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On the Causes of the Shrinking of Lake Chad
1973
1997
Huilin Gao1, Theodore Bohn1, Erika Podest2, Dennis P. Lettenmaier1
1Dept.
of Civil and Environmental Engineering, University of Washington
2Jet Propulsion Laboratory, California Institute of Technology
Approach
1. Description of the physical lake system
2. Hydrological model and its implementation
3. Historical reconstruction of Lake Chad using the model
4. Evaluation of irrigation effect versus climate effect
5. Role of lake bathymetry
6. Evaluate prospects for natural recovery and effects of
water diversion
Lake Chad basin and Lake Chad
The unique bathymetry of Lake Chad
 River Chari provides 98%
of the river inflow into Lake
Chad from the south;
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283
 When water is
effluent/deep, the lake
behaves as one lake;
 When water level retreats
below the barrier, the lake
splits into two parts.
282
Bol (gauge)
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280
279
278
Great barrier
277
Inflow
276
(m)
Inflow
The loss of Lake Chad: what happened?
Climate change? Human water usage? Lake bathymetry?
Can we quantify these by hydrological modeling?
?
10/31/1963
12/25/1972
01/31/1987
precipitation
02/06/2007
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)
Modeling approach
1. Simulate over the drainage basin
•
Calibrate model parameters using observed
discharge from 1952 to1963;
•
Run the model from 1952 to 2006.
2. Implement the model to represent Lake Chad dynamics
• Switch between one-lake and twolakes according to lake level;
•
Add the inflow as additional forcing.
Inflow
Validation of modeled lake depth
(observations from gauge and satellite altimetry)
12
Gauge
Lake depth (m)
10
8
6
4
2
0
(RMSE=0.37m)
One lake
Validation of modeled lake depth
(observations from gauge and satellite altimetry)
12
Gauge
10
(RMSE=0.37m)
Lake depth (m)
Satellite
8
(RMSE=0.48m)
One lake
6
4
2
0
south lake
north lake
Validation of modeled lake surface area
(observations from satellite imagery)
Landsat/aircraft images
Landsat/aircraft derived
water coverage
Bias: -8%
10/31/1963
Bias: 3%
12/25/1972
Bias: 5%
01/31/1987
VIC simulated
lake area
Irrigation impact
12
Lake depth (m)
10
8
6
4
2
0
not Irrigated
(south lake)
not Irrigated
(north lake)
Climate impact
An equilibrium of two lakes is favored by climatology
Climate impact
The threshold for an equilibrium of one lake is 38km3/yr inflow
Bathymetry impact
12
Simulation with the barrier removed
Lake depth (m)
10
8
no historical split
6
4
2
0
irrigated
If the lake did not split, Lake Chad still
would have shrunk dramatically
Irrigation vs split: lake volume
• Irrigation withdrawals played a key role
in preventing a recovery
• Lake bathymetry exacerbated the loss
of the lake
Irrigation vs split: lake volume
Lake volume would be
maximized if there was neither
irrigation nor split
Towards recovering
Maximum inflow from 1952 to 2006:
50 km3/yr
Observed inflow from 1997 to 2006:
24 km3/yr
Naturalized inflow from 1997 to 2006:
37 km3/yr
1963 level
Merge level
Inter-basin water transfer is
needed for a full recover of Lake
Chad
Conclusions
 The VIC model simulated lake dynamics are very consistent with
gauge data and satellite observations;
 The bifurcation of Lake Chad in 1972 occurred as a combined
consequence of the bathymetry of the lake and severe droughts;
 Absent irrigation, the lake would have merged around 2000;
 The averaged climatology from 1952 to 2006 does not favor a single
lake;
 Supplemental water transfer is required for the lake to recover to its
1963 size.
On the Causes of the Shrinking of Lake Chad
Thanks!
Huilin Gao1, Theodore Bohn1, Erika Podest2, Dennis P. Lettenmaier1
1Dept.
of Civil and Environmental Engineering, University of Washington
2Jet Propulsion Laboratory, California Institute of Technology
Modeling strategy and results
Analyzing Lake Chad from a hydrological perspective ……
10/31/1963
12/25/1972
01/31/1987
02/06/2007
i) Validate the modeled lake dynamics (1952-2006) using gauge and
satellite observations;
- How well could the model capture the loss of Lake Chad?
ii) 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?
iii) What it takes for a full recovery for the lake.
- Will Lake Chad be able to recover naturally?
VIC wetland algorithm
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.
(Bowling and Lettenmaier, JHM, 2010)
iii) Irrigation vs split: lake area & volume
iii) Irrigation impact: lake depth
12
Small impacts:
Delayed dry-out in north lake;
Increased lake level in south lake.
Lake depth (m)
10
8
not irrigated
(south lake)
historical split
6
4
2
0
Irrigated
(south lake)
Irrigated
(north lake)
not irrigated
(north lake)
i) Validation of modeled lake depth
(observations from gauge and satellite altimetry)
12
gauge
Lake depth (m)
10
8
One lake
6
4
2
0
south lake
north lake
ii) Split/bathymetry impact
12
Irrigation impact would be magnified if
the historical split of the lake did not
occur
Lake depth (m)
10
8
not irrigated
no historical split
6
4
2
0
irrigated