SPI is the standardised deviation from the long term mean. if you have rainfall zero, you will get negative SPI (i.e. lower than the climatological mean).

Thank Kenneth! I got your answer but how can we get SPI value if the precipitation is zero in June?

The study area is Cambodia. The monthly rainfall is from 1988-2013 as I have attached below. I want to calculate 1 month SPI in January.

Thank Kenneth!

This is not an answer. I also expect to have answer from Kenneth. I need a complete solution of getting SPI for 3 month (e.g., SPI at Feb) for data like Tith Nhim. Thank you.

Since the gamma function is undefined for x=0 and a precipitation distribution may contain zeros, the CDF becomes: 

H(x)=q+(1-q)G(x) where q is the probability of zero. Then The H(x) is transformed to the standard normal random variable Z with mean = 0 and variance = 1. This Z value is in fact SPI.

I recommend the paper of Naresh Kumar et al - 2009 - "On the use of Standardized Precipitation Index (SPI) for drought intensity assessment". 

I put the Table data suggested by Dr Costinel in the paper "On the use of Standardized Precipitation Index (SPI) for drought intensity assessment"

I have found slight differences in the calculation of different parameters for example I found for the mean 61.974 instead of 61.981 (in the paper) and all the other parameters are slightly different for example U = 0.2848 instead of 0.2846, scale 32.4618 instead 32.4544 in the paper, finally mean SPI 0.0117 instead of  -0.0111. Could someone verify the calculation of this paper, maybe I'm wrong!!!

hi Bakali, i got same values as u, but my spi is 0,0585..... can u tell me how u did urs, i have feeling am doing something wrong

hello there, how do you find q in the kumar's paper, where they say: H(x)=q+(1-q)G(x) where q is the probability of zero ? i.e. i need to find the q, how do i find it?

Hi

Please read this. This article has a free software for calculating 8 drought Indices. SPI CZI, MCZI, ZSI, RAI, PN,DI, EDI

Article Estimation of meteorological drought indices based on AgMERR...

Piet, if you computing a 3 months drought, q= number of 0/ 3. I.e. if there are 2 zeros q =2/3

Hi,

If you want to download an excel file, you can refer to:

faq.agrimetsoft.com

Then in the list of questions there is an excel file that actually works for you to get your exact response.

Also, take a look in the following link for applying the best way of the SPI meteorological drought index.

https://agrimetsoft.com/MDM.aspx

Regards,

Nasrin Salehnia

Hi

You can find a sample excel file for calculating SPI in:

https://agrimetsoft.com/faq/How%20to%20calculate%20standard%20precipitation%20index%20in%20excel.aspx

If you have further question you can add a new question in:

https://agrimetsoft.com/faq

Cheers

This software useful to calculate Standardized Precipitation Index (SPI)

http://drought.unl.edu/MonitoringTools/DownloadableSPIProgram.aspx

Hello

You can download ''Drinc'' freeware and enjoy !

Hi,

I calculated SPI based on recommended excel sheet, but i think there is some problem , please guide me

What problems are you having?

Sorry Please find the attach of your excel sheet. i added another tab so you look at it and see if it does make sense. if none, you can always find me on [email protected] will talk about it

R programming is the best way to calculate SPI for the desired period. Please don't use Excel for such a work. You can solve your problem using SPI package in R:

https://cran.r-project.org/web/packages/spi/spi.pdf

Moreover, you can plot your results in a simple way.

If you aren't familiar with R, you can start to learn it.

Introductory Statistics with R is a very good book to learn R.

You can also use an empirical probability function, instead of a theoretical distribution like Gamma, Pearson, etc.; this way you don't have to deal with zero precipitation problems.

This link may help to find the answer

https://www.researchgate.net/post/How_calculate_standard_precipitation_index_in_excel

It is easy to calculate by using the attached software. You need to create a file as same as the attached sample file using Microsoft Excel and NotePad.