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…

6

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…

7

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.

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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 …

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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…

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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

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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