Preliminary vegetation index products from Suomi NPP VIIRS illuminate the California drought
Tuesday, 16 December 2014
The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite has been collecting data for almost three years. NOAA and NASA have been collaborating to validate numerous VIIRS data products capturing atmospheric, land and ocean parameters. For vegetation monitoring, standard VIIRS products currently include a top-of-canopy Enhanced Vegetation Index (EVI), thereby providing an opportunity to observe the recent land surface expression of drought conditions in the state of California as well as creating the potential for modeling gross primary productivity (GPP), evapotranspiration (ET), and other carbon- and water- related variables. We mapped and compared 16-day composite VIIRS EVIs over California from January 2012 to 2014 with equivalent MODIS products from the Aqua satellite (the MYD13 product). Relative to MODIS EVI, VIIRS EVI showed a bias of less than 7% (larger values than MODIS) and a precision of 0.50 to 0.85%. The remaining small proportion of large errors seem to be related to cloud and snow cover. Substantial changes in the EVI time series can be clearly seen for both irrigated and semi-natural land as the recent drought deepened. We also used the VIIRS EVI as input to the Terrestrial Observation and Prediction System (TOPS) model to generate ET and GPP, which illuminate drought effects more specifically. As the VIIRS products become more robust, they will make a valuable addition to the arsenal of satellite sensors capturing synoptic trends in vegetation response to drought conditions.