Standardized Precipitation Index (SPI)

The Standardized Precipitation Index (SPI; McKee et al. 1993) is based solely on accumulated precipitation. Accumulated precipitation over different precipitation can be used to detect precipitation deficits and drought over short (i.e. weeks) and long (years) timescales. The intensity of the drought at different time scales can be measured using a traditional drought metric of the Standardized Precipitation Index(SPI). A SPI value near 0 represents precipitation near normal conditions, while positives or negatives values represent precipitation amounts above or below normal conditions. SPI is approximately the number of standard deviations the precipitation amount (accumulated over a specified time scale, i.e. 3-months, 6-month, 12-month, or 24-month) is above the mean precipitation amount. SPI values below -2 represent drought conditions. The main limitation of the SPI is that it is based entirely on precipitation and ignores other variables that affect atmospheric water demand such as solar radiation, temperature, humidity, and windspeed. While SPI has some limitations for detecting different types of drought, it is useful for evaluating precipitation anomalies at different time scales and complements other drought indices.

Climate Engine computes SPI using a parametric standardized probability based method. The Gamma distribution is used to obtain probabilities and then converted to SPI values using an inverse-normal distribution.

McKee, T. B., N. J. Doesken, and J. Kleist, 1993: The relationship of drought frequency and duration to time scales. Proc. Eighth Conf. on Applied Climatology, Boston, MA, Amer. Meteor. Soc., 179–184.