Agricultural Drought Indices - US Department of Agriculture

Figure 7. Standardized Soil Water Index maps (October 2010) for four different time steps (1 month,
3 months, 6 months, and 12 months).
A severity assessment is performed as per Table 2. SSWI above 0 characterizes a water period
greater than average and SSWI below 0 deals with drier soil conditions than average. Standard
conditions range from –0.99 to +0.99. An extremely dry event is defined by SSWI below –2.0.
Table 2. Standardized Soil Water Index (SSWI) categories.
Value of SSWI Category Quantile
≥ 2.00 Extremely humid >= 43 years
1.50 - 1.99 Very humid >= 15 years
1.00 - 1.49 Moderately humid >= 6 years
-0.99 to 0.99 Around normal 6 years = 6 years
-1.50 to -1.99 Very dry >= 15 years
≤ -2.00 Extremely dry >= 43 years
Drought Characterization
With this index, drought events can be characterized at local scale for each specific grid cell. For
instance, if we choose a drought threshold of 5%, Météo-France can identify different events over
the last fifty years, and derive duration, severity, and magnitude from each of them. Different
characteristics like the starting date, end date, or peak date can also be extracted (Vidal et al.
2010). A classification of drought events that have occurred in France over the last 50 years has
been performed in the framework of this project. Spatio-temporal characteristics have been
derived from the meteorological reanalysis. Most severe events drought events can be identified
on Severity–Area–Time Scale (SAT) curves inspired from depth-area-duration analysis of storm
precipitation (Vidal et al. 2010).
Figure 8. Spatio-temporal characteristics of drought events in France from 1958 to 2008 (from Vidal
et al. 2009).
92