Satellite-Based Observations of Inter-annual Variation of Vegetation Water Content and Productivity in Northern Asia During 1998-2001

The terrestrial biosphere was largely carbon neutral during the 1980s, but became a much stronger net carbon sink in the 1990s. It is also thought that the unusually large carbon sink in the early 1990s can be largely attributed to climate variability. We analyzed multi-temporal images (1-km spatial resolution, 10-day composites) from the SPOT-4 VEGETATION (VGT) sensor over the period of April 1, 1998 to September 30, 2001 to characterize spatial and temporal variations of vegetation and water indices for Northern Asia (40oN - 75oN, and 45oE - 179oE). Three remote sensing proxies were derived from the VGT data: Normalized Difference Water Index (NDWI), Normalized Difference Vegetation Index (NDVI), and Enhanced Vegetation Index (EVI). We calculated anomalies of NDWI, NDVI and EVI at different temporal scales, i.e., 10-day, monthly, seasonal and plant growing season (April to September), and compared these with inter-annual variations in precipitation and temperature from the National Climate Data Center Global History Climate Network. Both anomalies of precipitation and NDWI over plant growing season (April to September) for Northern Asia were highest in 1998 but declined from 1999 to 2001. NDVI and EVI anomalies did not correlate well with each other overall. The EVI anomaly over plant growing season (April to September) was highest in 1998, and declined from 1999 to 2001,while the NDVI anomaly over plant growing season was lowest in 1998 and highest in 2000 for North Asia. The EVI includes information from the VGT blue band to account for the effects of residual atmospheric contamination (e.g., aerosols) and soil/vegetation background, while the NDVI does not. Large fires occurred in eastern Russia and Northeastern China in 1998 and may have increased the atmospheric aerosol burden; high precipitation in that year may have been associated with increased atmospheric water vapor. Both of these effects would lower the NDVI value in 1998. This continental-scale study demonstrates that NDWI has a potential for operational monitoring of vegetation water content, and also supports previous theoretical and observational studies which show EVI to be an improved measure relative to NDVI, as NDVI is substantially affected by both atmospheric condition and soil/vegetation background