Analysis of the spatial distribution, trend, and persistence of meteorological and hydrological drought in the western region of turkey

Using monthly total precipitation and monthly average flow data from 13 meteorological observation stations and 8 stream observation stations spread across seven basins in Turkey within the Mediterranean Basin, this study sought to identify meteorological and hydrological droughts at various time scales (3, 6, and 12 months) and to uncover the relationship between these drought types. Drought intensities were calculated using the Standard Precipitation Index (SPI) and the Standard Streamflow Index (SSI). Drought trends were evaluated using the Mann-Kendall Rank Correlation Test (MK). To ascertain whether the trends possess long-term memory, the Hurst coefficient (H) and rescaled interval analysis (R/S) were employed. The analyses revealed that, in terms of moderate droughts, meteorological droughts in the study area were more severe during short time periods (3–6 months), whereas extreme and very extreme droughts were more noticeable during 12-month time scales. The analyses found that extremely severe hydrological droughts dominated the Büyük Menderes River-Aydın Bridge and Gediz River-Manisa stations in the study area for 3, 6, and 12 months. In the first period of the 6-month time scale (October-March) and the 12-month time scale, the spread of meteorological droughts into hydrological droughts happened with a one-year lag; in the other periods, the spread was concentrated more in lags 2 and 3. Drought trends are getting worse, according to trend analyses for both types. Moreover, the fact that long-term memory exists implies that these patterns will persist in the future in their current configuration. The Regional Average Mann-Kendall Rank Correlation Test (RAC) was used to evaluate the regional trends in droughts after station-based analyses. For the 3, 6, and 12 months in the study area, the RAC results (Z) showed statistically significant declining trends in drought intensities ( Z= -2.534, -2.894, -2.621) related to hydrological droughts. Values of Z= -0.635 for the 3-month first period (October-December), -0.103 for the second period (January-March), and Z= -0.402 for the 6-month first period (October-March) indicated statistically insignificant declining trends in terms of meteorological droughts.