in the rainy season; Mechanical excavation or dredging; Conventional hydraulic dredging; and Hydraulic dredging by use of gravity (Transport of sediment in pipeline or by free surface flow in channels or tunnels).
4.
Compensating for reservoir sedimentation;
Maintain long-term storage capacity
by raising the dam; Abandon or decommission the
silted reservoir and construct a new reservoir; and Import water from elsewhere.
silted reservoir and construct a new reservoir; and Import water from elsewhere.
4. Conventional hydro-technical technologies for reduction
of sediment yield (Gogus, 2007)
1.
Vegetative screens
Vegetation normally seeds on exposed
delta deposits of reservoirs to form natural
vegetative screens which
reduce inflow velocities and increase the roughness
coefficients, encouraging deposition above
the reservoir crest elevation within the
reservoir basin or immediately upstream.
2. Watershed structures
Several types of structures may be built in
a watershed for the specific purpose of
reducing sediment yield to a reservoir.
These include such structures as sedimentation
basins to trap sediment below eroding areas
and erosion control structures to halt the
production of sediment.
The reduction of channel erosion
normally requires a structure. In a watershed, where
the primary source of sediment is derived
from channel erosion, installation of control structures
may have an appreciable effect in reducing sediment yield to a reservoir.
Such structures include drop inlets and
chutes for reduction of gully erosion, stream-
bank revetment to reduce stream-bank
erosion, and sill or drop structures for
stream bed stabilization.
3. Watershed land -treatment measures
Land
treatment measures provide an effective and economical means of reducing
erosion and sediment yield where the
primary source of sediment is sheet (Raindrop, Rill and
Inter-rill) erosion. These measures include soil improvement, proper tillage
methods, strip cropping, terracing, and crop rotations and
others.
4. Control of wind erosion
Wind
erosion is a serious problem in areas of low precipitation, frequent drought,
and
where temperatures, evaporation, and wind
speeds are high.
5. Summary of available information (figures and values) on physical description and water quality parameters of Lake Tana.
NOTE
:-The available information (figures and values) on the physical
description of the lake (Length, Area, Volume , etc…); and water quality
parameters given in previous study & research documents differ & are
not consistent through. Based on a literature
review, summary of the comparisons of data on the Physiography of the Lake
(Table I) and some Selected Physical
and Chemical Water Quality Results from Previous Works is presented in Table
II.
Table I: Summary of Comparison of various
data and figures on the description of the
Physiography of Lake Tana (Sub-basin) based on Results
from Previous Works.
Parameters
|
Values
|
Date of
Sampling
|
Reference
|
||
· Surface
Area of Lake Tana (km2)
|
3000 – 3500
|
Late 1990 are...
|
BCEOM (1998)
|
||
3150
|
2003
|
E. Dejen (2003)
|
|||
3500
|
Ashine
(1998),
|
||||
3200
|
Wudneh
(1998),
|
||||
3675
|
Oduola
(2003)
|
||||
3600
|
www.worldlakes.org (2004).
|
||||
3050
|
estimated
|
Tesfahun &
Demisse(2005).
|
|||
85km wide x 65km
Long
84
long X 66km wide
|
Approximate
|
Tesfahun &
Demisse(2005).
Setegn (2008)
|
|||
· Catchments
Area of Lake Tana (km2)
|
15000 – 16500
|
1987 and 1997
|
BCEOM
|
||
16500
|
2003
|
E. Dejen 2003
|
|||
15054
15096
|
(Tesfahun &
Demisse(2005).
Setegn (2008)
|
||||
· Average depth (m)
|
8 – 9
|
Late 1990's
|
BCEOM
|
||
8 – 9
|
2003
|
E. Dejen 2003
|
|||
· Max.
