Satellite Detection of Water Stress Effects on Terrestrial Latent Heat Flux With MODIS Shortwave Infrared Reflectance Data

Abstract

The MODerate-resolution Imaging Spectroradiometer (MODIS) provides spatially contiguous measurements of terrestrial biophysical variables, which can be used to estimate the terrestrial latent heat flux (LE). MODIS-derived shortwave infrared reflectance (SWIR) metrics (SWIRs) are sensitive to the soil moisture and vegetation water stress. In this study, we used the MODIS-derived SWIRs with eddy covariance flux measurements obtained from 25 flux tower sites representing 10 different land cover types within China to evaluate the sensitivity of SWIRs to ground-measured evaporation fraction and LE. The water constraint metrics determined using the MODIS-derived SWIR generally corresponded better with the ground-measured evaporation fraction values than those obtained without using SWIR. The MODIS-derived SWIRs were used as proxies for the soil and vegetation water supply constraints in a revised Priestley-Taylor algorithm to estimate the terrestrial LE. The estimated LE using the MODIS-derived SWIRs generally corresponded well with the ground-measured LE (0.56 ≤ R2 ≤ 0.97) for most of the flux tower sites. Regional algorithm sensitivity analysis using the MODIS-derived SWIRs as water supply proxies demonstrated that water limitations reduce LE by more than 53% over China, and the atmospheric vapor pressure deficit and relative humidity are not sufficient to characterize both the atmosphere demand and water supply for LE estimation. Our results demonstrate the potential of using MODIS-derived SWIRs to characterize soil and vegetation water supply factors for determining LE, where the relatively high spatial and temporal resolutions (500 m and daily) are closer to the scale of the eddy covariance ground measurements.