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What does the proper threshold that I should to use it for SPI?

The Standardized Precipitation Index (SPI) that measures the deviation of the actual precipitation from the average conditions in a given area (Mckee et al. 1993). The SPI is calculated as follows: build a frequency distribution from the historical precipitation data (at least 30 years of data) at a location for a specified period (1, 3, 6, 9, 12, 24 or 48 months). Then, a theoretical probability density function (e.g., gamma distribution) is fitted to the empirical distribution of precipitation frequency for the selected time scale.

According to these assessment criteria, Standardized Precipitation Index (SPI) was found to be the best choice for detecting droughts with many advantages, including fewer data requirements, ability of computation for any timestep, and interpretability. Keyantash and Dracup (2002) also found SPI as a highly valuable estimator of drought severity. The main advantage of the SPI is that can be calculated for multiple time-scales.

In the various papers, the researchers have different idea about the threshold of drought but usually, we get the threshold between 0 to -0.99. For calculating meteorological drought through SPI, you can easily determine every correct threshold of SPI in the RDIT(Rain-based Drought Indices Tool). Agrimetsoft has added an option for writting the intent threshold of the SPI drought index, with this ability in the RDIT tool, the user can easily write the proper value, according to the published papers.

A drought event for time scale i is defined here as a period in which the SPI is continuously negative and the SPI reaches a value of -1.0 or less. The drought begins when the SPI first falls below zero and ends with the positive value of SPI following a value of -1.0 or less.