Standardized Precipitation Index (SPI)
Index name: Standardized Precipitation Index (SPI).
Ease of use: Green.
Origins: The result of research and work done in 1992 at Colorado State University, United States, by McKee et al. The outcome of their work was first presented at the 8th Conference on Applied Climatology, held in January 1993. The basis of the index is that it builds upon the relationships of drought to frequency, duration and timescales.
In 2009, WMO recommended SPI as the main meteorological drought index that countries should use to monitor and follow drought conditions (Hayes, 2011). By identifying SPI as an index for broad use, WMO provided direction for countries trying to establish a level of drought early warning.
Characteristics: Uses historical precipitation records for any location to develop a probability of precipitation that can be computed at any number of timescales, from 1 month to 48 months or longer. As with other climatic indicators, the time series of data used to calculate SPI does not need to be of a specific length. Guttman (1998, 1999) noted that if additional data are present in a long time series, the results of the probability distribution will be more robust because more samples of extreme wet and extreme dry events are included. SPI can be calculated on as little as 20 years’ worth of data, but ideally the time series should have a minimum of 30 years of data, even when missing data are accounted for.
SPI has an intensity scale in which both positive and negative values are calculated, which correlate directly to wet and dry events. For drought, there is great interest in the ‘tails’ of the precipitation distribution, and especially in the extreme dry events, which are the events considered to be rare based upon the climate of the region being investigated.
Drought events are indicated when the results of SPI, for whichever timescale is being investigated, become continuously negative and reach a value of -1. The drought event is considered to be ongoing until SPI reaches a value of 0. McKee et al. (1993) stated that drought begins at an SPI of -1 or less, but there is no standard in place, as some researchers will choose a threshold that is less than 0, but not quite -1, while others will initially classify drought at values less than -1.
Owing to the utility and flexibility of SPI, it can be calculated with data missing from the period of record for a location. Ideally, the time series should be as complete as possible, but SPI calculations will provide a ‘null’ value if there are insufficient data to calculate a value, and SPI will begin calculating output again as data become available. SPI is typically calculated for timescales of up to 24 months, and the flexibility of the index allows for multiple applications addressing events that affect agriculture, water resources and other sectors.
Input parameters: Precipitation. Most users apply SPI using monthly datasets, but the computer programs have the flexibility to produce results when using daily and weekly values. The methodology of SPI does not change based upon using daily, weekly or monthly data.
Applications: The ability of SPI to be calculated at various timescales allows for multiple applications. Depending on the drought impact in question, SPI values for 3 months or less might be useful for basic drought monitoring, values for 6 months or less for monitoring agricultural impacts and values for 12 months or longer for hydrological impacts. SPI can also be calculated on gridded precipitation datasets, which allows for a wider scope of users than those just working with station-based data.
Strengths: Using precipitation data only is the greatest strength of SPI, as it makes it very easy to use and calculate. SPI is applicable in all climate regimes, and SPI values for very different climates can be compared. The ability of SPI to be computed for short periods of record that contain missing data is also valuable for those regions that may be data-poor or lacking long-term, cohesive datasets. The program used to calculate SPI is easy to use and readily available. NDMC provides a program for use on personal computers that has been distributed to more than 200 countries around the world. The ability to be calculated over multiple timescales also allows SPI to have a wide breadth of application. Many articles relating to SPI are available in the science literature, giving novice users a multitude of resources to rely on for assistance.
Weaknesses: With precipitation as the only input, SPI is deficient when accounting for the temperature component, which is important to the overall water balance and water use of a region. This drawback can make it more difficult to compare events of similar SPI values but different temperature scenarios. The flexibility of SPI to be calculated for short periods of record, or on data that contain many missing values, can also lead to misuse of the output, as the program will provide output for whatever input is provided. SPI assumes a prior distribution, which may not be appropriate in all environments, particularly when examining short-duration events or entry into, or exit out of, drought. There are many versions of SPI available, implemented within various computing software packages other than that found in the source code distributed by NDMC. It is important to check the integrity of these algorithms and the consistency of output with the published versions.
Resource: The SPI program can be run on Windows-based personal computers. Download at: National Drought Mitigation Center – SPI Program. Global SPI data at 3-, 6-and 12-moth scales is available at NCAR/UCAR Research Data Archive. Visualization of global SPI data at 3-, 6- and 12-month scales is provided by the Tokyo Climate Center (TCC) ClimatView tool. Additional resources are available at the Flood and Drought portal developed by GEF, UN Environment, IWA and DHI.
