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Presented by Abeyou Wale, Amy S Collick, David G Rossiter, Simon Langan and Tammo S. Steenhuis at the Nile Basin Development Challenge (NBDC) Science Workshop, Addis Ababa, Ethiopia, 9–10 July 2013
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1
Realistic Assessment of Irrigation Potential in the Lake Tana Basin,
Ethiopia
Abeyou WaleAmy S Collick,
David G Rossiter, Simon Langan and
Tammo S. Steenhuis
Nile Basin Development Challenge (NBDC)
Science Workshop
Addis Ababa, Ethiopia
9 – 10 July 2013
Contents
1. Introduction
1.1. Study Area
1.2. Objectives
2. Method
3. Result
4. Conclusion and Recommendation
2
Introduction
3
Ethiopia has a large potential of water and land resources that could be easily developed for irrigation.
12 major river basin, annual runoff volume of 122 bm3 of water.
Water resources potential of Ethiopia is demonstrated by the under utilized, from 3.7 m ha of irrigation potential approximately 5 to 7% is only develop (Awulachew et al 2007).
The government of Ethiopia is planning to solve this paradox through agricultural led development program.
The study area is considered as development corridor of the national and regional government for 2011–2016 Plan for Accelerated and Sustained Development to End Poverty (PASDEP).
This study is aiming to give a close view of the surface irrigation potential of Lake Tana Basin.
Introduction…
4
Study area
Objectives
The general objective of this research is to assess the surface irrigation potential based on river discharge and land suitability in the Lake Tana Basin. The specific objectives of this study are:
Mapping of areas suitable for surface irrigation based on a GIS based multi-criteria evaluation technique.
Identifying medium and large-scale areas over 200 ha, those are suitable for irrigations and areas that can be irrigated with the existing river discharges.
Area 15,000 km2, lake covers 20%
North-West highlands of Ethiopia.
Method
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Factors affecting
surface irrigation:Climate conditionRiver proximityTopography (slope)Market outlets (Roads and Urban proximity)Soil typeLand cover
Method…
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Suitability classes
FAO (1976 and 1981) framework
Class S1
Highly
Suitable:
Land without significant limitations. This land is not perfect but
is the best that can be hoped for.
Class S2
Moderately
Suitable:
Land that is clearly suitable but which has limitations that either
reduce productivity or increase the inputs needed to sustain
productivity compared with those needed on S1 land
Class S3
Marginally
Suitable:
Land with limitations so severe that benefits are reduced and/or
the inputs needed to sustain production are increased so that
this cost is only marginally justified.
Class S4
Less
Suitable:
Land that cannot support the land use on a sustained basis, or
land on which benefits do not justify necessary inputs
Method…
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Mapping factors: Soil map
Soil groups
Suitability for Irrigation
FAO (2006)
Suitability
Eutric Leptosols Extremely gravelly and/or
stony
S4
Lithic Leptosols
Eutric Vertisols
Considerable agricultural
potential
S2
Haplic Alisols Poor natural soil fertility S3
Haplic Nitisols Very productive soils S1
EMWR
Highly suitable : 47.43%Moderately suitable: 27.11%Marginally suitable: 6.40%Not suitable: 19.06%
Method…
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Mapping factors: Land use mapLand use Description Suitability
Dominantly cultivated Agricultural land S1
Moderately cultivated Agricultural land S1
Forest Natural Forest S4
Grassland Grass land S2
Plantations Artificial forest S4
Shrub land Dominated by shrubs S3
Highly suitable : 73.8%Moderately suitable: 2.8%Marginally suitable: 2.9%Not suitable: 20.5%
Method…
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Market outlets Urban proximity 50, 000 population (2007 Census of Ethiopia)Euclidean distanceEqual ranging technique
Distance to town 0 to 84 kmHighly suitable : 0 to 21 kmModerately suitable: 21 to 42 kmMarginally suitable: 42 to 63 kmLess suitable: 63 to 84 km
Town proximity
Method…
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Market outlets Road proximity Paved roadEuclidean distance Equal ranging technique
Distance to road 0 to 62.5kmHighly suitable : 0 to 15kmModerately suitable: 15 to 31kmMarginally suitable: 31 to 46kmLess suitable: 46 to 62.5km
Road proximity
Method…
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SlopeDEM SRTM 90mReclassified based on FAO slope class
FAO (1999)
http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp
% Slope
Class
Suitability
Class
< 2 S1
2 to 4 S2
4 to 8 S3
> 8 S4
DEM Slope
Method…
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River proximity DEM SRTM 90mExtracting major drainage networks Euclidean distanceEqual ranging technique
Distance to road 0 to 27kmHighly suitable : 0 to 6.7kmModerately suitable: 6.7 to 13.5kmMarginally suitable: 13.5 to 20.2kmLess suitable: 20.2 to 27km
Major Perianal rivers
River proximity map
Method…
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Climate Condition (Rainfall deficit)
Aug
EToPpt
ET
ETET
ET
Rainfall deficit (1992 to 2006)8 Ppt stations4 Eto stationsAnnual rainfall deficit -440 to -810 mm
Method…
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Weighting of factors Ranking technique: involves ordering of decision factors in their relative order of
importance.
