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Climate change and Lake Chad: a 50-year study from land surface modeling
Huilin Gao, Theodore Bohn, Dennis P. Lettenmaier
Dept. of Civil and Environmental Engineering,University of Washington
1973 1997
Outline
1. Background and motivation
2. Introduction to the hydrological model and its lake/wetland algorithms
3. Modeling results
i) Simulated river runoff into Lake Chad;
ii) A 50-year simulation of Lake Chad dynamics and its validation;
iii) A set of experiments to test the impacts of human water usage versus lake bathymetry under the changing climate;
4. Conclusions
Lake Chad basin and Lake Chad
Largest endoreic basin in the world
- 2,500,000 km2
Fourth largest lake in Africa 40 years ago
Climatology and water resources over the Lake Chad basin
Provides 98% inflow for Lake Chad
12/25/1972 01/31/198710/31/1963 02/06/2007
The loss of Lake Chad: what happened?Climate change?
Can we quantify these by hydrological modeling?
?
Human water usage? Something unique?
precipitation
Variable Infiltration Capacity (VIC) model
Semi-distributed model driven by a set of surface meteorological data
Represents vegetation, has three soil layers with variable infiltration, non linear base flow
Simulated hydrology at each grid cell for all time steps
Well calibrated and applied in a number of large river basins over the continental US and the globe.
VIC lake algorithm
I: Evaporation from the lake is calculated via energy balance;
II: Runoff enters the lake from the land surface;
III: Runoff out of the lake is calculated based on the new stage;
IV: The stage is re-calculated.
(Bowling and Lettenmaier, JHM, 2010)
a) when the lake is at its maximum extent the soil column is saturated;
b) as the lake shrinks runoff from the land surface enters the lake;
c) evaporation from the land surface depletes soil moisture;
d) as the lake grows, water from the lake recharges the wetland soil moisture.
VIC wetland algorithm
(Bowling and Lettenmaier, JHM, 2010)
i) Simulate river runoff from the Lake Chad basin into the lake;
- How much is the irrigation water usage in the basin?
ii) Validate the modeled lake dynamics (1952-2006) using gauge and satellite observations;
- How well could the model capture the loss of Lake Chad?
iii) A set of experiments to test the impacts due to climate change, human water usage, and lake bathymetry.
- How much do these factors affect Lake Chad?
12/25/1972 01/31/198710/31/1963 02/06/2007
Modeling strategy and results
Analyzing Lake Chad from a hydrological perspective ……
i) VIC simulated runoff and the impact of irrigation water usage
5000
4000
3000
2000
1000
0
(m^3
/s)
observedsimulated
no irrigation lot irrigation1953 1963 1973 1983 1993 2003
Modeling and observation differences in recent years are due to irrigation
i) VIC simulated runoff and the impact of irrigation water usage
1954-1979 1983-2004
irrigation
5000
4000
3000
2000
1000
0
(m^3
/s)
4000
3000
2000
1000
0
(m^3
/s)
Feb May Aug Nov
4000
3000
2000
1000
0
(m^3
/s)
Feb May Aug Nov
observedsimulated
observedsimulated
observedsimulated
no irrigation lot irrigation1953 1963 1973 1983 1993 2003
Komadu
guLogone-Chari
(m)
ii) The unique bathymetry of Lake Chad River Chari provides 98%
of the river inflow into Lake Chad from the south;
Bol (gauge)
Great barrier
Great barrier
When water is effluent/deep, the lake behaves as one lake;
When water level retreats below the barrier, the lake splits into two parts.
284
283
282
281
280
279
278
277
276
ii) Validation of modeled lake depth (observations from gauge and satellite altimetry)
One lake
gauge
historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
ii) Validation of modeled lake depth (observations from gauge and satellite altimetry)
One lake
gauge
north lake
south lake
historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
ii) Validation of modeled lake depth (observations from gauge and satellite altimetry)
One lake
gauge
satellite
north lake
south lake
historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
Alt
imet
er a
nom
aly
(m)
Landsat/aircraft images
12/25/1972
01/31/1987
10/31/1963
VIC simulatedlake area
Landsat/aircraft derivedwater coverage
A
B
C
ii) Validation of modeled lake surface area (observations from satellite imagery)
(Landsat/aircraft water classification by E. Podest at JPL)
iii) The changing role of Lake Chad in basin water storage
Period 1Period 2
Period 3
Terr
estr
ial w
ater
sto
rage
(m
m)
iii) Irrigation impact: lake depth
Irrigated(north lake)
Irrigated(south lake)
historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
iii) Irrigation impact: lake depth
Irrigated(north lake)
not irrigated(north lake)
Irrigated(south lake)
not irrigated(south lake)
Small impacts:Delayed dry-out in north lake;Increased lake level in south lake.
historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
iii) Split/bathymetry impact: lake depth
irrigated
If the lake did not split, Lake Chad still would have shrunk dramatically
no split no historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
iii) Isolation/bathymetry impact: lake depth
irrigated
not irrigated
Irrigation impact would be magnified if the historical split of the lake did not occur
no historical split
12
10
8
6
4
2
0
Lak
e de
pth
(m)
iii) Irrigation vs split: lake area & volume
before split
after split
after split, no irrigation
iii) Irrigation vs split: lake area & volume
before split
no split, no irrigationno split
after split
after split, no irrigation
iii) Irrigation vs split: lake area & volume
before split
no split, no irrigationno split
after split
after split, no irrigation
before split
no split, no irrigation
no split
after splitafter split, no irrigation
Conclusions
During the last 50 years, Lake Chad has experienced the normal period, the transition period, and the small lake period.
The VIC lake/wetland model simulated lake dynamics are very consistent with gauge data and satellite observations;
Climate change (droughts in the 70’s and 80’s) is the main cause of the loss of Lake Chad;
The intensified human water usage has worsened the shrinking of Lake Chad;
More than anything else, the unique characteristic of Lake Chad’s bathymetry is the major factor preventing it from recovering (to the ‘big lake’).
Climate change and Lake Chad: a 50-year study from land surface modeling
Huilin Gao, Theodore Bohn, Dennis P. Lettenmaier
Dept. of Civil and Environmental Engineering,University of Washington
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