Long-term continuity in land surface phenology measurements: A comparative assessment of the MODIS land cover dynamics and VIIRS land surface phenology products

Vegetation phenology contributes to, and is diagnostic of, seasonal variation in ecosystem processes and exerts important controls on land-atmosphere exchanges of carbon, water, and energy. Satellite remote sensing provides a valuable source of data for monitoring the phenology of terrestrial ecosystems and has been widely used to map geographic and interannual variation in land surface phenology (LSP) over large areas. The Visible Infrared Imaging Radiometer Suite (VIIRS) land surface phenology product provides global data sets characterizing the annual LSP of terrestrial ecosystems, and is designed to support long-term continuity of LSP measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS). We used data from VIIRS and MODIS to evaluate the agreement and characterize the similarities and differences between LSP data from each instrument. Specifically, we compare data from the Collection 6 MODIS Land Cover Dynamics (LCD) product with data from the newly developed VIIRS LSP product over the most common land cover types in North America. To do this, we assessed the overall agreement between time series of vegetation indices from VIIRS and MODIS, evaluated the correspondence between retrieved phenometrics from each instrument, and analyzed sources of differences between phenometrics from the each product. As part of this analysis, we also compared phenometrics from MODIS and VIIRS with phenometrics derived from Landsat Analysis Ready Data and PhenoCam time series. Results show that two-band enhanced vegetation index (EVI2) values from VIIRS and MODIS are similar (R-2 > 0.81; root mean square deviation < 0.062), but that VIIRS EVI2 time series show more high frequency variation than time series from MODIS. Further, even though the VIIRS and MODIS products are generated using different instruments and algorithms, phenometrics from each product are similar and show only minor differences within and across land cover types. Systematic differences between phenometrics from the two products were generally less than one week (absolute bias 4.8 +/- 3.0 days), and RMSDs were less than two weeks for most phenometrics across different land cover classes (10.7 +/- 4.3 days). Comparison of VIIRS and MODIS LSP data with corresponding metrics estimated from Landsat and PhenoCam data consistently showed high agreement among the data sets. Overall, results from this analysis indicate that the VIIRS LSP product provides excellent continuity with the MODIS record. However, studies attempting to create high-fidelity long-term LSP time series by merging these products should exploit the overlap period of MODIS and VIIRS to estimate land cover-specific corrections for modest systematic bias in the MODIS LCD product relative to the VIIRS LSP product.