1 research outputs found
Estimating Vegetation Water content with Hyperspectral data for different Canopy scenarios: Relationships between AVIRIS and MODIS Indexes
Three linked leaf and canopy radiative transfer models were used to assess uncertainties in three vegetation architectures for the relationships
between canopy water content and Equivalent Water Thickness (EWT). The leaf radiative transfer model PROSPECT was linked to SAILH,
rowMCRM, and FLIM canopy reflectance models to generate synthetic spectra for a range of leaf and canopy parameters under closed, rowstructured,
and forest canopy architectures, respectively. Leaf water content (Cw) and leaf area index (LAI) were used to calculate canopy water
content. Leaf and canopy parameters that affect the retrieval of EWT, estimated by the MODTRAN-based fitting technique, were used to
investigate their influence on the water content estimates. Results showed a consistent relationship between retrieved EWT and canopy water
content for the different simulated architectural scenarios. It was shown that EWT was negatively affected by leaf dry matter and soil background.
Retrievals of EWT from hyperspectral Advanced Visible Infrared Imaging Spectrometer (AVIRIS) imagery at three study sites were then used for
cross-validation of the Moderate Resolution Imaging Spectrometer (MODIS) data, assessing the behavior of NDVI, EVI, NDWI, and SIWSI as
potential indicators of vegetation water content. All four MODIS indexes showed consistent agreement with retrievals of EWT from AVIRIS
imagery at the agricultural site and the savanna–shrub site, with EVI having the highest correlation. However, at the conifer forest study site the
two water indexes, NDWI and SIWSI, yielded better agreement with retrievals of EWT than NDVI and EVI. The performance of NDVI was
inconsistent across sites. This manuscript demonstrates the importance of canopy architecture when estimating EWT by showing that large errors
are obtained when EWT estimates derived from absorption feature curve-fitting are applied to different canopy types. These errors are propagated
in simple indexes that produce inconsistent results when applied to divergent vegetation conditions.Portions of this study were supported by National Aeronautics
and Space Administration (NASA) under Grant No. NNG0
4GQ42G “Global Estimation of Canopy Water Content” and
Contract No. NAG5-9360 “New EOS Data Products to Improve
Biogeochemical Estimates” and support from the Cotton Foundation,
#5002-008-00, “Evaluation of Advanced Hyperspectral
Remote Sensing for Detection of Cotton Water Stress and Irrigation
Refinement”. Financial support was provided to P.J Zarco-
Tejada under the “Ramón y Cajal” and “Averroes” Programs from
Spain are acknowledged. Financial support to D. Riaño under the
“Ramón y Cajal” Program of Ministry of Science and Technology
from Spain is acknowledged.Peer reviewe