1. Huilin Gao, Theodore Bohn, and Dennis P. Lettenmaier
Long term simulations of Lake Chad using the Variable Infiltration Capacity model
Huilin Gao, Theodore Bohn, and Dennis P. Lettenmaier
Department of Civil and Environmental Engineering, Box , University of Washington, Seattle, WA
Steve Burges
Retirement Symposium Mar 24-26, 2010 1 3
Case study of Lake Chad, Africa 4
Towards global lake simulations
Objective
Although lakes and reservoirs play a major role in the hydrology of the land surface over substantial areas of the globe, coherent information about their dynamics is largely lacking. The quality and completeness of information from in situ sources varies tremendously for different countries and regions. Recently, satellite data have provided some information about variations in lake surface elevation (from satellite altimeters) and surface extent (from visible and other sensors) for the largest lakes. Land surface models offer an alternative means of gaining insights into lake dynamics. Here we use a recent version of the Variable Infiltration Capacity (VIC) model which includes a lake/wetland module, to simulate the 57-year ( ) variations of lake level and surface area of Lake Chad, Africa. The objectives of this study are two-fold:
To test the VIC lake/wetland module, which was originally intended for application to smaller sized lakes in high latitudes regions, over a large lake in the tropics;
To gain insights into issues associated with simulating lakes and wetlands globally using the modified version of the VIC model and ancillary data sets.
Study area: the vanishing Lake Chad
We are in the process of simulating lakes and wetlands globally following the procedure outlined in the flowchart below. Large lakes and small lakes are separated using the Global Lakes and Wetlands Database (GLWD). Only the largest lakes are represented in the model’s river routing network, and small lakes are considered in aggregate as an effective land cover class within each grid cell. Similar to the approach used for Lake Chad, the large lakes are simulated using a constructed grid cell containing the whole lake (in most cases) with its forcings modified by the routed inflows.
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 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 due to a combination of severe droughts and increased irrigation water usage.
Figure 3 Geographic situation of the Lake Chad basin (figure cited from Coz et al., 2009)
II. Data and approach
According to the bathymetry, Lake Chad features a relatively deep northern half and a very shallow southern half, with most of its inflows from the Logone-Chari river system into the southern part. Therefore, we use two separate grid cells to represent the lake. The southern cell is modeled first, with its forcings modified by the inflows from the southern part of the Chad basin. The inflow for the northern part consists of the runoff from the southern part and the discharge from the Koma-
Global Lakes and Wetlands Database
small lakes
big lakes
Aggregate within grid cell
Grid cell simulation
parameterization
Big lakes downstream? NO
YES
Routing to outlet
Simulate the first downstream big lake
Modified routing network
Routing to the first
downstream big lake
-dugu river. For both grid cells, the depth-area relationship is from topography, and rpercent = 1. For the north wfrac = 0; for the south wfrac is calibrated. Forcings are from Sheffield et al. (2006).
Komadugu
Logone-Chari
(m) 2
VIC dynamic lake/wetland module
Figure 5 VIC simulated discharge to the north part (a), and south part (b) of the lake; lake depth-area profile based on bathymetry of the north part (c) and south part (d).
VIC lake algorithm
VIC wetland algorithm
Figure 4 Lake bathymetry from DEM.
III. Results
Figure 9 Flowchart of the global lake simulation plan.
Results from the modeled lake level for the southern part (Fig. 6) suggest two things. First, the lake level and its variations were significantly reduced during the droughts in the 1970’s and 1980’s. 5
Summary and future work
In this study we used a recent version of the VIC macroscale hydrology model with a lake/wetland module, in combination with remotely sensed altimetry data, to simulate and verify lake level and area variations in Lake Chad, Africa. The 57-year ( ) results are consistent with both observations and known climate change in the area.
Further steps toward global simulations are being taken, as shown in a strategy for global implementation of the model. Future work will focus on the parameterizations of lakes globally, and modification of the river networks to incorporate large lakes. Irrigation water usage will be a critical term for the model to handle to achieve realistic results.
Second, the modeled results are fairly consistent with observations from satellite altimetry. The lake level for the northern part of the lake indicates a disappearance in the 1980’s, with a decrease in the lake size of about 50% from the mid 60s to the mid 70s. The modeled lake water coverage maps (based on lake level and bathymetry) demonstrate a good coherency with the available satellite imagery, except for a low bias in the southern part in the 1963 and 1972 comparison.
Figure 2. Schematic for the 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 and 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, 2009).
Figure 6 Modeled lake level (south part) and its comparison with observations .
Figure 7 Modeled lake level (north part) .
Figure 1. Schematic of the 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, and IV: The stage is re-calculated (Bowling and Lettenmaier, 2009).
10/31/1963
Major module characteristics:
Multi-layer energy balance lake model of Hostetler et al as modified by Bowling and Lettenmaier (2009)
Dynamic lake area allows seasonal inundation of adjacent wetlands
Currently not part of channel network
Lake/wetland parameters:
Lake depth-area profile;
Wfrac (Width of lake outlet, as a fraction of the lake perimeter);
Rpercent (Fraction of grid cell runoff that enters lake) 7
References
Birkett, C.M., 2000, Synergistic remote sensing of Lake Chad: Variability of basin inundation. Remote Sensing of Environment, 72,
Bowling and Lettenmaier, 2009: Modeling the effects of lakes and wetlands on the water balance of Arctic environments Journal of Hydrometeorology (accepted).
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,
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,
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,
12/25/1972
01/31/1987
Figure 8 Modeled lake surface area and the comparisons with satellite images in selected days.
This project is supported by NASA Grant NNX08AN40A – “Developing Consistent Earth System Data Records for the Global Terrestrial Water Cycle”