Two indices: standardized precipitation index (SPI) and relative precipita-tion index (RPI) analyze the same feature, which is either excess or shortage of precipitation. Having known that, the authors compared SPI and RPI with the use of monthly precipitation sums. The choice of this time period was intentional, since in the literature there has been no classification of precipitation conditions for monthly precipitation sums with the use of RPI so far. Therefore, the aim of the study was to determine the relative precipitation index values for monthly precipitation sums based on standardized precipitation index. The described method of transformation from SPI to RPI is innovative and has no equivalent in the literature. Having used the data from Agro- and Hydrometeorology Observatory in Wrocław-Swojec, the authors proposed the method of identification of precipitation conditions for monthly precipitation sums in this region. ...
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 ...
