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Identifying suitable areas for water harvesting in the Upper Blue Nile Basin, Ethiopa

Authors: Yihun Dile Taddele, Louise Karlberg, Johan RockströmStockholm Environment Institute

Pitting Strip catchment tillageMeskat-type system

What are they?Contour bunds

Semi-circular bunds

Floodwater harvesting within the stream bed Ephemeral Stream Diversion

Hillside sheet/rill runoff utilization

Dry spells

Flooding

Drought

Land degradiationWhy Water Harvesting

Then what?

- with water harvesting- without water harvesting

Bridging dry spells and droughts, and creats a resilient farming system which can produce more crop per drop

Avoid flooding problems by flattening the hydrograph: Ex-situ water harvesting Supports a sustained stream flow downstream: both in-situ and ex-situ

Improves soil fertility

BUT?

Precipitation DEM Land Cover Soil

Surface Analysis

Slope map Land coverrastar map

Land coverrastar map

Vector to grid conversion

ReclassifiedPrecipitation

Reclassified slop map

Reclassified land cover

ReclassifiedSoil

Reclassifying into 5 classes

WH suitability maps

Weighted Overlay Analysis

Suitability Criteria

Suitability values 1 2 3 4 5 Precipitation(mm) <200 >1200 200-400 800-1200 400-800Slope(%) >20 12-20 8-12 2-8 <2 Land cover Bushland, forest N.A N.A plantations cultivated land woodland, grassland irrigated land swamp, shurbland

Soil Leptosols Arenosols Vertisols Luvisols, Cambisols Regosols Acrisols, Alisols Fluvisols, Nitisols

Percent Influence Assignment

Percent Influence

Parameter

Precipitation

Slope

Land use

Soil

Condition - 1

25

25

25

25

Condition - 2

30

30

20

20

Condition - 3

35

25

20

20

Condition - 4

25

35

20

20

Condition - 5

35

30

20

15

S = ∑wi*xi

S = final suitability score; wi is weight of factor i and xi is the criterion score of factor i

Suitability Analysis

Equal inflience assignment for precipitation, slope, land use and soil

30% influence assignment for Precipitation & slope, and 20% for land use & soil

Results

35%, 25%, 20% and 20% influence assignmentFor precipitation, slope, land use & soil resp.

25%, 35%, 20% and 20% influence assignmentFor precipitation, slope, land use & soil resp.

Results

Results

Suitability level

Condition-1

Condition-2

Condition-3

Condition-4

Condition-

5

5 0 0 0 0 0

4 23 11 8 10 6

3 57 61 61 62 61

2 16 23 28 23 29

1 0 1 0 1 0

% precipitation slope

Discussion and conclusion

The Upper Blue Nile has a large potential for water harvesting: 6% to 24%, most suitable and over 50% moderate suitability

The results of the study are highly sensitive to precipitation amounts

If higher value is given for other land use types such as open woodland, the potential suitable area will increase considerably

soil type: Luvisols, Cambisols, Vertisols increases water avilability for livestock – increases farmers tendency to invest in agriculture – which has nexus effect on increasing on water productiity

Thank You

Questions?