References:
Guttman, N.B., 1998: Comparing the Palmer Drought Index and the Standardized Precipitation Index. Journal of the American Water Resources Association, 34: 113–121. DOI: 10.1111/j.1752-1688.1998.tb05964.x. (For more information on this paper, please contact the IDMP HelpDesk).
Guttman, N.B., 1999: Accepting the Standardized Precipitation Index: A Calculation Algorithm. Journal of the American Water Resources Association, 35: 311–322. DOI: 10.1111/j.1752-1688.1999.tb03592.x. (For more information on this paper, please contact the IDMP HelpDesk).
Hayes, M., M. Svoboda, N. Wall and M. Widhalm, 2011: The Lincoln Declaration on Drought Indices: Universal Meteorological Drought Index Recommended. Bulletin of the American Meteorological Society, 92(4): 485–488. DOI: 10.1175/2010BAMS3103.1.
McKee, T.B., N.J. Doesken and J. Kleist, 1993: The Relationship of Drought Frequency and Duration to Time Scales. Proceedings of the 8th Conference on Applied Climatology, 17–22 January 1993, Anaheim, CA. Boston, MA, American Meteorological Society.
World Meteorological Organization, 2012: Standardized Precipitation Index User Guide, (WMO-No. 1090), World Meteorological Organization, Geneva, Switzerland.
Wu, H., M.J. Hayes, D.A. Wilhite and M.D. Svoboda, 2005: The Effect of the Length of Record on the Standardized Precipitation Index Calculation. International Journal of Climatology, 25(4): 505–520. DOI: 10.1002/joc.1142.
Currently used by: Argentina, Austria, Belize, Bosnia and Herzegovina, Brazil, Bulgaria, Canada, Chile, Croatia, Cyprus, Dominican Republic, Germany, Greece, Hong Kong, Iran, Israel, Jamaica, Jordan, Kazakhstan, Libya, Lithuania, Macedonia, New Zealand, Pakistan, Peru, Slovenia, Spain, Sri Lanka, Switzerland, Tanzania, Thailand, Trinidad and Tobago, Turkey, Ukraine, USA.
70 Responses
How can I calculate the daily SPI index, and how to interpret it?
Hello everyone!!
I have used monthly precipitation using the monthly rainfall data, the SPI index is generated used the tool on WMO website. Pls guide me as to how to convert the monthly SPI index values to an Yearly index. Please guide
Dear Vidya,
Thank you for your question. We recommend to calculate the annual SPI based on annual precipitation values.
Kind regards,
Valentin
Hello! I have input my data and setup for generating the output. When I generated it is done completely but no output. Thus result at all.
Dear Jabiru,
thank you for your question. Can you please clarify what output is missing?
Best regards,
IDMP TSU
This software uses a parametric or non parametric method? Which distribution is used in this software to calculate SPI?
I have used the program to generate 1-month SPI values. I found that the month of July (highest monthly rainfall in study area) across multiple years has a SPI of exactly zero which seems like a strange result. Are there any specific reasons for this?
Dear Harsh Upadhyay
You can check the SPI user guide here for validation of the method and expected values: https://www.droughtmanagement.info/literature/WMO_standardized_precipitation_index_user_guide_en_2012.pdf
There is also additional guidance on the NDMC page for the SPI tool: https://drought.unl.edu/monitoring/SPI.aspx
Best regards
IDMP TSU
Dear Belkys
Thank you for your comment. With SPI time scale I understand that you mean the run lengths of 1,3,6,12,… months. The choice of run length depends on what time scale you want to look at – short, medium or long-term moisture conditions. Further info on this is given here: Standardized Precipitation Index User Guide. Note that a data series of monthly or weekly precipitation of at least 30 years is required in order to calculate robust probabilities.
Best regards
IDMP TSU
Hello, could someone explain to me how to use scale time based on the data I have, and what spi to generate?
