The objective of this study was to exploit the synergy between SMOS and SMAP based on vegetation optical depth (VOD) to improve brightness temperature (TB) simulations and land surface soil moisture (SM) retrievals in arid regions of the world. In the current operational algorithm of SMAP (level 2), vegetation water content (VWC) is considered as a proxy to compute VOD which is calculated by an empirical conversion function of NDVI. Avoiding the empirical estimation of VOD, the SMOS algorithm is used to retrieve simultaneously SM and VOD from TB observations. The present study attempted to improve SMAP TB simulations and SM retrievals by benefiting from the advantages of the SMOS (L-MEB) algorithm. This was achieved by using a synergy method based on replacing the default value of SMAP VOD with the retrieved value of VOD from the SMOS multi angular and bi-polarization observations of TB. The insitu SM measurements, used as reference SM in this study, were obtained from the International Soil Moisture Network (ISMN) over 180 stations located in arid regions of the world. Furthermore, four stations were randomly selected to analyze the temporal variations in VOD and SM. Results of the synergy method showed that the accuracy of the TB simulations and SM retrievals was respectively improved at 144 and 124 stations (out of a total of 180 stations) in terms of coefficient of determination (R2) and unbiased root mean squared error (UbRMSE). Analyzing the temporal variations in VOD showed that the SMOS VOD, conversely to the SMAP VOD, can better illustrate the presence of herbaceous plants and may be a better indicator of the seasonal changes in the vegetation density and biomass over the year.
Bibliographical noteFunding Information:
Many thanks go to Amen Al-Yaari for his helpful comments and Fernandez-Moran for providing some MATLAB codes. The authors would like to acknowledge the CATDS (https://www.catds.fr/), the Jet Propulsion Laboratory and NASA Reverb exploring tools (https://reverb.echo.nasa.gov), and ISMN (https://ismn.geo.tuwien.ac.at/) for making respectively the SMOS, SMAP, and insitu SM measurements data freely available. We are very thankful to the anonymous reviewers for their comments on this manuscript. The authors actually thank Iranian National Science Foundation and CNES/TOSCA for their supports in this research.
© 2017 Elsevier B.V.
Copyright 2018 Elsevier B.V., All rights reserved.
- Vegetation optical depth