Hello there,

I am currently exploring the use of two drought indexes (SPI and SRI) for monitoring purposes. In addition, I would like to make SPI and SRI forecasts using predicted rainfall and discharge, respectevely.

For monitoring purposes, I know that you have to have at least 20 years of observed data,. Thereafter, you adjust your measurement to a Gamma distribution and then, you normalized the transform data using a standard deviation of 1 and a mean of zero. The question arise when we want to make prediction of the SPI, since I do not know if i have to adjust the 20 years of observation + monthly predictions data into the gamma distribution or should i obtain the gamma parameters with the 20 years of observed data only, and then make the SPI forecast using those gamma parameters