| Drought Monitoring |  |
| Drought Monitoring | |
6.C. STANDARDIZED PRECIPITATION INDEX (SPI)
The Standardized Precipitation Index (SPI) is designed to quantify the precipitation deficit for multiple time scales. These time scales reflect the impact of drought on the availability of the different water resources. The SPI is calculated by taking the difference between the actual and mean precipitation for a particular time scale, then dividing by the standard deviation. A drought event is defined as any time the SPI is continuously negative and reaches an intensity where the SPI is -1.0 or lower. The drought event ends when the SPI becomes positive. Each drought event therefore has a duration defined by its beginning and end, and an intensity for each month that the event continues, as shown by example in Table 3.
| Table 3: SPI Values for Recent Colorado Drought | ||
| SPI Values | Drought Category | Time in Category |
| 0 to -.99 | Mild Drought | 34.1% |
| -1.0 to -1.49 | Moderate Drought | 9.2% |
| -1.5 to -1.99 | Severe Drought | 4.4% |
| -2.00 or less | Extreme Drought | 2.3% |
Table 3 also shows the percent of time that the SPI was in each of the drought categories based on an analysis of available station data in Colorado. The standardization allows the SPI to determine the rarity of a current drought as well as the probability of the precipitation necessary to end the current drought.
Current and archive values of the SPI for various time frames can be obtained from the Western Regional Climate Center Website