Sperling Drought Index Methodology
With the Sperling Drought Index, we sought to provide an easy-to-understand measure of the current drought status by looking at the long-term effects of weather patterns. A score of 100 is the normal - scores over 100 indicate dry conditions, under 100 indicate wetness.
We used statistics from the National Climatic Data Center to create the Sperling Drought Index. We specifically considered long-term precipitation trends and patterns, and the Palmer drought indices. These metrics are particularly valuable in that they attempt to measure the duration and intensity of long-term drought-inducing circulation patterns. We used a rolling average, weighted more heavily towards recent precipitation trends.
Since the hydrological effects of drought such as groundwater and reservoir levels take longer to develop and recover, we based the Sperling Drought Index on those metrics which respond more slowly to immediate weather conditions.
Accordingly, a short-term wet spell may have little direct effect on the Sperling Drought Index.
| Drought score | Description |
|---|
| over 120 | Extreme Drought |
| 120-115 | Severe Drought |
| 115-110 | Moderate Drought |
| 110-105 | Mild Drought |
| 105-102.5 | Temp Dry Spell |
| 102.5-97.5 | Near Normal |
| 97.5-95 | Temp Wet Spell |
| 95-90 | Mildly Wet |
| 90-85 | Moderately Wet |
| 85-80 | Very Wet |
| under 80 | Extremely Wet |
We created our index so that a score of 100 represents the climatic normal. Values greater than 100 represent drier conditions, and values less than 100 indicate wetter conditions.
It would not be accurate to use the Sperling Drought Index to quantify differences in drought. For example, if one place has a Drought Score of 110 compared to another with a Drought Score of 95, it would be misleading to say the place has "15% more drought," or faces a "15% greater drought risk." The effects of drought vary greatly between different places, and cannot be reduced to a single measure.
For more information about the measurement of drought, consult the NCDC resources for the Standardized Precipitation Index (SPI), the Crop Moisture Index (CMI), the Palmer Z Index, the Palmer Drought Severity Index (PDSI) and the Palmer Hydrological Drought Index (PHDI).