Investigating the Long-Term Spatial and Temporal Characteristics of Vegetative Drought in the Contiguous United States

Multipronged statistical approaches were applied to give spatiotemporal insight into the long-term patterns of vegetative drought over the contiguous United States (CONUS) from a weekly NOAA/AVHRR VHI product of 1981-2017. Based on the analysis results, the following characteristics have been found: First, the category of the most frequently occurring drought is the moderate drought (D1), and the drought frequency successively decreases from abnormally dry conditions (D0) to exceptional drought (D4). The frequencies of moderate drought (D1), severe drought (D2), and extreme drought (D3) in the West Mountains region are consistently higher than the frequency of the corresponding severity levels in other regions, while the frequency of exceptional drought (D4) in the Great Plains is the highest. The frequencies of three worst drought levels (D2, D3, and D4) in the East region are consistently the lowest in the CONUS. Second, there are gradual changes in the trends of vegetative drought from March to October and the change patterns are significantly different for newly identified four vegetation classes (evergreen forest, deciduous forest, grass, and crop). There is more increase in VHI values in March-May and less in June-October for deciduous forest and grass. Evergreen forest presents similar results except more in October. There are obviously more pixels with significant increase in VHI values in March, April and July, August for crop. Third, the trajectories of the centroids of vegetative drought have significantly moved southwestward in the past 37 years. With drought severity levels increasing, the centroid movement got larger and there were regular translations toward southwest.