I'm trying to calculate the SPI from CHIRPS monthly mean precipitation data, because it's too large I cut it down to my area of interest and here it is: https://www.dropbox.com/s/jpwcg8j5bdc5gq6/chirps_mensual_v1.nc?dl=0 I did this to open it:
require(utils)
require(colorRamps)
require(RNetCDF)
require(rasterVis)
require(rgdal)
library(ncdf4)
library(raster)
datos2 <- nc_open("Datos/chirps_mensual_v1.nc")
ppt_array <- ncvar_get(datos2, "precip")
#I'm only taking complete years so I took out two months from 2018
ppt_mes <- ppt_array[ , ,1:444]
I know there is a SPI library but I don't know how should I format the data in order to use it. So I tried to do it without the function by fitting the gamma distribution but I dont' know how to do it for this data base.
Does anyone know how to calculate SPI either with the function or by fitting the distribution?