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www.iwmi.org

Water for a food-secure world

Xueliang Cai

29/04/2014, Johannesburg, South Africa

Flow Regulating Functions

of Natural Ecosystems for

Dam synchronization in the

Zambezi River Basin

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Water for a food-secure world

Outline

• Introduction

• The Zambezi river basin

• Flow duration curves for analysing

hydrological functions

• Results

• Conclusions

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Water for a food-secure world

Introduction – why does it matter

• Africa, and to less extent Asia, landscape largely

characterized with natural vegetation and untamed areas;

• Forests, wetlands and floodplains big influence on

hydrological processes;

• Natural ecosystems into

water resources planning

and management (e.g.

dam operations) for green

economy;

• Lack of understanding on

hydrological functions of

ecosystems.

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Water for a food-secure world

Mixed findings of wetland hydrological functions

• 30/66: headwater wetlands reduce flood peaks, but 27 concluded the other way around.

• 11/20: headwater wetlands increased flood event volumes.

• 48/77: wetlands increase evaporation or reduce river flow.

• 47/71: wetlands reduce downstream flows during dry periods but in 20% of cases verse visa.

• 23/28: floodplains reduce or delay downstream floods

Bullock and Acreman, 2003, based on review of 169 studies

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Water for a food-secure world

ESA GlobCover

GLWDThe Zambezi

River Basin

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Water for a food-secure world

The Zambezi

River Basin

102 stations with 25 years

or more data

18 sites identified

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Water for a food-secure world

The method

Downstream

gauge

Upstream

gauge

Reference

gauges

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Water for a food-secure world

Establishing reference (no ecosystem) flow duration curve

Standardized FDCs derived from mean daily flow measured at gauges located in the

vicinity of the Luswishi floodplain

0.01

0.1

1

10

0.1 2 12 22 32 42 52 62 72 82 92 99.3

Q/Q

me

an

% time flow exceeded

Normalised reference FDC

Regional FDC (avg) GRDC 1591500 (Reference) FRIEND 60334250 (Reference)

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Water for a food-secure world

Transferring the reference flow duration curve to the

site of interest

0.1

1

10

100

1000

0.1

0.5

0.9 4 8

12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

99.1

99.5

99.9

Flo

w (m

3 s-1 )

% time flow exceeded

Reference FDC GRDC 1591440 (Downstream)

Comparison of the “reference” FDC and the observed FDC at the gauge

downstream of the Luwishi floodplain

FDCdestination = FDCreference * Qdes. mean

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Water for a food-secure world

Generating the reference flow time series

Pdes = (Pupstream + Pdownstream)/2

0.1

1

10

100

1000

0.1 2 12 22 32 42 52 62 72 82 92 99.3

Q/Q

me

an

% time flow exceeded

Reference FDC downstream of floodplain

Regional FDC (avg)

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Water for a food-secure world

Results

0

20

40

60

80

100

120

1-O

ct-8

4

1-N

ov-

84

1-D

ec-

84

1-J

an

-85

1-F

eb

-85

1-M

ar-

85

1-A

pr-

85

1-M

ay-

85

1-J

un

-85

1-J

ul-

85

1-A

ug

-85

1-S

ep

-85

1-O

ct-8

5

1-N

ov-

85

1-D

ec-

85

1-J

an

-86

1-F

eb

-86

1-M

ar-

86

1-A

pr-

86

1-M

ay-

86

1-J

un

-86

1-J

ul-

86

1-A

ug

-86

1-S

ep

-86

Flo

w (

m3s-1

)

Daily flow with and without floodplain: HY1984 and HY1985

Without floodplain (simulated) With floodplain (observed) Upstream floodplain (observed)

Flood plains

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Water for a food-secure world

ResultsBase flow index Mean annual minimum (m3s-1)

1-day 10-day

With floodplain 0.994 2.96 3.04

Without floodplain 0.886 2.02 2.13

Return

period

(yrs)

Flood Magnitude (m3s-1) %

reduction

With

floodplain

Without

floodplain

1.1 27.3 37.0 26.3

1.5 41.0 62.0 33.9

2 47.3 73.3 35.5

5 56.2 94.3 37.2

10 65.2 104.6 37.7

25 71.4 115.3 38.0

50 75.4 121.9 38.2

100 78.9 127.8 38.3

200 82.0 133.0 38.3

0

20

40

60

80

100

120

140

1 10 100

Pe

ak

flo

od

flo

w (

m3s-1

)

Return period (yrs)

Flood Frequency

With floodplain Without floodplain (simulated)

Extrapolated

0

20

40

60

80

100

120

140

1 10 100

Pe

ak

flo

od

flo

w (

m3s-1

)

Return period (yrs)

Flood Frequency

With floodplain Without floodplain (simulated)

Extrapolated

Flood plains

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Water for a food-secure world

ResultsHeadwater wetlands

0.00001

0.0001

0.001

0.01

0.1

1

10

100

0.1

0.5

0.9 4 8

12

16

20

24

28

32

36

40

44

48

52

56

60

64

68

72

76

80

84

88

92

96

99

.1

99

.5

99

.9

Q/Q

me

an

% time flow exceeded

Regional FDC (avg) - inc 65312102 FRIEND 65312602 (downstream)

