Interannual covariability in remotely sensed vegetation index and actual evapotranspiration over northern Eurasia

The covariability between the interannual changes of Normalized Difference Vegetation Index (NDVI) and actual evapotranspiration was examined. This study employed NDVI datasets from the Pathfinder AVHRR Land (PAL) data and the Global Inventory Monitoring and Modeling Studies (GIMMS) group to reduce the uncertainty that may be involved in the NDVI time series. The analysis was carried out for the northern Asia region from 1982 to 2000. 19-year interannual change in PAL-NDVI and GIMMS-NDVI were both compared with interannual change in actual ET which was estimated from model-assimilated atmospheric data and gridded precipitation data. For both NDVI datasets the annual maximum correlation with ET occurs in June, which is near the central period of the growing season. A significant positive correlation between NDVI and ET interannual changes was observed over most of the vegetated land area in June in PAL and GIMMS-NDVIs. These results suggest that the control of interannual change in ET is dominated by interannual change in vegetation activity. Based on analyses of temperature, precipitation, and NDVI interannual changes, it was found that the study area is roughly divided into two regions: the warmth dominant northmost region and the wetness dominant southern region. These results indicate that the vegetation interannual change and the resultant ET interannual change are controlled by warmth and wetness in these two regions, respectively.