Dear Yeluma
The SPI is one of several indices used in the US Drought Monitor. The National Drought Mitigation Center has more information on the USDM: https://drought.unl.edu/Home.aspx
Additionally, please find information and references in the following websites:
https://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices
https://www.drought.gov/data-maps-tools/us-gridded-standardized-precipitation-index-spi-nclimgrid-monthly
Publications (open source):
https://journals.ametsoc.org/view/journals/bams/80/3/1520-0477_1999_080_0429_mtduts_2_0_co_2.xml?tab_body=pdf
Best regards
IDMP TSU
Dear Yeluma
The SPI is one of several indices used in the US Drought Monitor. The National Drought Mitigation Center has more information on the USDM: https://drought.unl.edu/Home.aspx
Additionally, please find information and references in the following websites:
https://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/indices
https://www.drought.gov/data-maps-tools/us-gridded-standardized-precipitation-index-spi-nclimgrid-monthly
Publications (open source):
https://journals.ametsoc.org/view/journals/bams/80/3/1520-0477_1999_080_0429_mtduts_2_0_co_2.xml?tab_body=pdf
Best regards
IDMP TSU
Please can I get some articles on the application of spi index in drought determination in the US
i need spi softwere
Dear Berecha Teressa
Here you go: https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx
You can download the software and documentation by clicking on “Program and Documentation” on the right-hand side of the page.
Best regards
IDMP TSU
Dear Shani Khan
The SPI program uses two input parameters – the date and precipitation amount. Please consult the documentation of the SPI tool for detailed information on how to format input files. The documentation is part of the software package that can be downloaded here: https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx.
Regarding your analysis of various stations, you can generate the SPI for each station by preparing an input file for each station.
Best regards
IDMP TSU
The link is not working any idea to get it
Dear Angad
The website is temporarily offline, but will go online soon again. The link is correct, please try again to see the status.
Best regards
IDMP TSU
tus datos históricos tienes q ordenarlos en dos columnas (Fecha; Precipitacion (mm)) para que puedas trabajarlos. saludos
Dear Shafique,
Thank you for your question. The severity of the SPI is more negative number. Please note the SPI is standardized. Also, you will need to compare the SPI calculations with the historical occurrences and impacts.
Best regards,
IDMP TSU
Dear Shafique,
Thank you for your question. Please let us know if you are still having a problem using the SPI downloadable file with Windows 10. We will check with the US National Drought Mitigation Center who are the developers of the software to see there are any recurring issues. Here is the link to their website.
Best regards,
IDMP TSU
Dear Sachini,
Thank you for your question. This is a detailed question. We will email you directly if there is still a problem.
Best regards,
IDMP TSU
How to prepare the data sheet for calculating SPI?
Hi
i want to calculate the SPI for 6 month time scale. I have 10 different rainfall stations of one region. can anyone tell me how to arrange the data for calculation of SPI. I have the data for same time e.g 1981 to 2020 for all stations.
Hi how to extract severity and duration after calculating SPI
If this comment is in page kindly send again
How to obtain Severity and Duration variables from calculated SPI. Anyone can help me
Dear all anyone tell me how to seprate duraton and severity variables from calculated SPI.
I have successfullu calculate the SPI values from different stations but dont know how to identiify the sevrity and duration vaiables.
If I click on executable file then SPI generator file is seen and not understand about documentation how to carry input file and SPI are obtain
If I click on executable file then SPI generator file is seen and not understand about documentation how to carry input file and SPI are obtain
Sir how to install SPI software in pc. i am using window 10 in pc. If i download the document and programming file then no setup is seen in that folder. kindly help me about that. Thankuu
I’m going to compare drought events between two cities. There I’m using only 12 years of data(2007-2018).
For one town spi values related to February is NA(for the whole period only February data). I don’t why. But the software returned the February data for the other town. Please help me in this regard.
How to calculate SPI for each grid from gridded data in nc file?
Dear Nurul,
Thank you for your question. Have taken a look at the SPI Guide?
Best regards,
IDMP TSU
Dear Abela Bekele
Thank you for your comment.
As for a model that will deliver the best SPI predictions 3 months ahead of time, this is dependent on which model works best for your local context. Your national Meteorlogical Service may have information on which model performs best in your region of Ethiopia. Once you have decided on a model, you can use the SPI generator application (https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx) to run gridded projected precipitation values for your chosen time period. In this case, the program treats gridded data as station data.
You can use the precipitation data from your local station to calculate the SPI. Please note that for a viable result, you will need good monthly precipitation data for at least the past 25-30 years. A longer data track is always better in order to get more reliable results.
