Ariyanto, Dwi Priyo, et al. “Comparing the Accuracy of Estimating Soil Moisture Using the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI)”. SAINS TANAH - Journal of Soil Science and Agroclimatology, vol. 17, no. 1, June 2020, pp. 23-29, https://doi.org/10.20961/stjssa.v17i1.41396.