HR: 1340h
AN: B43C-1451    [Abstracts]
TI: The use of Soil Reflectance Database and NDVI Time Series to compute an Adjusted Green Vegetation Fraction
AU: * Montandon, L M
EM: montando@colorado.edu
AF: Department of Geological Sciences, University of Colorado, 2200 Colorado Ave, Boulder, CO 80309-0399, United States
AU: Small, E E
EM: eric.tilton@gmail.com
AF: Department of Geological Sciences, University of Colorado, 2200 Colorado Ave, Boulder, CO 80309-0399, United States
AB: The green vegetation fraction (Fg) is an important climate and hydrologic model parameter. Common methods to calculate Fg are simple linear mixing models between two NDVI end-members: bare soil NDVI (NDVI_o) and full vegetation NDVI (NDVI_∞). A common assumption is that NDVI_o is close to zero and as a result it is generally chosen from the lowest observed NDVI values. However, the mean soil NDVI computed from 2906 samples is much larger (NDVI=0.21) and is highly variable (standard deviation=0.1). Because underestimation of NDVI_o yields overestimations of Fg that are higher than 0.2 for any pixel with 0.2<NDVI<0.4, it is important to evaluate alternative methods to estimate NDVI_o when no information on local soils is available. Here we describe and test a method to improve Fg estimation by combining a global soil database and time series of NDVI derived from MODIS Terra data. Fg is computed for each pixel using a subset of the soils database that respects the linear mixing model condition NDVI_o ≤NDVI_h, where NDVI_h is the historical minimum for that pixel. Using in situ measurements of soil NDVI, from sites that span a range of land cover types, we show that this method improves the temporal Fg estimates in areas with no information on soil reflectance. We also estimate the spatial distribution of Fg errors by employing our method for the conterminous U.S. and comparing the new Fg with that derived using the standard method to calculate Fg.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0480 Remote sensing
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting