The document analyzes the flow regulating functions of natural ecosystems like floodplains, headwater wetlands, and miombo forest on the Zambezi River Basin in Africa. It finds that floodplains decrease flood magnitudes and increase low flows. Headwater wetlands increase flood magnitudes and decrease low flows. Miombo forest, when covering over 70% of a catchment, decreases flood magnitudes and low flows. The approach uses flow duration curves and hydrological modeling to compare observed flows with and without these ecosystems. The results help understand how natural ecosystems influence hydrological processes and could inform water resources planning and dam operations.

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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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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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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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ESA GlobCover
GLWD
The 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/Qmean
% time flow exceeded
Normalised reference FDC
Regional FDC (avg) GRDC 1591500 (Reference) FRIEND 60334250 (Reference)

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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
Flow(m3s-1)
% time flow exceeded
Reference FDC GRDC1591440 (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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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/Qmean
% time flow exceeded
Reference FDC downstream of floodplain
Regional FDC (avg)

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Results
0
20
40
60
80
100
120
1-Oct-84
1-Nov-84
1-Dec-84
1-Jan-85
1-Feb-85
1-Mar-85
1-Apr-85
1-May-85
1-Jun-85
1-Jul-85
1-Aug-85
1-Sep-85
1-Oct-85
1-Nov-85
1-Dec-85
1-Jan-86
1-Feb-86
1-Mar-86
1-Apr-86
1-May-86
1-Jun-86
1-Jul-86
1-Aug-86
1-Sep-86
Flow(m3s-1)
Daily flow with and without floodplain: HY1984 and HY1985
Without floodplain (simulated) With floodplain (observed) Upstreamfloodplain (observed)
Flood plains

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Results
Base 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
Peakfloodflow(m3s-1)
Return period (yrs)
Flood Frequency
With floodplain Without floodplain (simulated)
Extrapolated
0
20
40
60
80
100
120
140
1 10 100
Peakfloodflow(m3s-1)
Return period (yrs)
Flood Frequency
With floodplain Without floodplain (simulated)
Extrapolated
Flood plains

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Results
Headwater 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/Qmean
%time flow exceeded
RegionalFDC (avg) - inc 65312102 FRIEND65312602 (downstream)
0
50
100
150
200
250
300
350
400
450
500
1-Oct-85
1-Nov-85
1-Dec-85
1-Jan-86
1-Feb-86
1-Mar-86
1-Apr-86
1-May-86
1-Jun-86
1-Jul-86
1-Aug-86
1-Sep-86
1-Oct-86
1-Nov-86
1-Dec-86
1-Jan-87
1-Feb-87
1-Mar-87
1-Apr-87
1-May-87
1-Jun-87
1-Jul-87
1-Aug-87
1-Sep-87
Flow(m3s-1)
Daily flowwith and withoutheadwaterwetlands: HY1984 and HY1985
Withoutheadwaterwetlands(simulated) Withheadwaterwetlands(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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Results
Headwater 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
Peakfloodflow(m3s-1)
Return period (yrs)
With headwater wetlands (observed) Without headwater wetlands (simulated)
Extrapolated

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Results
Miombo 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(m3s-1)
% time flow exceeded
Regional FDC (avg) Downstream (65312505)
0
5
10
15
20
25
1-Oct-72
1-Nov-72
1-Dec-72
1-Jan-73
1-Feb-73
1-Mar-73
1-Apr-73
1-May-73
1-Jun-73
1-Jul-73
1-Aug-73
1-Sep-73
1-Oct-73
1-Nov-73
1-Dec-73
1-Jan-74
1-Feb-74
1-Mar-74
1-Apr-74
1-May-74
1-Jun-74
1-Jul-74
1-Aug-74
1-Sep-74
Flow(m3s-1)
Daily flowwith and withoutforest: HY1972 and HY1973
Withoutforest(simulated) Withforest(observed)

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Results
Miombo 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
Peakfloodflow(m3s-1)
Return period (yrs)
With forest (observed) Without forest (simulated)
Extrapolated

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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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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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Thank you!