Depth (m)
|
14
|
Late 1990's
|
BCEOM
|
||
14
|
2003
|
E. Dejen 2003
|
|||
15
|
2008
|
Setegn (2008)
|
|||
Storage
Volume (BCM)
|
28 - 29.2
|
1997
|
BCEOM
|
||
28
|
1988
|
Wassie, 2005
|
|||
· Shore
line length (km)
|
-
|
-
|
BCEOM
|
||
385
|
1976
|
Wassie, 2005
|
|||
· Altitude
(masl)
|
1786
|
Late 1990'
|
BCEOM
|
||
1830
|
2003
|
E. Dejen 2003
|
|||
1800
|
2008
|
Setegn (2008)
|
|||
1783.52
|
Datum (hydrology
Department)
|
MoWR
(Personal
communication)
|
|||
· Annual
Soil loss in the Lake Tana Sub basin (Tone/ha)
|
30 – 100
|
1980's and 1990's
|
BCEOM, 1908
|
||
31 – 50
|
2001
|
E. Dejen 2003
|
|||
Data Source:- Modified from MoWR (2008);
Ligdi (2008, 2010c)
Table II: Summary of Comparison of various data and figures of some Selected Physical and Chemical Water Quality Parameter Results of Lake Tana from Previous Works.
Parameters
|
Measured
Figures/values
|
Measurement
Year
|
Source
/
Reported
by
|
·
PH Values
|
7.50
- 8.87
|
1940
and 1997
|
BCEOM
|
6.8
- 8.3
|
2004
|
Wassie,
2005
|
|
·
EC (µS/cm)
|
105
– 234
|
1997
|
BCEOM
|
115
– 148
|
2004
|
Wassie,
2005
|
|
·
Dissolved oxygen (mg/l)
|
3.3
- 10.8
|
1997
|
BCEOM
|
5.9
- 7.3
|
2000
– 2002
|
E.
Dejen 2003
|
|
·
Chlorophyll 'a' (mg/m3)
|
3.7
- 6.2
|
1986
and 1988
|
BCEOM
|
3.4
- 12.9
|
2000
– 2002
|
E.
Dejen 2003
|
|
·
Biomass (mg C/m3)
|
129
|
1997
|
BCEOM
|
·
Transparency (cm)
|
31
– 182
|
1997
|
BCEOM
|
·
Temperature (°c)
|
18.3
- 26.2
|
1997
|
BCEOM
|
20
-27
|
2000
– 2002
|
E.
Dejen 2003
|
·
TDS (mg/l)
|
151
– 174
|
1925
and 1940
|
BCEOM
|
148
-178
|
2000
– 2002
|
E.
Dejen 2003
|
·
Calcium (mg/l)
|
18
– 27
|
1925
and 1940
|
BCEOM
|
·
Magnesium (mg/l)
|
9
– 10
|
1925,
1940 and 1995
|
BCEOM
|
·
Chloride (mg/l)
|
8
|
1925
and 1940
|
BCEOM
|
·
Silica (mg/l)
|
22
|
1940
|
BCEOM
|
·
Hardness (mg/l) as CaCo3)
|
85
|
1995
|
BCEOM
|
·
Total Alkalinity (mg/l)
|
104
|
1940
|
BCEOM
|
·
Turbidity (NTU)
|
-
|
-
|
-
|
13
– 85
|
2000
- 2002
|
E.
Dejen 2003
|
·
Annual Soil loss in the
Lake Tana Sub basin (Tone/ha)
|
30
– 100
|
1980's
and 1990's
|
BCEOM,
1908
|
31
– 50
|
2001
|
E.
Dejen 2003
|
Data Source:- MoWR,(2008); Ligdi (2008,2010c)
SOURCE:
Ligdi,E.E., (2011). Ecohydrology & Sustainable Sediment Management in Inland water bodies in Ethiopia. :
A Review with a case Study Site at Lake Tana in the Upper Blue Nile (Abbay) Basin in Ethiopia..A case
Study Report.UNESCO-IHP;FRIEND/Nile Project, Phase II;Eco-Hydrology Component. Addis Ababa, Ethiopia
A Review with a case Study Site at Lake Tana in the Upper Blue Nile (Abbay) Basin in Ethiopia..A case
Study Report.UNESCO-IHP;FRIEND/Nile Project, Phase II;Eco-Hydrology Component. Addis Ababa, Ethiopia