Best regards
IDMP team
which models is more appropriate to forecasting drought a head of time of three month through using SPI? specially for the semi-arid areas of Oromia pastoral areas of Ethiopia, How will I get SPI soft were? could I use the rain fall data from local station to get SPI result of my interest area?
please attach to me SPI software packages
Dear Endale
Thank you for your comment.
For example, you can find the R-based SPI program here: https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx
Best regards
IDMP team
Hi
How to take timescales in computing spi? Taking them in the cumulative probability or in precipitation data? Can you share me a theoretical step by step computation
Dear Shahla,
Thank you for your email. If we understand your request correctly, it is recommended to use 30 years of precipitation data. You can use 10 years but then the analysis will not be robust.
Please look at the references at the bottom of the SPI web page. Specifically, look at the Standardized Precipitation Index User Guide published by WMO.
Best regards,
IDMP Team
i am thankful for the realization of this simplified software
Hi
How to calculate spi for missing data and zero rainfall
Dear Santoshchougale.27
Thank you for your comment.
The Standardized Precipitation Index (SPI) User Guide provides an explanation of the data needed in order to calculate the SPI. You can find it here: https://library.wmo.int/pmb_ged/wmo_1090_en.pdf
This extract of the above-mentioned publication lists the key points on how the SPI works:
• Precipitation is normalized using a probability distribution function so that values of SPI are
actually seen as standard deviations from the median.
• A normalized distribution allows for estimation of both dry and wet periods.
• Accumulated values can be used to analyse drought severity (magnitude).
• At least 30 years of continuous monthly precipitation data are needed but longer-term
records would be preferable.
• SPI timescale intervals shorter than 1 month and longer than 24 months may be unreliable.
• It is spatially invariant in its interpretation.
• Its probability-based nature (probability of observed precipitation transformed into an index)
makes it well suited to risk management and triggers for decision-making.
Kind regards,
IDMP Team
Hi Jaldesa
About drought forecasting, First of all, you should obtain precipitation values of the future period via outputs of CMIP5 models, or weather generators, or etc., then you can put them in the MDM (https://agrimetsoft.com/mdm.aspx) tool or other tools for calculating rain-based drought indices.
When you extract your specific data from CMIP5 models under RCP scenarios, you should use statistical downscaling methods (https://agrimetsoft.com/sd-gcm.aspx) or dynamical downscaling methods for finer resolutions then apply them in your drought monitoring tool for the future period. Also, be careful for implementing calibration and validation steps
If you have further question let me know (salehnia61@gmail.com).
Regards,
Nasrin
Hi Robert Stefanski and Hassan
I checked the DrainC tool and compare it. Also, I focused on the formula I think the result of Drainc is not true. But the Drought Mitigation Center software for SPI is similar to DMAP and it is true.
Hi Jaldesa Doyo
If you want to predict drought so you need to future climate data. You can use weather generators such as KNN-WG.
The best prediction for drought prediction is GCM models. because GCM models have been run by dynamic models based on RCP scenarios. RCP scenarios consider climate change.
You can use SD-GCM tool for downscaling GCM data. If you want to use coarse scale so you can use DMAP V1.1.
Good Luck
Hi Eremugo Isaac
You can use these two indexes:
SWSI (Surface Water Supply Index), Garen, 1993
SDI (Streamflow Drought Index), Nalbantis and Tsakiris, 2009
You can use this tool for calculating:
https://agrimetsoft.com/drought%20monitoring%20and%20prediction.aspx
I can not add here an excel file but if you want I can send to you an excel file for calculating SWSI.
Cheers
I am to get this vital information from you, My question is that , how we can calculate future drought / drought forecast for drought monitoring by SPI and RDI?
Dear Jaldesa,
SPI and RDI are indices and therefore you can use weather / climate forecasts to calculate the indices in the future.
Another example is provided by the USA. Please see the following examples of a drought monitoring map linked to the drought forecast.
US Drought Monitor – https://droughtmonitor.unl.edu/
Drought Forecast – https://www.cpc.ncep.noaa.gov/products/expert_assessment/sdo_summary.php
Best regards,
IDMP Team
Dear Team,
Kindly share with me a guide for Stream-flow Drought Index
Best Regards
Isaac
Dear Eremugo Issac,
Here is the refernce:
Nalbantis, I. and G. Tsakiris, 2009: Assessment of hydrological drought revisited. Water Resources Management, 23(5): 881–897. DOI 10.1007/s11269-008-9305-1.
