TAMMO S. STEENHUIS, DAWIT ASMARE, MOHAMMAD ENKAMIL, CHRISTIAN

GUZMAN, TIGIST Y. TEBEBU, HAIMANOTE BAYABIL, ASSEFA D. ZEGEYE, SEIFU

TILAHUN CHARLOTTE MACALISTER AND SIMON LANGAN

EVALUATING BEST MANAGEMENT PRACTICES FOR DECREASING

DOWNSTREAM SEDIMENT LOAD IN A DEGRADING BLUE NILE BASIN

NILE BASIN DEVELOPMENT CHALLENGE (NBDC) SCIENCE WORKSHOP

ADDIS ABABA, ETHIOPIA, 9–10 JULY 2013

What is the effect of improved rainwater

productivity in Ethiopia on the discharge

and sediment load downstream?

QUESTIONS TO BE ANSWERED RAINWATER PRODUCTIVITY EFFECTS ON SEDIMENT AND DISCHARGE

What was the discharge and sediment

concentration in the past?

Is there a trend?

What will be discharge and concentration in

the future with improved rainwater productivity

PAST DISCHARGE AND SEDIMENT

CONCENTRATION TRENDS

Very little data available, therefore

• Use mathematical model to relate the existing

discharge and sediment concentrations with rainfall

• Assume that changes in parameters reflects trends

• Assume that the main impact on the hydrology and

sediment load is an increase in degraded areas in the

landscape

• Climate changes (that has been minimal over the past)

are included in the mathematical model

WHAT WE DID Obtained discharge and sediment concentration data for Blue Nile at the Sudanese border

Gumura

Anjeni

Debre Mawi

Calibrated model to historical and recent data

Used historical parameters for current period and visa versa to predict change

Parameter Efficient Distributed Model (PED model)

BASICS OF THE MODEL

Basics of Model: Rainfall Intensities generally

greater than infiltration rates for unsaturated soils

Debre Mawi

Maybar

Ethiopia

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0.00 0.20 0.40 0.60 0.80 1.00Infi

ltra

tio

n r

ate

or

rain

fall

in

ten

sit

y c

m/h

r

Probabaility of Exceedance

Rainfall Intensity Lowest Infiltration Rate

Median infiltration rate

Model Basics

Consequently,

Only those areas that saturate

during storm produce runoff

Regional groundwater rising to

surface in valley bottoms

Degraded soils with shallow low-

permeable sub soils

1

2

3

4

5

TESTING OF MODEL BASICS, DEBRE MAWI

Rainfall intensity vs.

storm runoff

R2 <0.4

Total rainfall vs.

storm runoff,

R2 > 0.59

Wee

kly

run

off

(m

m)

Rainfall intensity Weekly precipitation

Location of runoff source and infiltrating areas

Hill slope Areas

Degraded soils

Saturated

Surface runoff

infiltration

interflow

RUNOFF PLOTS (MAYBAR) SURFACE RUNOFF DECREASES

WITH STEEPNESS

16 37 43 64

slope of land

Runoff Coefficients

HYDROLOGY MODEL

There is a constant area for storm outflow after the

threshold is exceeded.

In the remaining part of the watershed, rain

infiltrates and becomes stream flow at some point

The threshold value can be obtained by simulating

a water balance. The threshold value is exceeded

when the soil exceeds field capacity.

EROSION MODEL

• Surface runoff interflow and baseflow from three areas

<30 days; 30 - 60 days >60 days

H = 1 H decreases 1→0 H=0

Model has been validated

in several small watersheds

GUMARA 1,500 KM2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1980 1985 1990 1995 2000 2005 2010

po

rtio

n o

f w

ater

shed

are

a

GUMARA: INPUT DATA

Well drained

hillsides

Max Stor Saturated Area 90 mm

Max Stor Degraded Area 30 mm

Max Stor Hillside 250 mm

base flow half life (t1/2) 20 days

interflow (τ*) 35 days

Degraded

Hillsides Periodically saturated areas

0

2

4

6

8

10

12

14

16

01/01/1987 01/01/1988 01/01/1989 01/01/1990 01/01/1991 01/01/1992

Dis

char

ge(

mm

/day

)

