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Soil Moisture Drought Severity

Note: The plots below show weekly drought severity maps over the continental US, using soil moisture fields from the UW Experimental Surface Water Monitor. These maps are derived using similar algorithms to those used in Andreadis et al. (JHM 2005). Soil moisture at each model grid cell is expressed as a percentile of the grid cell's model climatology (1915-2004). This enforces a nominal unit range that allows for the comparison of hydrologic conditions across the CONUS domain.

The algorithm starts by partitioning the domain in drought areas (using a threshold of 20%). The initial drought clusters are formed by grouping drought grid cells that belong in spatially contiguous blocks. After that, drought clusters are merged based on their distance from each other, and their minimum area. A temporal persistence constraint is used to ensure plausible drought recovery; if a grid cell is out of drought at the current timestep, its drought status for the previous 3 weeks is examined and if its drought transition probability exceeds 50%, the grid cell is set back in drought. The transition probabilities have been calculated beorehand from its grid cell's climatology. A spatial smoothing step that involves examining the neighborhood of each grid cell at a pre-defined radius is applied to produce the final map.

List of the models used for this analysis are:

(Click on thumbnails below for larger image)