Best regards,
IDMP TEAM
It is good teaching about SPI
hi
I used 3 tools for calculating SPI:
1- DrainC
2-DMAP
https://agrimetsoft.com/drought%20monitor.aspx
3-Drought Mitigation Center software for SPI
https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx
But I faced two difference between the calculated values of SPI in items1 Veras Item2,3
Which of them is true?
All of them has a research paper in peer-reviewed journals.
Thanks
Dear Hassan,
Thank you for your email. It is always go to look at different formulas. Are there significant differences in the three methods? This may be an interesting journal article. However, it not a question of which one of them is true but which one best represents drought in your country and region.
We do know that the Drought Mitigation Center software for SPI is widely used.
Best regards,
IDMP
Hello!
I need to have SPI software temple to download please if you have send to me. And Mann-Kendal software temple.
Best regards,
Dear Befirdu Zeleke,
Thank you for your email. Each index has a section where you can find the software (if it exists).
The SPI software can be found at: https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx
We are not familiar with the Mann-Kendal software. It is not listed in the IDMP Handbook on Drought Indicators and Indices.
Could you please find more us information on this index??
Best regards,
IDMP
Dear,sir
Please,Can I have you?How do the download for spi software?
Dear Nwet Nwet,
Thank you for your email. Each index has a section where you can find the software (if it exists).
The SPI software can be found at: https://drought.unl.edu/droughtmonitoring/SPI/SPIProgram.aspx
Best regards,
IDMP
Hello!
Can you explain me or give me reference about -0.8 SPI value as a threshold of moderate drought? It will serve to research in Peru.
Thanks a lot.
Dear Kris Correa Marrou,
The threshold on any drought index including the SPI have to be referenced to historical events and impacts.
Please look at the following publications:
SPI guide at https://www.droughtmanagement.info/literature/WMO_standardized_precipitation_index_user_guide_en_2012.pdf;
the Handbook on Drought Indices at https://www.droughtmanagement.info/find/guidelines-tools/handbook-drought-indicators-and-indices/
and the National Drought Management Policy Guidelines at https://www.droughtmanagement.info/find/guidelines-tools/guidelines/
Best regards,
IDMP Team
Hello!
Can you explain me or give me a reference about -0.8 SPI value as a threshold of moderate drought? It will serve to research in Peru.
Thanks a lot.
my question regarding this SPI 1,is what are the softwares we can calculate SPI on 2, for how many year that we have to calculate that the result have to be accurate 3, what are the accurate classes for the drought in pastoralist areas e.g moderate ,severy and others or standard numbers are
Dear Galgalo Wako,
Here are the answers to your questions.
1) Please find the SPI Guide at https://www.droughtmanagement.info/literature/WMO_standardized_precipitation_index_user_guide_en_2012.pdf
2) According the above guide, the minimum dataset is 30 years of rainfall data. Here the exact text from page 7 of the SPI guide:
3) The SPI can be used for many applications but you will need to know the historical droughts and impacts. We will need to obtain the previous droughts impacting pastoralists then you could determine which classes or thresholds to use for your country or region.
IDMP Team
how you culcul the spi index with the logiciel arcgis ?
Dear Meka,
Thank you for your comment. Please look at the Standardized Precipitation Index
User Guide at https://library.wmo.int/pmb_ged/wmo_1090_en.pdf Please look at page 13-15 which discusses mapping capabilities.
Best regards,
IDMP
softwares about standardized preciptation index and detail statsitcal calculation and working procedure about standardized streamflow indx(ssfi), streamflow drought index(sdi), drought reconenesance index(dri) ,percent normal precipitation, deciles
Please be sure to read the description of all of the indices. Much of information that you have requested is on the web page of each index.
For the SPI: http://drought.unl.edu/MonitoringTools/DownloadableSPIProgram.aspx
https://library.wmo.int/pmb_ged/wmo_1090_en.pdf
DRI software is available at http://drinc.ewra.net/
Good day sir I would like to know how are SPI values interpreted once calculated and classified.
Dear Thabang,
Please take a look at Tables 1 and 2 in the SPI Guide. The classification scheme can also be adjusted to your actual needs. This is where you will need to know the historical impacts on drought in your country or location.
Best regards,
IDMP TSU