1. 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

2. 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

3. Lake Chad basin and Lake Chad Largest endoreic basin in the world -2,500,000 km 2 Fourth largest lake in Africa 40 years ago

4. Climatology and water resources over the Lake Chad basin Provides 98% inflow for Lake Chad

5. 12/25/1972 01/31/1987 10/31/1963 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

6. 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.

7. 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)

8. 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)

9. 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 01/31/1987 10/31/1963 02/06/2007 Modeling strategy and results Analyzing Lake Chad from a hydrological perspective ……

10. i) VIC simulated runoff and the impact of irrigation water usage 5000 4000 3000 2000 1000 0 (m^3/s) observed simulated no irrigationlot irrigation 1953 1963 1973 1983 1993 2003 Modeling and observation differences in recent years are due to irrigation

11. 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 observed simulated observed simulated observed simulated no irrigationlot irrigation 1953 1963 1973 1983 1993 2003

12. Komadugu Logone-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  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

13. ii) Validation of modeled lake depth (observations from gauge and satellite altimetry) One lake gauge historical split 12 10 8 6 4 2 0 Lake depth (m)

14. 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 Lake depth (m)

15. 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 Lake depth (m) Altimeter anomaly (m)

16. Landsat/aircraft images 12/25/1972 01/31/1987 10/31/1963 VIC simulated lake area Landsat/aircraft derived water coverage ABCABC ii) Validation of modeled lake surface area (observations from satellite imagery) (Landsat/aircraft water classification by E. Podest at JPL)

17. iii) The changing role of Lake Chad in basin water storage Period 1 Period 2 Period 3 Terrestrial water storage (mm)

18. iii) Irrigation impact: lake depth Irrigated (north lake) Irrigated (south lake) historical split 12 10 8 6 4 2 0 Lake depth (m)

19. 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 Lake depth (m)

20. 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 Lake depth (m)

21. 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 Lake depth (m)

22. iii) Irrigation vs split: lake area & volume before split after split after split, no irrigation

23. iii) Irrigation vs split: lake area & volume before split no split, no irrigation no split after split after split, no irrigation

24. iii) Irrigation vs split: lake area & volume before split no split, no irrigation no split after split after split, no irrigation before split no split, no irrigation no split after split after split, no irrigation

25. 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’).

26. 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 Thanks! Questions?