Validating Long-term Consistency of MODIS EVI Time Series Using Ground-based Radiation Flux Data
Abstract:The Enhanced Vegetation Index (EVI) time series from Moderate Resolution Imaging Spectrometer (MODIS) has exceeded a decade in length. It is, thus, desirable to evaluate how well the time series captures inter-annual variability of vegetation phenology. Previous studies calculated a two-band version of the EVI (EVI2) from tower radiation flux data and used it to validate satellite VI time series. Differences in view angle, bandpass, and spatial representativeness between flux and satellite data, however, may lead to landcover-dependent biases when they are compared directly. The objective of this study was to validate long-term consistency of MODIS EVI time series with radiation flux-derived EVI2 time series by comparing phenological metrics derived from these datasets.
Ten years of MODIS EVI and ground-based EVI2 (Tower EVI2) were obtained for 10 AmeriFlux sites. Asymmetric double logistic functions were fitted to each of VIs, from which SOSs were derived. After the Gram-Schmidt orthogonalization of the derived SOSs, the standard deviation (SD) in horizontal direction (inter-annual variability) was compared with SD in perpendicular direction (differences) to assess consistency of MODIS EVI in tracking vegetation dynamics.
Temporal profiles of MODIS EVI showed analogous patterns with those of tower EVI2 across five biomes although site specific differences were seen in the VI amplitude. Cross plots of SOS from MODIS and Tower VIs closely aligned to the 1:1 line (slope > 0.865, R2>0.896). The SD in inter-annual variability (≈ 20 days) was more than twice larger than the SD of SOS difference averaged for five biomes (≈ 9 days). MODIS consistently captured SOSs with 2.7-4.9-day differences at deciduous broad leaf forest and clopland sites, and also agreed well at a wooded savanna site (< 6 days). Grassland sites showed more than a week difference due to a failure in model fitting of the year with subtle VI amplitude and the year with multiple growing seasons.
These results indicate that MODIS EVI time series was capable to capture long-term variability in surface vegetation dynamics at four biomes other than arid grassland in North America. Further efforts on data screening and fitting on multiple growing seasons could improve results on arid grassland sites.