1. Handbook of mathematical functions with formulas, graphs, and mathematical tables;Abramowitz M.,1965
2. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements;Allen R. G,1998
3. SPEI: Calculation of the Standardized Precipitation-Evapotranspiration Index;Beguería S,2017
4. Standardized precipitation evapotranspiration index (SPEI) revisited: parameter fitting, evapotranspiration models, tools, datasets and drought monitoring;Beguería S;International Journal of Climatology,2014
5. Inadequacy of the gamma distribution to calculate the Standardized Precipitation Index;Blain G. C;Revista Brasileira de Engenharia Agrícola e Ambiental,2015