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Explanation of Standard Precipitation Index (SPI)

The Standard Precipitation Index (SPI) is a relatively new drought index based only on precipitation. It's an index based on the probability of precipitation for any time scale. Some processes are rapidly affected by atmospheric behavior, such as dry land agriculture, and the relevant time scale is a month or two. Other processes have longer time scales, typically several months, such as the rate at which shallow wells, small ponds, and smaller rivers become drier or wetter. Some processes have much longer time scales, such as the rate at which major reservoirs, or aquifers, or large natural bodies of water rise and fall, and the time scale of these variations is on the order of several years.

The SPI was formulated by Tom Mckee, Nolan Doesken and John Kleist of the Colorado Climate Center in 1993. The purpose of SPI is to assign a single numeric value to the precipitation that can be compared across regions with markedly different climates. The standardization of the SPI allows the index to determine the rarity of a current drought.

The Standardized Precipitation Index (SPI) was designed to show that it is possible to simultaneously experience wet conditions on one or more time scales, and dry conditions at other time scales. Consequently, a separate SPI value is calculated for a selection of time scales.

Click here to view maps showing Standardized Precipitation Index (SPI) drought maps for the state for current month.

Interpretation of SPI at different time scales

A 1-month SPI typically compares well to the percent of normal precipitation for the month. However, as with other SPI time scales, it is actually considered to be a more accurate representation of monthly precipitation for a given location because the long-term precipitation record (over 30 years or more) is fitted to a probability distribution. It is then transformed into a normal distribution so that the median SPI for the location and period is zero; that is, half of the historical precipitation amounts are below the median and half are above the median. Positive SPI values indicate greater than median precipitation (i.e. wet conditions), and negative values indicate less than median precipitation (i.e. dry conditions). The 1-month SPI is a short-term value and during the growing season can be important for correlation of soil moisture and crop stress.

The 3-month SPI provides a comparison of the precipitation over a specific 3-month period with the precipitation totals from the same 3-month period for all the years included in the historical record. In other words, a 3-month SPI at the end of February compares the December-January-February precipitation total in that particular year with the December-February precipitation totals of all the years. A 3-month SPI reflects short- and medium-term moisture conditions and provides a seasonal estimation of precipitation.

It is important to compare the 3-month SPI with longer time scales. A relatively normal 3-month period could occur in the middle of a longer-term drought that would only be visible at longer time scales. Looking at longer time scales would prevent a misinterpretation that any "drought" might be over.

The 6-month SPI compares the precipitation for that period with the same 6-month period over the historical record. The 6-month SPI indicates medium-term trends in precipitation and is still considered to be more sensitive to conditions at this scale than the Palmer Index. A 6-month SPI can be very effective showing the precipitation over distinct seasons. Information from a 6-month SPI may also begin to be associated with anomalous streamflows and reservoir levels.

The 9-month SPI provides an indication of precipitation patterns over a medium time scale. Droughts usually take a season or more to develop. SPI values below -1.5 for these time scales are usually a good indication that fairly significant impacts are occurring in agriculture and may be showing up in other sectors as well. Some regions of the country may find that the pattern displayed by the map of the Palmer Index closely relates to the 9-month SPI maps. For other areas, the Palmer Index is more closely related to the 12-month SPI. The Palmer Index maps are updated each week, although the patterns usually do not change significantly. The SPI maps are updated at the end of each month.

A 12-month SPI is a comparison of the precipitation for 12 consecutive months with the same 12 consecutive months during all the previous years of available data. The SPI at these time scales reflect long-term precipitation patterns. Because these time scales are the cumulative result of shorter periods that may be above or below normal, the longer SPIs tend toward zero unless a specific trend is taking place.

SPIs of these time scales are probably tied to streamflows, reservoir levels, and even groundwater levels at the longer time scales. In some locations of the country, the 12-month SPI is most closely related with the Palmer Index, and the two indices should reflect similar conditions.

Additional information concerning the SPI can be found on the following web sites:


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