On the Causes of the Shrinking of Lake Chad 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 1973 1997

Approach Description of the physical lake system Hydrological model and its implementation Historical reconstruction of Lake Chad using the model Evaluation of irrigation effect versus climate effect Role of lake bathymetry Evaluate prospects for natural recovery and effects of water diversion

Lake Chad basin and Lake Chad Largest endoreic basin in the world Fourth largest lake in Africa 40 years ago 2,500,000 km2

The unique bathymetry of Lake Chad River Chari provides 98% of the river inflow into Lake Chad from the south; 284 283 282 281 280 279 278 277 276 When water is effluent/deep, the lake behaves as one lake; When water level retreats below the barrier, the lake splits into two parts. Bol (gauge) Great barrier Inflow (m) Inflow

The loss of Lake Chad: what happened? Can we quantify these by hydrological modeling? ? Human water usage? Lake bathymetry? Climate change? 10/31/1963 12/25/1972 01/31/1987 02/06/2007 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)

Modeling approach 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 two-lakes according to lake level; Add the inflow as additional forcing. Inflow

Validation of modeled lake depth (observations from gauge and satellite altimetry) 12 10 8 6 4 2 Gauge (RMSE=0.37m) One lake Lake depth (m)

Validation of modeled lake depth (observations from gauge and satellite altimetry) 12 10 8 6 4 2 Gauge (RMSE=0.37m) Satellite (RMSE=0.48m) One lake Lake depth (m) south lake north lake

Validation of modeled lake surface area (observations from satellite imagery) Landsat/aircraft derived water coverage VIC simulated lake area Landsat/aircraft images Bias: -8% 10/31/1963 Bias: 3% 12/25/1972 Bias: 5% 01/31/1987

Irrigation impact Lake depth (m) 12 10 8 6 4 2 not Irrigated Lake depth (m) 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 Lake depth (m) 12 10 8 6 4 2 Simulation with the barrier removed no historical split Lake depth (m) 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 Inter-basin water transfer is needed for a full recover of Lake Chad 1963 level Merge level

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.

Thanks! On the Causes of the Shrinking of Lake Chad 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 Thanks!

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 10 8 6 4 2 Small impacts: Delayed dry-out in north lake; Increased lake level in south lake. not irrigated (south lake) historical split Lake depth (m) Irrigated (south lake) not irrigated (north lake) Irrigated (north lake)

i) Validation of modeled lake depth (observations from gauge and satellite altimetry) 12 10 8 6 4 2 gauge One lake Lake depth (m) south lake north lake 26

ii) Split/bathymetry impact 12 10 8 6 4 2 Irrigation impact would be magnified if the historical split of the lake did not occur not irrigated no historical split Lake depth (m) irrigated