Relating Optical Indices to Carbon and Water Fluxes in a Chaparral Ecosystem

Between 2001 and 2003, spectral reflectance coupled with CO₂ and H₂O flux data were collected at Sky Oaks Biological Field Station, a chaparral-dominated ecosystem in southern California. The reflectance data were collected by walking along a transect (early 2001) and semi-automated 100 meter tram system installed at the site (after mid 2001), while CO₂ and H₂O flux data were gathered with an eddy covariance flux tower. Over the study, which included a normal wet (2001), an extremely dry (2002), and a recovery year (2003), the water band index (WBI) was more closely correlated with ecosystem H₂O flux than CO₂ flux. In the wet year, WBI was closely correlated with both the H₂O and CO₂ fluxes, but when a record drought struck in 2002, the correlation between WBI and CO₂ disappeared as vegetation died off. Also, WBI is dynamic over time, precipitation conditions, and between species in the region. These results suggest that the ecosystem average WBI is an overall more robust estimator of the H₂O flux than CO₂ flux at the ecosystem level and water fluxes can be directly estimated from optical remote sensing.