A multi-scale Analysis of Dynamic Optical Signals in a Southern California Chaparral Ecosystem: a Comparison of Field, AVIRIS and MODIS Data

Using field data, Airborne Visible Infrared Imaging Spectrometer (AVIRIS) imagery, and Moderate-Resolution Imaging SpectroRadiometer (MODIS) data, a multi-scale analysis of ecosystem optical properties was performed for Sky Oaks, a Southern California chaparral ecosystem in the SpecNet and FLUXNET networks. The study covered a four-year period (2000-2004), which included a severe drought in 2002 and a subsequent wildfire in July 2003, leading to extreme perturbation in ecosystem optical properties. Two vegetation greenness indices (Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI)) and a measure of the fraction of photosynthetically active radiation absorbed by vegetation (fPAR) were compared across sampling platforms, which ranged in pixel size from 1 meter (tram system in the field) to 1000 m (MODIS satellite sensor). For the EVI, there was excellent agreement between MODIS, AVIRIS and the ground measurements (tram system). AVIRIS and tram-based NDVI and fPAR values were in close agreement. However, MODIS NDVI and fPAR values were consistently higher than those determined from the field and the aircraft sensor, and these differences could not be entirely attributed to differences in sampling scale. Interestingly, MODIS fPAR derived from backup algorithms (NDVI driven) was closer to the AVIRIS and tram fPAR under the cloudy conditions. This suggests that derivation of fPAR directly from vegetation indices could work better than the currently deployed dominant algorithms incorporating look-up tables for biome type. These results appear consistent with other recently published results that indicate that MODIS overestimates fPAR and thus NPP for terrestrial ecosystems, and demonstrates the need for proper validation of MODIS terrestrial biospheric products by direct comparison against optical signals at other spatial scales. The study also demonstrates the utility of in-situ field sampling (e.g. tram systems) and hyperspectral aircraft imagery for proper interpretation of satellite data taken at coarse spatial scales.