1. Introduction: Most investigations on the SPI (Standard Precipitation Index) have focused on drought analysis yet. A few researchers including Seiler et al. (2002) and Du et al. (2012) using monthly data showed SPI2, SPI12, and SPI24 have the potential to forecast floods occurrence. The present research aimed at using SPI for flood occurrence forecasting. The results of the study are important from two points of view: first, the applied model uses just one parameter that imposes less uncertainty through decreasing errors in preparing and introducing data to the model, and second, the model uses the most commonly available data with acceptable length everywhere; i. e. precipitation. While in drought analysis monthly scale is common, in this research the SPI was used in daily scale to fit the purpose of the study. For the case study, two different watersheds were selected; one in a humid area (Kasilian watershed in the north of Iran) and the other in a semi-dry region with a river of rainfall-snowy hydrologic regime (Kailak Darahsi watershed in the north-west of Iran)...