Temporal and spatial patterns in vegetation and atmospheric properties from AVIRIS

Little research has focused on the use of imaging spectrometry for change detection. In this paper, we apply Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data to the monitoring of seasonal changes in atmospheric water vapor, liquid water, and surface cover in the vicinity of the Jasper Ridge, CA, for three dates in 1992. Apparent surface reflectance was retrieved and water vapor and liquid water mapped by using a radiative-transfer-based inversion that accounts for spatially variable atmospheres. Spectral mixture analysis (SMA) was used to model reflectance data as mixtures of green vegetation (GV), nonphotosynthetic vegetation (NPV), soil, and shade. Temporal and spatial patterns in endmember fractions and liquid water were compared to the normalized difference vegetation index (NDVI). The reflectance retrieval algorithm was tested by using a temporally invariant target. Atmospheric analysis showed a strongly negative linear between water vapor and elevation with significant seasonal variation in water vapor. Comparison of AVIRIS estimates of specific humidity to ground-based measures showed good correspondence for all three dates. Analysis of surface properties showed that GV, NDVI, and liquid water varied in response to green vegetation and were highly correlated. However, whereas the NDVI peaked between 0.7 and 0.85 in forests, liquid water continued to vary by as much as a factor of two. Seasonal patterns included senescence in herbaceous and nonconiferous vegetation, potential leaf growth in coniferous forests, and a general increase in shadows. This resulted in seasonal declines in NDVI, GV, and liquid water for nonforested vegetation and increases in NPV. Nonconiferous forests showed similar declines in liquid water and GV and increases in shade and NPV, but they showed an increase in NDVI. In coniferous forests, liquid water and NDVI increased seasonally, matching an interpretation of continued growth, but GV decreased owing to increased shade. The combination of retrieved surface reflectance, atmospheric modeling, and mapping of liquid water demonstrates the utility of imaging spectrometry for change detection. SMA with the use of reference end-members is an effective method for monitoring surficial changes. Temporal patterns in NDVI that contradict trends of GV and liquid water in nonconiferous forests raise additional questions about the NDVI. Liquid water may be more appropriate for analysis of high-leaf-area, shadowed forests because it overcomes the problem of saturation with NDVI. (C) Elsevier Science Inc., 1997.