Introduction to a MODIS Global Terrestrial Evapotranspiration Algorithm Qiaozhen Mu Maosheng Zhao Steven W. Running Numerical Terradynamic Simulation Group, Dept. of Ecosystem and Conservation Sciences, The University of Montana
Input MODIS data (Albedo, LAI/FPAR, Land cover) Daily Meteorological data (Temperature, VPD, S ) ET = f (RS, MD)
Algorithm Data Requirements Meteorological data Daily Incoming Solar Radiation (S ) Daily Average Temperature (Ta) Daily Minimum Temperature (Tmin) Daily Vapor Pressure (VP) Remote sensing data (MODIS) Land Cover Type (LC) Albedo: the 10th band of White-Sky-Albedo from MOD43C1 (α) Fraction of Absorbed Photosynthetically Active Radiation (FPAR) Leaf Area Index (LAI)
Evapotranspiration: Evaporation from Wet Canopy Surface Plant transpiration Soil Evaporation Total Daily Evapotranspiration Daytime ET Nighttime ET
Vegetation Cover Fraction (F C ) Wet surface fraction
Flowchart of the improved MODIS ET algorithm. LAI: Leaf Area Index; FPAR: Fraction of Photosynthetically Active Radiation
Penman-Monteith (P-M) equation R [ S (1 )] [( ) T ] 4 n a s a 4 2 a 10.261exp[ 7.7710 (273 Ta) ] S 0.97 de dt sat a f P, RH, ressure T a e f sat T a f E ( T a ) R a R G C e e R a n RS p sat C p P ressure 622 (2501 2.361 Ta ) Surface Resistance
Evaporation from wet canopy surface
Surface Resistance for Plant Transpiration Plant Transpiration
Evaporation from Soil Surface
Methodology for global implementation MODIS data temporally-filling gaps with contamination or missing 1.00 X1.25 GMAO meteorology data spatially interpolated GMAO data to MODIS pixels
Average Annual MODIS ET
Climatological Zonal Mean of Global Annual ET
Histogram of Global Annual Evapotranspiration Total global annual ET: 62.8 10 3 km 3 Reported total global annual ET: 65.5 10 3 km 3 (Oki & Kanae, 2006) Average MODIS ET over vegetated land surface : 568 ± 378 mm yr -1 Average precipitation: around 750 mm yr -1 Actual ET over the global land surface: around 750*60%=450mm yr -1 Barren/deserts take up 24% of the Earth's land surface MODIS ET over the entire land surface: 568*(100-24)/100=431.68 mm yr -1
Average Annual Precipitation
(precipitation-et)/precipitation
MODIS ET Seasonality
Validations at Global Watersheds Pseudo ET observations = Annual precipitation - stream flow Assumption: soil water is at equilibrium in a long term (>=five years). Distribution of the 234 watersheds. Each watershed is filled with yellow color and green edge.
Average annual MODIS ET estimates vs. pseudo ET measurements (ET OBS) from the 234 watersheds over 2000-2006
FLUXNET: more than 500 towers globally from about 30 regional networks
Methodology for tower validations 46 AmeriFlux tower sites Obtained 3-km x 3-km cutouts of the MODIS data for each tower ET algorithm was driven by Tower observed meteorology data GMAO data
LBA Tapajos KM67 Mature Forest, Brazil Tropical rain forest (EBF) RMSE: 0.72 1.28 MAE: 0.02 0.02 AVG: 3.08 Donaldson, FL,USA slash pine (ENF) RMSE: 1.30 1.49 MAE: 0.29 0.45 AVG: 2.68
Willow Creek, WI,USA sugar maple Little Prospect Hill, MA, USA Red oak, Red pine, White pine RMSE: 1.25 1.37 MAE: 0.11 0.17 AVG: 1.35 RMSE: 0.59 0.76 MAE: 0.10 0.29 AVG: 0.97
Tonzi Ranch, CA,USA Blue oak, foothill pine, grass Walnut River, KS,USA C3/C4 mixed grassland RMSE: 0.68 0.75 MAE: 0.24 0.27 AVG: 1.86 RMSE: 0.67 0.68 MAE: 0.34 0.41 AVG: 1.13
Bondville, IL,USA Annual rotation between corn (C4) and soybeans (C3) RMSE: 0.96 1.03 MAE: 0.77 0.63 AVG: 1.82
Average daily MODIS ET vs. ET measurements
Daily ET estimates and ET measurements Crop DBF ENF EBF Grass CSH TOWERmet GMAOmet OBS ET AVG: 1.23 mm/d 1.32 mm/d 1.34 mm/d MAE: 0.33 mm/d 0.31 mm/d MF W L OSH
Validation of Global MODIS ET Algorithm in Tropical Savannas Hydrological Model Eddy Covariance Data
Tropical Biome: Brazilian Cerrados (Savannas) PDG - Eddy flux tower in Cerrado sensu stricto USE - Eddy flux tower in Sugar Cane Plantations
Rio Grande Basin
PDG Site MGB x EC MOD16 x EC R: 0.85 0.79 RMSE: 0.60 mm/d 0.78 mm/d Water deficit(i) = Precipitation(i) Evapotranspiration (i) + Water deficit (i-1) if P < ET MOD12 PDG site classification: EBF Ground truth: 90% savanna + 10% woody savanna
USE Site MGB x EC MOD16 x EC R: 0.96 0.74 RMSE: 0.27 mm/d 0.75 mm/d Water deficit(i) = Precipitation(i) Evapotranspiration (i) + Water deficit (i-1) if P < ET MOD12 USE site classification: cropland Ground truth: Cropland (sugar-cane)
Monthly Evapotranspiration
MOD16 spatial validation MOD12 Land Use and Land Cover Classification
Mean BIAS (Annual ET) Mean BIAS = MOD16 - MGB Annual average ET for different Land Cover Forest: Savannas: Cropland: Cropland/Natural Veg. mixed: 3.07 mm/d 1.95 mm/d 2.09 mm/d 2.15 mm/d
Conclusions Global terrestrial annual total ET (62.8 10 3 km 3 ) agrees with reported 65.5 10 3 km 3. MODIS ET can explain 84% of the variations of the pseudo ET measurements at the 234 watersheds. Daily MODIS ET compares favorably with flux tower measurements (ET OBS): tower-driven GMAO-driven MAE: 0.33mm/d 0.31mm/d 24.6% 24.1% (in the 10-30% range of the accuracy of ET OBS) R: 0.86 0.86