| Remotely sensed measurements from two sensors were examined for their capability of providing estimates of vegetation canopy water content. Equivalent Water Thickness (EWT), a physically based retrieval derived from AVIRIS (Advanced Visible Infrared Imaging Spectrometer) data, was compared to in situ measurements and was validated using a modeling approach. Retrievals of EWT showed promising agreement with in situ canopy water content measurements while the theoretical study also demonstrated that EWT was sensitive to change of canopy water content. However, both studies also showed that plant dry matter content and soil background reflectance can cause errors in estimates of EWT. These errors in EWT were further examined, in studies that used EWT as reference to validate band-ratio indexes derived from the Moderate Resolution Imaging Spectrometer (MODIS). The agreements between AVIRIS EWT and MODIS indexes were tested on different vegetation types from boreal forest to semi-arid savanna. At one agriculture site and one semi-arid shrubland, all MODIS vegetation and water indexes were correlated to AVIRIS EWT, while MODIS water indexes had better regression agreement with AVIRIS EWT than vegetation indexes at two conifer forest sites. Seasonal variation of MODIS indexes was further explored by comparing to micrometeorological data. Time series of MODIS water indexes and NEE (net ecosystem exchange) at a temperate conifer forest were found to follow the same temporal trajectory, a pattern that was the strongest when the forest was a carbon sink. Contrary, MODIS vegetation indexes did not yield similar seasonal patterns with NEE. At the semi-arid site, MODIS indexes showed clear seasonal pattern, which responded well to precipitation. |
