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Standardized Precipitation Index

This map shows the Standardized Precipitation Index (SPI) for multiple monthly accumulation periods for Colombia and can be used to identify the intensity of drought or excess of rainfall for each month.

SPI values lower than -1 indicate a condition of drought, the more negative the value the more severe the situation. SPI values higher than +1 indicate more humid conditions compared to a normal situation. When the SPI has a value between -0.5 and +0.5 the situation is identified as normal. The table below can be used to interpret the SPI value.

Table 1: Range of Standardized Precipitation Index

SPI

Colors closer to red/brown in the map indicate a dry condition while colors closer to blue indicate a humid condition compared to the normal condition that specific area.

The SPI is available for different accumulation periods: 1, 3, 6, 9, 12, 24, 36 and 48 months. This allows evaluating the duration of drought or humid condition at different time scales. You can select the time scale in the menu analysis.

How is the SPI calculated?

The Standardized Precipitation Index (SPI; McKee 1993) is the number of standard deviations that observed cumulative precipitation deviates from the climatological average. It can be calculated for any time scale; various monthly and multi-monthly time scales are shown here, from 1 to 48 months. To compute the index, a long-term time series of precipitation accumulations over the desired time scale are used to estimate an appropriate probability density function. The analyses shown here are based on the Pearson Type III distribution (i.e., 3-parameter gamma) as suggested by Guttman (1999). The associated cumulative probability distribution is then estimated and subsequently transformed to a normal distribution. The result is the SPI, which can be interpreted as a probability using the standard normal distribution (i.e., users can expect the SPI to be within one standard deviation of the mean about 68% of the time, two standard deviations about 95% of the time, etc.) The analyses presented in this maproom utilize the FORTRAN code made available by Guttman (1999).

Extreme values of the SPI based on this CHIRPS dataset should be interpreted with caution as studies have indicated that the SPI should be based on a time series of at least 50 years in length.

Shading starts at +/- 0.5 with green/blue and yellow/orange shades indicating unusually wet and dry conditions, respectively.

References

Guttman, N. B., 1999: Accepting the Standardized Precipitation Index: A calculation algorithm. J. Amer. Water Resour. Assoc.., 35(2), 311-322.
McKee, T. B., N. J. Doesken, and J. Kliest, 1993: The relationship of drought frequency and duration to time scales. In Proceedings of the 8th Conference of Applied Climatology, 17-22 January, Anaheim, CA. American Meteorological Society, Boston, MA. 179-184.

Dataset Documentation

Standardized Precipitation Index
Data: Daily CHIRPS data with improved temporal downscaling procedure here.
Analysis Standardized Precipitation Index (SPI) using 1981-present climatological base period

Dataset

Access the dataset used to create this map.

How to use this interactive map

Helpdesk

Contact J.CARDONA@CGIAR.ORG with any technical questions or problems with this Map Room.