Drought classification and parameter identification based on actual precipitation index

Drought index studies are intensively in the literature, particularly based on various alternetive procedures for the standard precipitation index (SPI). This index requires transforming the original probability distribution function (PDF) into a standard normal (Gaussian) PDF with eight linguistical classifications. It is unfortunate that the SPI-based standard normal PDF cannot reveal statistical parameters regarding the actual durations, means, standard devistions and possible serial correlation coefficients of rainy and dry periods. The main objective of this paper is to explain the concept of the actual precipitation index (API) currently available in the literature and to determine the actual statistical parameters of rainy and dry periods within each categorization. Confusion has been expressed in the open literature that the SPI is based on statistical standardization operation, which is In fact, SPI is dependent on a completely invalid.statistical standardization procedure. In fact, the SPI relies on a probalistic standardization procedure that preserves the original unity of the hydro-meteorological variable after the probabilistic transformation to a normal (Gaussian) PDF with zero mean and unit standard deviation. After describing the APIs according to the classical SPI approach, its advanteges and disadvantages are presented comparatively. Long term application of the methodology is presented for hydro-meteorological time series from Romania, United States and Türkiye. • The paper is concerned with the application of innovative “Actual Precipittion Index” (API) method to the long term hydro-meteorological time series from Romania, United States and Turkey. • The classical “Spatial Precipitation Index” (SPI based approch cannot reveal the statistical parameters about actual durations of wet and dry spells, their means, standard deviations and possible serial correlation coefficients. • After explaining the API over the classical SPI approach, the pros and cons are presented in a comparative way. • This paper focuses on the main suggestion that a distinction should be considered in any classifications of wet, dry and drought index.