Pairwise comparison (Saaty 1977): each factor will be matched head-to-head (one-to-one) and a comparison matrix is prepared to express the relative importance.
Importance Definition Explanation
1 Equal importance Two factors contribute equally to the objective
3 Somewhat more
important
Experience and judgement slightly favour one
over the other
5 Much more
important
Experience and judgement strongly favour one
over the other.
7 Very much more
important
Experience and judgement very strongly favour
one over the other. Its importance is
demonstrated in practice.
9. Absolutely more
important
The evidence favouring one over the other is of
the highest possible validity.
2,4,6,8 Intermediate values When compromise is needed
Method…
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Pairwise comparison (Saaty 1977):
Factors SO LU RiP UP RoP RD SL
Wt.
Pairwi
se
Wt.
Rankin
g
River Proximity
(RiP) 3 7 1 6 2 4 2 32 20
Road Proximity
(RoP) 2 6 1/2 5 1 2 2 22 18
Slope (SL) 3 5 1/2 4 1/2 3 1 19 17
Soil (SO) 1 4 1/3 4 1/2 2 1/3 12 15
Rain deficit (RD) 1/2 3 1/4 2 1/2 1 1/3 8 14
The rows indicate the strength of the factors Given a the factor (on the left) and another (on top) how much
strongly important is the first factor for surface irrigation area suitability than the second?
To evaluate the credibility of the pairwise matrix consistency was checked, the result indicated the judgment to be trustworthy
Result
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Preliminary Suitability Map
Weights are distributed to four classes of suitability by equal interval ranging technique:
Ranking Pairwise
𝑆 = 𝑓𝑖𝑤𝑖
𝑛
𝑖=1
Constraint map
Ranking Pairwise
Result…
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Ranking
Pairwise
Optimal sights: Preliminary Suit. > 85
Majority pixel filter Area > 200ha
Ranking Pairwise
Suitable
Area (ha)
No Large
scale area
No Medium
Scale
Percentage of
suitable area
Gilgle Abay 54,894 4 78 12
Ribb 31,780 3 4 16
Gumara 24,805 2 16 14
Megech 19,029 2 8 19
Total 130,508 11 106
60,750 ha5% of the land
130,500 ha11% of the land
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Water Availability
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Watershed Irrigation potential of Q90 (ha)
Gilgel Abay 0.607 (2,040 to 2,780 ha)
Gumara 0.577 (750 to 1,020 ha)
Ribb 0.086 (129 to 175 ha)
Megech 0.088 (64 to 87 ha)
Conclusion and recommendation
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Conclusion
Nearly 11% of the land in the Lake Tana Basin is suitable for surface irrigation.
However, by analyzing 27 years of river discharge, less than 3% of the potential irrigable area (or less than 0.25% of the basin area) could be irrigated consistently by 90 percentile available flow.
The main limitation for irrigation in Lake Tana Basin is the available water and not land suitable for irrigation. The irrigation potential around lake Tana can be met by construction of reservoirs or by pump systems using water from the Lake Tana.
Conclusion and recommendations
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Recommendation
In order to improve the result of this study: Chemical property of soil and soil depth has to be considered.
The study can also be done crop specific.
22
አአአአአአአ!
Thank you!
23
አአአአአአአ!
Thank you!
Anticipated Outcomes
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Background
Area of interest
Problem statement
Objectives
Approach
General description of study area
General description of spatial data needed
Proposed analytical methods
Spatial analysis flow diagram
Graphics
Maps
References
Introduction …
25
Irrigation Irrigation: is the artificial application of water to soil to assist
the production of crops
if crop water requirement is met by rainfall irrigation is not required
Surface irrigation: water is distributed over the field by gravity, the water is introduced at a highest point
Introduction…
26
Blue Nile Basin
Contributes more than 60% to the Nile River
In Ethiopia second largest watershed covering 20% of country area, contributing 27% the country irrigation potential
Accounts 50% the total surface runoff, more than 50% of ground water potential
Until recently there is only one water resource structure to control flow of water downstream
The Irr. potential developed is below the national less than 2%
Method…
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Constraint map
Limit the application of surface irrigation
Constraints include water body's, urban areas, forest and protected areas.
Constraint map has a value of 1 and 0, value of zero is assigned for constraints areas.
0
00
00
00
0
0
00
0
000
00
Method…
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Rainfall deficit (1992 to 2006)
• 8 rainfall station
• 4 potential evaporation
• Interpolation by Thiessen
Polygon method
• Rainfall deficit map is computed
ET
ET
ET
ET
2
2
34.01
273
900)(408.0
U
eeUT
GR
ETasn
o
Method
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Factors affecting surface irrigation Factors included in this study :
• Climate condition (temperature, humidity, rainfall wind speed etc)
• Water availability (river proximity)
• Topography (slope)
• Market outlets (Roads and Urban proximity)
• Soil type
• Land cover
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