[ncl-talk] SPI/SPEI ( log-logistic distribution )

[Dennis Shea]

Good question!
--

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.

doi:10.1002/joc.3887

I have not looked carefully at this but it contains ....

"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 ..."

---

So, while not the "recommended" distribution, apparently, the Pearson III
is acceptable.

The SPI uses monthly precipitation and the SPEI uses the climatic water
balance (precipitation minus potential evapotranspiration).

---
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.

If you do try this, I would suggest using several different reference
evapotranspiration methods:

http://www.ncl.ucar.edu/Document/Functions/crop.shtml

refevt_*

thornthwaite

---
Good luck



On Thu, Feb 22, 2018 at 9:40 AM, Jian Peng <jian.peng at ouce.ox.ac.uk> wrote:

> Dear all,
>
> I am wondering if it is possible to use ncl function ‘dim_spi_n’ to
> calculate SPEI. I have my own precipitation and potential
> evapotranspiration datasets. The only concern is the distribution function.
> It seems that dim_spi_n only supports Gamma and Pearson type III
> distribution. However, the log-logistic distribution is widely recommended
> for SPEI calculation.
>
> Could you please let me know if there are any possibilities to use log-logistic distribution
> or other alternative solutions? Many thanks indeed!
>
> Best regards,
> Jian
>
>
>
>
>
>
>
>
>
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