Nonparametric Standardized Precipitation Index (SPI)

The commonly used Standardized Precipitation Index (SPI) relies on a representative parametric probability distribution function (e.g., Gamma). However, a parametric distribution function may not fit the data, especially in continental/global scale studies. This code computes SPI based on a nonparametric framework that does not require assuming a representative parametric distribution. The most attractive feature of the framework is that it leads to statistically consistent drought indicators based on different variables. A user guide and sample inpute data are included in the package.
This code is also part of the Standardized Drought Analysis Toolbox (SDAT):
http://amir.eng.uci.edu/sdat.php
References:

Hao Z., AghaKouchak A., Nakhjiri N., Farahmand A., 2014, Global Integrated Drought Monitoring and Prediction System, Scientific Data, 1:140001, 1-10, doi: 10.1038/sdata.2014.1.

Farahmand A., AghaKouchak A., 2015, A Generalized Framework for Deriving Nonparametric Standardized Drought Indicators, Advances in Water Resources, 76, 140-145, doi: 10.1016/j.advwatres.2014.11.012.

Hao Z., AghaKouchak A., 2014, A Nonparametric Multivariate Multi-Index Drought Monitoring Framework, Journal of Hydrometeorology, 15, 89-101, doi:10.1175/JHM-D-12-0160.1.