A new approach to validate MODIS time-series VIs were examined by comparing over five years of vegetation start of season (SOS) estimated from up to ten years (2003-2012) of MODIS and ground based VIs across five different biomes. Daily ground based NDVI and a two band EVI (EVI2) were calculated from incoming and outgoing photosynthetically active radiation and global solar radiations at ten AmeriFlux sites located in North America. Obtained daily ground based VIs and eight-day composited MODIS VIs (NDVI, EVI, and EVI2) were fitted to asymmetric double logistic function in each year, and SOSs were calculated using two methods; inflection point of first derivative (SOSinf) and half amplitude (SOSratio) of fitted curves.
Although site specific differences were seen in the direction and range, SOS from MODIS and ground VIs showed an analogous pattern over time across five biomes. MODIS SOSratio plotted against ground SOSratio closely aligned with 1:1 line (slope > 0.80, R2>0.85), whereas SOSinf showed apparently narrower angle caused by the large variability in arid grassland sites (slope ≈ 0.5, R2<0.60). Comparing SOSs from both datasets after the Gram-Schmitt orthogonalization, the standard deviation (s.d.) of SOS differences were smaller than s.d. of interannual variability except for evergreen needle leaf forest and arid grassland sites. In particular, MODIS tracked ground based SOSs with a week differences on average at deciduous broad leaf forest sites (3-7days) and clopland sites (6-9days) regardless of the method. SOSratio at wooded savanna site and temperate grassland site also showed good agreement (5-7days). These results indicate that MODIS VIs are capable to monitor inter-annual variability of surface vegetation at four biome types with half amplitude method. The narrow slopes suggested that the early in the year, the early MODIS detects SOS, and vice versa. The further effort on time series screening and the increase of number of data point could improve the issue seen in evergreen forest and arid grassland sites.
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