Monitoring drought effects on vegetation water content and fluxes in chaparral with the 970 nm water band index

The goal of this study was to explore the utility of the 970 nm water band index (WBI) in estimating evapotranspiration and vegetation water status for a semiarid shrubland ecosystem. Between 2001 and 2003, spectral reflectance coupled with CO2 and water flux data were collected at Sky Oaks Biological Field Station, a chaparral-dominated ecosystem in southern California, and one of the sites within the SpecNet network. The reflectance data were collected either by walking along a 100 m transect or by using a semi-automated tram system installed later at the site along the same 100 m transect. CO2 and water flux data were gathered with an eddy covariance flux tower adjacent to the tram system. The 970 nm WBI and normalized difference vegetation index (NDVI) were derived from the spectral reflectance. The two indices were expressed both as points approximately a meter apart along the transect and as whole-transect averages, where all of the reflectance values along the transect were averaged together, simulating a large pixel. This study encompassed a wet year with normal precipitation (2001), a 100-year record drought (2002), and a recovery year (2003), allowing for comparison over time and between precipitation regimes. Species-specific responses to wet and dry periods were evident in the reflectance spectra, providing a basis for separating species based on their optical properties. The WBI was significantly correlated with the NDVI revealing a strong link between canopy water content and green canopy structure; however this relationship varied with species and water status, providing evidence for the independence of these two optical indices. The WBI was also strongly linked to surface-atmosphere fluxes, explaining 49% of the variance in the water vapor flux, and 24% of the carbon dioxide fluxes. These results suggest that WBI or other similar water status indices may be useful variables in modeling CO2 and water fluxes when combined with other physiological, environmental, and atmospheric factors. (c) 2006 Published by Elsevier Inc.