Observed…Predicted…

GUMARA WATERSHED

VALIDATION NS=0.6

GUMARA DISCHARGE CALIBRATION 1981-1986 VALIDATION 1987-1992

y = 1.04x + 0.81 R² = 0.85

0

50

100

150

200

250

300

350

0 100 200 300 400

Pre

dic

ted

Flo

w(m

m/m

on

th)

Observed Flow(mm/month)

y = 0.9237x + 1.692 R² = 0.8505

0

50

100

150

200

250

0 50 100 150 200 250

Pre

dic

ted

Flo

w(m

m/m

on

th)

Observed Flow(mm/month)

GUMARA VALIDATION/CALIBRATION SEDIMENT CONCENTRATION

1981-1992 1994 -2005

y = 0.9997x R² = 0.8513

0

1

2

3

4

5

6

7

8

9

10

0 5 10

Pre

dec

ted

sed

imen

t co

nc,

g/L

Observed sediment conc, g/L

y = 1.03x + 0.22 R² = 0.85

0

1

2

3

4

5

6

7

8

9

10

0 5 10Pre

dic

ted

Sed

imen

t co

nce

ntr

atio

n (

g/l)

Measured Sediment Concentration (g/l)

0

2

4

6

8

10

12

14

16

18D

isch

arg

e(m

m/d

ay)

Observed flow(mm/day)

Predicted flow(mm/day)

degraded

GUMARA EFFECT OF DEGRADATION

4% DEGRADED SOIL VS 14% DEGRADED AREAS

Cumulative discharge Cumulative soil loss

0

1

2

3

4

5

6

7

8

9

Cu

mu

lati

ve d

isch

arg

e, m

Axis Title

not degraded

degraded

0

10

20

30

40

50

60

70

80

90

100

Cu

mu

lati

ve s

edim

ents

loss

, To

ns

not degraded

degraded

BLUE NILE

Discharge 2003, degraded area = 0.22

Blue Nile watershed, 170,000 km2

0

50

100

150

200

250

300

350

400

450

5000

5

10

15

20

25

30

35

40

45

50

1-Ja

n-0

3

2-M

ar-0

3

1-M

ay-0

3

30-J

un

-03

29-A

ug

-03

28-O

ct-0

3

27-D

ec-0

3

Pre

cip

tati

on

(m

m/1

0-d

ays)

Dis

char

ge

(mm

/10-

day

s)

2003 calibration 1993 calibration 1993 calibartion

Observed Precip

1960

SEDIMENT CONCENTRATION 2003 DEGRADED

AREA =0.22 BLUE NILE WATERSHED 180,000 KM2

0

50

100

150

200

250

300

350

400

450

5000

1

2

3

4

5

6

7

8

9

10

1-Ja

n-0

3

2-M

ar-0

3

1-M

ay-0

3

30-J

un

-03

29-A

ug

-03

28-O

ct-0

3

27-D

ec-0

3

Pre

cip

tati

on

(m

m/1

0-d

ays)

Sed

imen

t co

nce

ntr

atio

n (

g/l)

Observed Predicted 2003

Predicted 1993 Precipitation

SEDIMENT CONCENTRATION 1993 DEGRADED AREA =0.18

BLUE NILE WATERSHED 180,000 KM2

0

50

100

150

200

250

300

350

400

450

5000

1

2

3

4

5

6

7

8

9

10

1-Ja

n-9

3

2-M

ar-9

3

1-M

ay-9

3

30-J

un

-93

29-A

ug

-93

28-O

ct-9

3

27-D

ec-9

3

Pre

cip

tati

on

(m

m/1

0-d

ays)

Sed

imen

t co

nce

ntr

atio

n (

g/l)

