<div dir="ltr">Good question!<br><div>--<br><div><br>Beguería, S., Vicente-Serrano, S. M., Reig, F. and Latorre, B. (2014), 
Standardized precipitation evapotranspiration index (SPEI) revisited: 
parameter fitting, evapotranspiration models, tools, datasets and 
drought monitoring. Int. J. Climatol., 34: 3001–3023. 
<br><br>doi:10.1002/joc.3887<br><br></div><div>I have not looked carefully at this but it contains ....<br></div><div><br>"The SPEI R package allows selection of: (1) two probability distributions
 for calculation of SPEI (log-logistic [recommended] or Pearson III 
[commonly used to obtain the Standardized Precipitation Index, SPI ..."<br><br>---<br></div><div><br></div><div>So, while not the "recommended" distribution, apparently, the Pearson III is acceptable. <br><br>The SPI uses monthly precipitation and the SPEI uses the climatic water balance (precipitation minus potential evapotranspiration).<br><br>---<br></div><div>Keep in mind that both the SPI and SPEI are *statistically* based indices while the PDSI (scPDSI) is a *physically* based (conceptually-simple) water balance model.<br><br></div><div>If you do try this, I would suggest using several different reference evapotranspiration methods:<br><br><a href="http://www.ncl.ucar.edu/Document/Functions/crop.shtml">http://www.ncl.ucar.edu/Document/Functions/crop.shtml</a><br><br></div><div>refevt_*<br><br></div><div>thornthwaite<br><br>---<br></div><div>Good luck<br></div><div><br></div><div><br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Feb 22, 2018 at 9:40 AM, Jian Peng <span dir="ltr"><<a href="mailto:jian.peng@ouce.ox.ac.uk" target="_blank">jian.peng@ouce.ox.ac.uk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">



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<font style="font-size:14px" face="Times">Dear all,</font>
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<div><font style="font-size:14px" face="Times">I am wondering if it is possible to use ncl function ‘<span style="line-height:12pt">dim_spi_n</span><span style="line-height:16px">’</span><span style="line-height:12pt"> to calculate SPEI. I have my own </span><span style="line-height:16px">precipitation</span><span style="line-height:12pt"> and
 potential evapotranspiration datasets. The only concern is the distribution function. It seems that </span></font><span style="font-family:Times;font-size:14px;line-height:16px">dim_spi_n</span><span style="font-size:14px;font-family:Times;line-height:12pt"> only
 supports Gamma and </span><span style="font-size:14px;font-family:Times;background-color:rgb(255,255,255)">Pearson type III distribution. However, the log-logistic distribution is widely recommended for SPEI calculation.</span></div>
<div><font face="Times"><span style="background-color:rgb(255,255,255);font-size:14px"><br>
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<div><font face="Times"><span style="background-color:rgb(255,255,255)"><span style="font-size:14px">Could you please let me know if there are any possibilities to use </span></span><span style="font-size:14px;background-color:rgb(255,255,255)">log-logistic distribution
 or other alternative solutions? Many thanks indeed!</span></font></div>
<div><font face="Times"><span style="background-color:rgb(255,255,255);font-size:14px"><br>
</span></font></div>
<div><font face="Times"><span style="background-color:rgb(255,255,255);font-size:14px">Best regards,</span></font></div>
<font face="Times"><span style="font-size:14px">Jian</span></font><br>
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