5000

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observedsimulated

little irrigation lot irrigation1953 1963 1973 1983 1993 2003

A study of lakes and wetlands in Africa from land surface modelingHuilin Gao, Theodore Bohn, and Dennis P. Lettenmaier

Department of Civil and Environmental Engineering, Box 352700, University of Washington, Seattle, WA 98195 (hgao@u.washington.edu)

1 Objective

VIC dynamic lake/wetland module2

4 Case study of Lake Chad, Africa

This project is supported by NASA Grant NNX08AN40A – “Developing Consistent Earth System Data Records for the Global Terrestrial Water Cycle”

6 References

5 Conclusions

4Lakes and wetlands are of great importance to the socio-economics of the

African continent. However, information about variations in surface water

storage in even the largest lakes is surprisingly sparse. Satellite altimeters now

provide some information about variations in lake surface elevation and

surface extent information is available from visible and other sensors. But for

most lakes the variations of lake water storage (regardless of size) at

interannual and interseasonal time scales is mostly unknown, and the

variations due to long term climate change are beyond the present state of

knowledge. We use a modified version of the Variable Infiltration Capacity

(VIC) macroscale hydrology model, in combination with remote sensing data,

to help understand water storage variations in lakes and wetlands across

Africa. Lake Chad is used as a representative lake in this study. The

objectives are two-folds:

1.To evaluate the land surface model by reconstructing the lake historically;

2.To analyze the impacts of climate change, human impact, and lake

bathymetry on lake variations.

Figure 2. Schematic for the wetland algorithm. (Bowling and Lettenmaier, 2010).

VIC lake algorithmVIC wetland algorithm

Figure 1. Schematic of the VIC lake algorithm.

I. Study area: the vanishing Lake Chad

Figure 5. Lake bathymetry from DEM.

Located in Central Africa with an area of 2,500,000 km2, the Lake Chad basin is the largest

endoreic basin in the world. Lake Chad is shared by four countries: Chad, Niger, Nigeria and

Cameroon. The hydrologically active part of the basin is mainly drained by the Chari–Logone

river system, and to a much lesser extent, by the Komadugu River.

In the 1960s Lake Chad had an area of more than 26,000 km², making it the fourth largest lake

in Africa. By 2000 its extent had fallen to less than 1,500 km². This is due to a combination of

severe droughts, increased irrigation water usage, and the unique lake bathymetry.

II. Approach

III. Reconstruction of historical Lake Chad

Figure 6. Observed and simulated lake inflow at N’Djamena.

Birkett, C.M., 2000: Synergistic remote sensing of Lake Chad: Variability of basin inundation. Remote Sensing of Environment 72, 218-236.

Bowling and Lettenmaier, 2010: Modeling the effects of lakes and wetlands on the water balance of Arctic environments. Journal of Hydrometeorology 11, 276-295.

Coe, M. T., and Foley, J. A., 2001: Human and natural impacts on the water resources of the Lake Chad basin. Journal of Geophysical Research-Atmospheres 106, 3349-3356.

Le Coz, M., Delclaux, F., Genthon, P., and Favreau, G., 2009: Assessment of Digital Elevation Model (DEM) aggregation methods for hydrological modeling: Lake Chad basin, Africa. Computers & Geosciences 35, 1661-1670.

Lehner, B., and Doll, P., 2004: Development and validation of a global database of lakes, reservoirs and wetlands. Journal of Hydrology 296, 1-22.

Sheffield, J., Goteti, G., and Wood, E. F., 2006: Development of a 50-year high-resolution global dataset of meteorological forcings for land surface modeling. Journal of Climate 19, 3088-3111.

The VIC model-simulated lake dynamics are consistent with gauge data

and satellite observations. The bifurcation of Lake Chad in 1972 occurred

as a combined consequence of the lake bathymetry and severe droughts.

Failure of the lake to merge back into a single lake following wetter

conditions in the 1990s is a result of irrigation withdrawals – without

irrigation, the lake would have merged in 1999, although it would have

again bifurcated in 2004. The 1952 to 2006 climatology does not favor a

single lake. It takes about 106% of the climatological precipitation and

115% of the historical mean inflow to avoid a split in the lake, even

without irrigation.

3 Lakes and wetlands in Africa

Inflow

Continued

IV. On the causes of the shrinking of Lake Chad

Figure 3. Lakes and wetlands in Africa (according to Global Lakes and Wetland Database).

The most notable African lakes are

along the Great Rift Valley, including

Lakes Victoria, Tanganyika, and

Malawi. Three of the world’s ten

largest wetlands are located in Africa:

Congo River basin (rank 4), Lake

Chad basin (rank 8), and River Nile

basin (rank 9).

The Lake Chad basin was selected for

this study not only because of its

extensive wetland but also because

Lake Chad has undergone dramatic

changes during the last few decades.

Figure 4. Geographic situation of the Lake Chad basin (figure cited from Coz et al., 2009)

First, model parameters (at 1 degree grid cells) were calibrated

using observed discharge from 1952 to1963. Using the

calibrated parameters, the model was run from 1952 to 2006 to

get the naturalized inflow for the lake. The difference between

naturalized inflow and observation represents the irrigation

water withdrawal. Forcings are from Sheffield et al. (2006).

One important feature of Lake Chad’s bathymetry is a ‘great barrier’ that runs between the

northern and southern parts of the lake. When water is effluent/deep, the lake behaves as one

lake; When water level retreats below the barrier, the lake splits into two parts. To represent

Lake Chad dynamics properly, the model was implemented to allow switching between one-

lake mode and two-lake mode according to lake level.

Figure 8. Modeled lake area compared with Landsat water cover images.

Figure 7. Modeled lake level compared with gauge and satellite observations.

Modeled results agree well with both gauge

(RMSE=0.37m) and altimetry observations

(RMSE=0.48m), and with aerial photograph and

Landsat satellite images (biases -8%, 3%, and 5% of

1963 area on 10/31/1963, 12/25/1972, and

1/31/1987, respectively).

Before the lake split in 1972, seasonal variations in the lake’s level

were about 1 m. Afterward, the seasonal variations increased to about

2 m in the south lake due to the reduction of lake size. The north lake’s

level continually decreased until 1986 when it dried out completely.

Water reappeared in the north lake in 1999 after a few years of wet

weather.

To study the causes of the shrinking of Lake Chad, a set of experiments were conducted to: 1) Estimate the effect of

irrigation on the observed lake changes; 2) Explore the response of the lake if the barrier between its two sections were to

be breached; and 3) Investigate Lake Chad’s steady-state response to observed climate change.

Figure 9. Impact of irrigation water usage on lake level.

Figure 10. Impact of lake bathymetry on lake level.

Figure 11. Lake level under different climate scenarios.

1) Without irrigation

withdrawals, the north lake

not only would have survived

the droughts in the 1980s but

would have merged with the

south lake again for a brief

period around 2000.

2) Although not as

significant as irrigation

withdrawals, the lake

bathymetry exacerbated the

loss of lake area and volume.

3) Absent irrigation, the

lake would still split under

mean climate state.

Depending on the climate

conditions, both the one-

lake and two-lake modes

can reach steady states.