Online Drought Indicators and Indices

Standardized Precipitation Evapotranspiration Index (SPEI) 


Index name: Standardized Precipitation Evapotranspiration Index (SPEI).

Ease of use: Yellow.

Origins: Developed by Vicente-Serrano et al. at the Instituto Pirenaico de Ecologia in Zaragoza, Spain.

Characteristics: As a relatively new drought index, SPEI uses the basis of SPI but includes a temperature component, allowing the index to account for the effect of temperature on drought development through a basic water balance calculation. SPEI has an intensity scale in which both positive and negative values are calculated, identifying wet and dry events. It can be calculated for time steps of as little as 1 month up to 48 months or more. Monthly updates allow it to be used operationally, and the longer the time series of data available, the more robust the results will be.

Input parameters: Monthly precipitation and temperature data. A serially complete record of data is required with no missing months.

Applications: With the same versatility as that of SPI, SPEI can be used to identify and monitor conditions associated with a variety of drought impacts.

Strengths: The inclusion of temperature along with precipitation data allows SPEI to account for the impact of temperature on a drought situation. The output is applicable for all climate regimes, with the results being comparable because they are standardized. With the use of temperature data, SPEI is an ideal index when looking at the impact of climate change in model output under various future scenarios.

Weaknesses: The requirement for a serially complete dataset for both temperature and precipitation may limit its use due to insufficient data being available. Being a monthly index, rapidly developing drought situations may not be identified quickly.

Resources: SPEI code is freely available at the SPEI website by the Consejo Superior de Investigaciones Científicas (CSIC) and the calculations are also described in the literature. Additional resources are available at the Flood and Drought portal developed by GEF, UN Environment, IWA and DHI.

Reference: Vicente-Serrano, S.M., S. Begueria and J.I. Lopez-Moreno, 2010: A multi-scalar drought index sensitive to global warming: the Standardized Precipitation Evapotranspiration Index. Journal of Climate, 23: 1696–1718. DOI: 10.1175/2009JCLI2909.1.

Currently used by: Peru, Switzerland.

  1. haregeweyen
    May 17, 2021 Reply

    Hello, how many years of data is needed for calculating SPEI?

    1. Katrin Ehlert
      May 21, 2021 Reply

      Dear Haregeweyen

      Thank you for your question. The SPEI is calculated in analogy to the SPI. The resulting values are sensitive to the quantity and reliability of the data used to fit the distribution. A time series of 30-50 years is recommended.

      Best regards
      IDMP TSU

  2. Vincent Uche
    April 28, 2021 Reply

    Hello, I surfed the internet in search of the Intensity scale for SPEI, then I stumbled on this interactive group. Could you please send me the intensity scale. Thanks

    1. Katrin Ehlert
      May 6, 2021 Reply

      Dear Vincent Uche

      Thank you for your question. The SPEI is calculated in analogy to the SPI, but the SPEI uses the weekly or monthly difference between precipitation and PET. Therefore, the values of both indices can generally range from -2 to 2. The lower the value, the higher the precipitation/PET deficit. The severity of a drought however is also connected with the recorded impacts at a certain SPEI value.

      Best regards
      IDMP TSU

  3. Frrancisco Saimone
    April 12, 2021 Reply

    I got all procedure from the Vicente-Serrano et al 2010 paper, I am trying to following it to my station but struggling with no success. Can someone provide a simple example of one year in excel to use the formulas and calculate SPEI?

    1. Katrin Ehlert
      April 15, 2021 Reply

      Dear Francisco Saimone

      Please refer to to the SPEI package for the R statistical software (freeware). This package will allow you to calculate the SPEI and there are also several data examples included:

      Best wishes
      IDMP Team

  4. Papaghiuc Lidia Maria
    February 5, 2021 Reply

    Hello !
    I really appreciate your services
    and I’m interested if you can expand the calculation area for 2019 and 2020.
    Please for Romania (NE): weather stations: Cotnari, Iasi, Botosani and Roman.
    All the best !
    My name is: Papaghiuc Lidia Maria, Iasi, Romania

  5. Ali
    November 5, 2020 Reply

    The above-mentioned web page ( was blocked.
    Could you please check the page?
    Thanks in advance.

    1. Robert Stefanski
      February 16, 2021 Reply

      Dear Ali,

      We just checked. The web page is still there. This is a link to a IDMP partner page but not directly under the management of IDMP.

      Bets reagrds,
      IDMP TSU

    2. Chin-Hsien
      April 2, 2021 Reply

      I can’t visit the page and it shows Web Page Blocked too. How may I resolve this issue?

  6. hosein
    November 13, 2019 Reply

    Why we calculate difference between precipitation and potential evapotranspiration?

    1. Katrin Ehlert
      December 6, 2019 Reply

      Dear Hosein

      Thank you for your comment to the SPEI. The SPEI uses the basis of the standardized precipitation index (SPI), which permits to identify the deviation of precipitation values from a long-term average. By adding a temperature component, the SPEI accounts for the effect of temperature on the development of drought conditions – high temperatures have been empirically shown to aggravate drought conditions. Temperature is included into the SPEI as a proxy for potential evapotranspiration (PET). By calculating the difference between precipitation and PET a simple climatic water balance is established, providing a simple measure of the water surplus or deficit of a month (if calculated on a monthly basis).

      You can find more information on this by visiting the SPEI website:

      Best regards
      IDMP team

  7. M.H.Gholizadeh
    December 11, 2016 Reply

    please send SPEI index for me.

    with the best wishes

    1. Robert Stefanski
      December 13, 2016 Reply

      Dear M.H. Gholizadeh,

      The SPEI code is freely available and the calculations are also described in the literature at
      The reference paper is: Vicente-Serrano, S.M., S. Begueria and J.I. Lopez-Moreno, 2010: A multi-scalar
      drought index sensitive to global warming: the Standardized Precipitation Evapotranspiration
      Index. Journal of Climate, 23:1696–1718.

      This information is also find on page 23 of the Handbook and n the SPEI page.

      Best regards,

      Bob Stefanki

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