0

50

100

150

200

250

300

350

400

450

5001

-Oct

-85

1-N

ov

-85

1-D

ec-

85

1-J

an

-86

1-F

eb

-86

1-M

ar-

86

1-A

pr-

86

1-M

ay

-86

1-J

un

-86

1-J

ul-

86

1-A

ug

-86

1-S

ep

-86

1-O

ct-8

6

1-N

ov

-86

1-D

ec-

86

1-J

an

-87

1-F

eb

-87

1-M

ar-

87

1-A

pr-

87

1-M

ay

-87

1-J

un

-87

1-J

ul-

87

1-A

ug

-87

1-S

ep

-87

Flo

w (

m3s-

1)

Daily flow with and without headwater wetlands: HY1984 and HY1985

Without headwater wetlands (simulated) With headwater wetlands (observed)

Percentile Flow (m3s-1) % difference

With

wetlands

Without

wetlands

99 0.0 0.1 -

95 0.0 0.5 -

90 0.0 0.8 -

75 0.2 2.1 980.4

50 2.2 5.3 142.1

25 20.1 17.1 -15.3

10 70.3 46.2 -34.3

5 107.0 73.5 -31.4

1 152.9 208.2 36.2

Bua River in Malawi

Total catchment area: 4,777 km2

Area of wetlands: 823 km2 (17.2% of

total catchment)

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Water for a food-secure world

ResultsHeadwater wetlands

BFI Mean annual minimum (m3s-1)

1-day 10-day

With wetlands 0.96 0.028 0.032

Without wetlands 0.74 0.389 0.443

Return

period

(yrs)

Flood Magnitude

(m3s-1)

%

reductio

nWith

wetlands

Without

wetlands

1.1 31.9 15.0 -112.7

1.5 75.0 24.8 -202.4

2 96.5 47.8 -101.9

5 140.5 161.8 13.2

10 164.4 272.2 39.6

25 190.8 439.6 56.6

50 208.3 579.2 64.0

100 224.3 728.3 69.2

200 239.2 885.9 73.0

0

100

200

300

400

500

600

700

800

900

1000

1 10 100

Pe

ak

flo

od

flo

w (

m3s-1

)

Return period (yrs)

With headwater wetlands (observed) Without headwater wetlands (simulated)

Extrapolated

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Water for a food-secure world

ResultsMiombo forest

Luchelemu River in Malawi

Total catchment area: 261 km2

Area of wetlands: 244 km2 (93.5% of

total catchment)

0.01

0.1

1

10

100

0.1 2 12 22 32 42 52 62 72 82 92 99.3

Flow

(m3 s-1

)

% time flow exceeded

Regional FDC (avg) Downstream (65312505)

0

5

10

15

20

25

1-O

ct-7

2

1-N

ov

-72

1-D

ec-

72

1-J

an

-73

1-F

eb

-73

1-M

ar-

73

1-A

pr-

73

1-M

ay

-73

1-J

un

-73

1-J

ul-

73

1-A

ug

-73

1-S

ep

-73

1-O

ct-7

3

1-N

ov

-73

1-D

ec-

73

1-J

an

-74

1-F

eb

-74

1-M

ar-

74

1-A

pr-

74

1-M

ay

-74

1-J

un

-74

1-J

ul-

74

1-A

ug

-74

1-S

ep

-74

Flo

w (

m3s-1

)Daily flow with and without forest: HY1972 and HY1973

Without forest (simulated) With forest (observed)

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Water for a food-secure world

ResultsMiombo forest

BFI Mean annual minimum (m3s-1)

1-day 10-day

With forest 0.79 0.552 0.628

Without forest 0.67 0.465 0.508

Return

period

(yrs)

Flood Magnitude

(m3s-1)

%

reductio

nWith

forest

Without

forest

1.1 6.5 7.9 17.7

1.5 9.6 17.0 43.3

2 11.7 19.7 40.6

5 16.8 30.6 45.1

10 20.4 38.3 46.7

25 25.3 48.5 47.8

50 29.1 56.2 48.2

100 33.1 64.2 48.4

200 37.3 72.4 48.5

0

10

20

30

40

50

60

70

80

1 10 100

Pe

ak

flo

od

flo

w (

m3s-1

)

Return period (yrs)

With forest (observed) Without forest (simulated)

Extrapolated

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Water for a food-secure world

Conclusions• A simple yet effective approach proposed and

tested capable of application in the Zambezi with

limitations;

• In the Zambezi:

• floodplains decrease the magnitude of flood

flows and increase low flows;

• headwater wetlands increase the magnitude of

flood flows and decrease low flows;

• miombo forest, when covering more than 70%

of the catchment, decrease the magnitude of

flood flows and also decrease low flows.

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Water for a food-secure world

Conclusions

Some further potential developments to separate

other factors:

• Land use, topography, climate soil, geology;

• Society development (Population, infrastructure,

farming, deforestation);

• Groundwater contribution.

• And feed into synchronized

operations.

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Water for a food-secure world

Thank you!