Does the ZSI (Z-Score Index) is equal to SPI (Standardized Precipitation Index) in calculations?

Unfortunately, in some cases has been seen that the Standardized Precipitation Index (SPI) is equal to ZSI, but it is not true, and the ZSI is occasionally confused with SPI. So, be careful in the equations of SPI and ZSI. The SPI is the most popular drought index (Karabulut, 2015) and is a widely recognized index for characterizing meteorological droughts (Hayes et al., 1999; Deo, 2011). McKee et al. (1993,1995) defined SPI suitable for different timescales (1, 3, 6, 12, 24 and 48 months), and the output values ranged from -2.0 to 2.0. Positive SPI values indicate precipitation higher than the median and negative values indicate precipitation lower than the median of long term precipitation records.

Drought periods are characterized by relatively high negative deviations. A drought event starts when SPI value reaches a negative value and ends when SPI becomes positive again Because precipitation data may be fitted by a gamma distribution, the SPI is calculated by using a probability density function of the gamma distribution. This drought index has been applied in order to determine drought duration and severity (Paulo et al., 2003). In its original version, a long precipitation record at a station is fitted to a probability (gamma) distribution, which is then transformed into a normal distribution so that the mean SPI is zero.

However, ZSI is more analogous to CZI, but without the requirement for fitting precipitation data to either gamma distribution or Pearson type III distribution. In ZSI equation, we should subtract the mean monthly precipitation (mm) from precipitation in a specific month (mm) then the outcome divide to the standard deviation of any time scale (mm), and so there is not any distribution in its calculation.(Drought Indices Paper). Since the standardized ZSI and SPI are performed in similar manners (McKee et al., 1993), they have similar interpretations of results. Thus, the ZSI values could be compared with the same thresholds as that of the SPI method (from -2 to 2).

For calculating the SPI and ZSI you can use three different products of Agrimetsoft, namely (Meteorological Drought Monitor Tool (MDM)), (Rain based Drought Indices Tool (RDIT)), and (Drought Monitor And Prediction (DMAP)).

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