Evaluation of optical remote sensing to estimate actual evapotranspiration and canopy conductance

摘要

We compared estimates of actual evapotranspiration (ET) produced with six different vegetation measures derived from the MODerate resolution Imaging Spectroradiometer (MODIS) and three contrasting estimation approaches using measurements from eddy covariance flux towers at 16 FLUXNET sites located over six different land cover types. The aim was to assess optimal approaches in using optical remote sensing to estimate ET. The first two approaches directly regressed various MODIS vegetation indices (VIs) and products such as leaf area index (LAI) and fraction of photosynthetically active radiation (fPAR) with ET and evaporative fraction (EF). In the third approach, the Penman-Monteith (PM) equation was inverted to obtain surface conductance (G_s), for dry plant canopies. The G_s values were then regressed against the MODIS data products and used to parameterize the PM equation for retrievals of ET. Jack-Knife cross-validation was used to evaluate the various regression models against observed ET. The PM-G_s approach provided the lowest root mean square error (RMSE), and highest determination coefficients (R~2) across all sites, with an average RMSE=38Wm~(-2) and R~2=0.72. Direct regressions of observed ET against the VIs resulted in an average RMSE=60Wm~(-2) and R~2=0.22, while the EF regressions an average RMSE=42Wm-2 and R~2=0.64. The MODIS LAI and fPAR product produced the poorest estimates of ET (RMSE>44Wm~(-2) and R~2<0.6); while the VIs each performed best for some of the land cover types. The enhanced vegetation index (EVI) produced the best ET estimates for evergreen needleleaf forest (RMSE=28.4Wm~(-2), R~2=0.66). The normalized difference vegetation index (NDVI) best estimated ET in grassland (RMSE=23.8Wm~(-2) and R~2=0.68), cropland (RMSE=29.2Wm~(-2) and R~2=0.86) and woody savannas (RMSE=25.4Wm~(-2) and R~2=0.82), while the VI-based crop coefficient (K_c) yielded the best estimates for evergreen and deciduous broadleaf forests (RMSE=27Wm~(-2) and R~2=0.7 in both cases). Using the ensemble-average of ET as estimated using NDVI, EVI and K_c we computed global grids of dry canopy conductance (G_c) from which annual statistics were extracted to characterise different functional types. The resulting G_c values can be used to parameterize land surface models.