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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; 284 283 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) 281 280 279 278 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