The effect of theoretical probability distribution on the standardized precipitation evapotranspiration index (SPEI) calculation algorithm for the Euphrates-Tigris Basin

One of the largest issues endangering our planet today is global warming, which shows up in different ways in different parts of the world. Since our country is located in the semi-arid climate zone, the threat of drought is felt more, especially in the Central Anatolia and Southeastern Anatolia regions. Low annual average rainfall and inadequate water resources in these regions put great pressure on the agricultural sector. The Standardized Precipitation Evapotranspiration Index (SPEI) was used in this study to analyze the drought in the Euphrates-Tigris Basin. For this purpose, the climate parameters (precipitation, wind speed, maximum and minimum air temperature, sunshine durations and relative humidity) required for the study were provided by 19 meteorological stations operated by the General Directorate of State Meteorology (MGM). The Penman-Monteith and Thornthwaite methods were used to estimate the necessary plant water consumption in the SPEI calculation. SPEI calculation was made according to the theoretical probability distributions that best fit the difference value series created depending on rainfall and plant water consumption for drought analysis, as well as the traditional method (Generalized Log-Logistic). Moments, maximum likelihood, and L-moments methods were applied to determine the parameters of the distributions under consideration. The most suitable theoretical probability distributions for different time periods (3-, 6-, 9-, 12-, and 24-h) of the stations were mostly Generalized Logistic (GLOG), Weibull, and Generalized Extreme Value (GEV). Therefore, estimating SPEI based on a single theoretical distribution poses a risk in terms of reliable drought analysis. The findings led to the following conclusions: Adıyaman, Ağrı, Bingöl, Bitlis, Hakkari, and Şırnak stations all showed transitions between GEV and GLOG distributions. The Penman-Monteith method was unable to determine a suitable distribution at the Diyarbakır, Kilis, and Malatya stations, but Weibull and GLOG distributions were found to be appropriate with the Thornthwaite method. At Erzincan station, neither approach was able to produce an appropriate distribution. Based on these findings, it was concluded that the time scale and the chosen theoretical probability distribution are important in drought analysis with SPEI. In addition, the methods used in parameter estimation of theoretical probability distributions were found to be important in drought analysis. An essential foundation for assessing the Euphrates-Tigris Basin's drought risks and water resource sustainability has been established by this study.