Summary:

In the study two methods of counting SPI (standardized precipitation index) are given (McKee i in. 1993, 1995). The first of them is based on the fact that em-pirical distributions of monthly precipitation sums are usually gamma distributed (Kaczmarek 1970). The verification of goodness-of fit test hypothesis of empirical distribution of monthly precipitation sums in the years 1964-2009 in Wrocław-Swojec, with gamma distribution, confirmed that. The second method of SPI eval-uation relies on counting the value: , where F is a distribution function of gamma distribution with parameters estimated on the basis of analyzed data set, and is a distribution function of standardized normal distribution (McKee i in. 1993,1995). In practice, the following fact is often taken into account: for a random variable X gamma distributed, variable has ap-proximately normal distribution (Krishnamoorthy K. i in. 2008). In order to esti-mate SPI, other transformations are tested: and .. With the use of those two methods, SPI for monthly precipitation sums in the years 1964-2009 were evaluated. Then, precipitation conditions for monthly precipitation sums, assessed by SPI counted with the use of gamma or normal distribution, were compared. It has been shown that the precipitation conditions may be classified with the use of SPI based both on gamma and normal distributions.