Observed Predicted 1993 degr. frac. 0.18

Predicted 2003 degr. frac. 0.22" Precipitation

CUMULATIVE DISCHARGE BLUE NILE SUDAN BORDER

0.0

0.5

1.0

1.5

2.0

1-Ja

n-98

1-M

ay-9

8

29-A

ug-9

8

27-D

ec-9

8

26-A

pr-9

9

24-A

ug-9

9

22-D

ec-9

9

20-A

pr-0

0

18-A

ug-0

0

16-D

ec-0

0

15-A

pr-0

1

13-A

ug-0

1

11-D

ec-0

1

10-A

pr-0

2

8-A

ug-0

2

6-D

ec-0

2

5-A

pr-0

3

3-A

ug-0

3

1-D

ec-0

3

Cu

mu

lati

ve D

isch

arg

e (m

) 2003 degr frac 0.22

1993 degr frac 0.18

1963 degr frac 0.10

10% difference is equal to 6 BCM

which can irrigate 500,000 ha

CUMULATIVE SEDIMENT LOSS BLUE NILE SUDAN BORDER

0

10

20

30

40

50

1-Ja

n-98

1-M

ay-9

8

29-A

ug-9

8

27-D

ec-9

8

26-A

pr-9

9

24-A

ug-9

9

22-D

ec-9

9

20-A

pr-0

0

18-A

ug-0

0

16-D

ec-0

0

15-A

pr-0

1

13-A

ug-0

1

11-D

ec-0

1

10-A

pr-0

2

8-A

ug-0

2

6-D

ec-0

2

5-A

pr-0

3

3-A

ug-0

3

1-D

ec-0

3

Cu

mu

lati

ve s

oil

loss

(To

ns/

/ha)

2003 degr frac 0.22

1993 degr frac 0.18

1963 degr frac 0.10

Need more data

ARE THESE RESULTS

REASONABLE?

ALMOST NO EFFECT ON DISCHARGE

LARGE EFFECT ON SEDIMENT

ANJENI: TERRACES INSTALLED IN 1986 AND 1987

EFFECT ON DISCHARGE AND LAND USE

ANJENI INSTALLATION OF TERRACES 1986-1987

DISCHARGE

0

20

40

60

80

100

120

140

160

180

2000

5

10

15

20

25

30

31-D

ec-8

3

26-S

ep-8

6

22-J

un-8

9

18-M

ar-9

2

13-D

ec-9

4

8-S

ep-9

7

4-Ju

n-00

Dai

ly S

trea

m F

low

(m

m/d

ay)

Measured Flow

Predicted Flow

precipitation

Installation

of terraces

SEDIMENT CONCENTRATION AT WATERSHED OUTLET

0.00

10.00

20.00

30.00

40.00

50.00

60.00

5/31/1984

10/13/1985

2/25/1987

7/9/1988

11/21/1989

4/5/1991

8/17/1992

12/30/1993

Sed

imen

t co

nce

ntr

atio

n,

g/l

Installation

of terraces

Rain water Management

Practices for Erosion

Control

EFFECT OF INSTALLATION OF TERRACES

•

• Virtually no effect on total runoff and distribution

between various discharge components

• Reduces sediment by what can be stored behind the

terraces. Once terraces are level, sediment

concentration are nearly back to old levels

EFFECTIVENESS OF

INFILTRATION FURROWS

AMOUNT OF SOIL SAVED IS

WHAT CAN BE STORED IN

FURROWS

PERMANENT PLANT COVER ON DEGRADED AREAS

This will stop erosion and provides income

Preventing head cuts moving upstream

Shaping the gully below angle of repose

Planting trees around gullies

Gully check dams

Planting Trees

Only by reversing the degradation of the land

further increases in sediment load can be

prevented

Structural soil and conservation measures are

only effective for a limited time to control

erosion

Arresting gully formation will save land and

reduce sediment in streams

No-till will conserve soil, but might increase soil

degradation due to increased pesticide use

Supporting Publications at

soilandwater\bee\cornell.edu

Search for soilandwater Ethiopia Cornell