The Trends of the Standardized Precipitation and Evapotranspiration Index (SPEI) in Turkey

This paper examines drought trends in Turkey due to climate change. Therefore, forecasting of periods of drought, which is one of the major negative side effects of climate change, is urgent. The authors use data on drought indicators according to the SPEI (Standardized Precipitation and Evapotranspiration Index) for the period from 1901 to 2015 obtained at intervals of 1, 3, 6, and 12 months. The data have been processed by several statistical tests: the Mann-Kendall test to determine the direction and significance of the trend, the Theil-Sen estimator to assess the trend slope coefficient, and the Pettitt test to determine the most probable year of change. The period under study is divided into two intervals (1901-1981 and 1982-2015), which are compared. The GIS program is used to isolate spatial characteristics from the results we obtained. It is concluded that Turkey is divided into two main zones of drought trends. The largest one allocated to the northern regions is characterized by an increase in moisture, and a decrease in moisture is observed in the southern regions. The trend towards dry periods was more significant until 1981, but the situation has changed since 1981 due to the trend towards excess moisture, except for southeastern Turkey, where a trend towards droughts persists.