Trends in drought analysis research differ temporally and spatially (Burn and Hag Elnur 2002). In this study, different drought values were obtained based on provinces and regions. The results are presented separately and comparatively according to both trend analysis methods (Appendices 1–12).
3.2.1. Results of Mann–Kendall trend analysis
The areal trend conditions of the provinces obtained by the MK tests are shown in Table 3 and Fig. 4.
When the MK test results were spatially evaluated, no significant increasing or decreasing trends were observed in most classes, except for the near normal and moderately dry classes. Specifically, in the Marmara and Aegean Regions, there was no significant trend in any drought classes. A decreasing trend was determined in five provinces in the extremely wet drought class, and they were located in the Eastern Anatolia, Southeastern Anatolia, Black Sea, and Mediterranean Regions. Only the province of Giresun, located in the Black Sea Region, had an increasing trend. Considering the very wet drought class, a decreasing trend was observed in six provinces, and an increasing trend was observed in one. Most of the provinces with a decreasing trend were located in the eastern part of Turkey. Sivas, in the Central Anatolia Region, was the only province with an increasing trend in this drought class. When the drought class moderately wet was examined, a significant decreasing trend was observed in eight provinces, four of which are in the Eastern Anatolia Region. In contrast, a significant increasing trend was detected in only one province, Şırnak, in the Southeast Anatolia Region. When the near normal drought class was evaluated, it was determined that 13 provinces had an increasing trend, and one province had a decreasing trend. Provinces with an increasing trend were concentrated in the inner parts of the country. Tokat, located in the Black Sea Region, was the only province with a decreasing trend. When the moderately dry drought class was examined, a decreasing trend was found in six provinces and an increasing trend was found in three provinces. It is striking that most of the provinces with a decreasing trend also had an increasing trend in the near normal class, which is an upper drought category. All three provinces with an increasing trend were located in the Black Sea Region. Finally, in the very dry drought class, a decreasing trend was determined in the provinces of Elazığ and Malatya in the Eastern Anatolia Region.
3.2.3. Regional comparison of trend analysis results
Drought analysis studies in Turkey are primarily in small-scale areas, such as regions, provinces, or basins, with few studies covering all of Turkey. Among the regional studies, drought analyses have been carried out in all geographical regions except the Marmara Region. The present study found no significant trend in the Marmara Region according to the MK test. However, according to the ITA, especially in Istanbul, there was a decreasing trend in the near normal class and an increasing trend in the moderately wet drought class. Again, in Kırklareli and Tekirdağ, there was a decrease in the moderately dry drought class and an increase in the near normal class. Additionally, a change from near normal to moderately dry was observed in Bilecik province.
Black Sea Region
The study determined that the areas belonging to extremely wet and very wet drought classes in the Black Sea Region were generally near normal and moderately dry classes according to the results of both trend analyses. In Yetmen's (2014) study using the SPI in the central Black Sea Region with data from 1975–2008, Yetmen determined that drought levels increased in the Amasya, Çorum, and Tokat provinces. According to the results of this study, while the trend was decreasing in the near normal class in Amasya, Çorum, and Tokat, there was an increase in the moderately dry class. Similarly, in a study by İnan et al. (2021), in which SPI and Percentage of Normal Index (PNI) values were examined on the monthly average precipitation in the Black Sea Region between 1961–2019, they emphasized that drought increased in the regions with the highest rainfall in Turkey. However, they stated that there was no significant drought trend. On the other hand, Zeybekoglu et al. (2018) evaluated the 24-month SPI values of the Sinop province for different periods between 1930–1981 with the ITA method and found no significant drought trend. Additionally, in this study, similar trend results were obtained for Sinop for each trend analysis method.
Aegean Region
Mersin et al. (2022) stated that drought conditions increased in Denizli, Muğla, and Aydın using SPI, SPEI, PNI, and DPI drought indices between 1973–2020 in the Aegean Region. In this study, while humidity decreased in Denizli, an increase was observed in near normal and moderately dry classes. Likewise, Muğla ranged from extremely wet levels to near normal drought conditions. Aydın, on the other hand, moved from the very wet class to the moderately wet class. In another study, Apak (2009) concluded that drought increased in the Aegean Region. Similarly, in this study, the provinces in the Aegean Region generally ranged from extremely wet, very wet, and moderately wet classes to near normal and moderately dry drought classes. Güner Bacanli (2017) evaluated drought in the Aegean Region according to the MK and Spearman's rho trend tests using the monthly precipitation values between 1960–2013 at 1-, 3-, 6-, 9-, 12-, and 24-month periods. He found no significant trend in the provinces of İzmir, Denizli, Aydın, and Uşak. He also determined no significant trend in all drought classes in the provinces specified according to the MK trend test, while there was a significant trend only in the near normal and moderately wet drought classes in the ITA. Güner Bacanli (2017) also stated in his study that drought increased in Kütahya, Muğla, and Manisa, while there was a decreasing trend in Afyon. In the current study, while there was no significant trend according to the MK test, a decrease was observed in the very wet and moderately wet classes for the Muğla province and an increase in the near normal class according to the ITA. On the contrary, there was a significant trend from the near normal class to the moderately wet class in the Manisa province. Last, this study observed a decrease in the drought class in the Afyon province.
Southeastern Anatolia Region
Dikici (2020) used the Decile Index, SPI, SPEI, and Standardized Runoff Index for 1-, 3-, 6-, 9-, 12-, and 48-month periods between 1970–2016 within the borders of the Asi basin in the Southeastern Anatolia Region. As a result, Dikici determined that the drought in the region has increased in recent years. In similar studies, both Komuscu (1999) calculated 3-, 6-, 12-, and 24-month SPI values between 1940–1990, and Gumus et al. (2021) stated that drought increased in the Şanlıurfa and Kilis provinces. Likewise, while there was no significant trend according to the MK test conducted in our study, according to the results of the ITA test, a transition from humid classes to arid classes is observed in these provinces. According to the results of a similar study conducted by Tonkaz (2006) using 1-, 2-, 3-, 6-, 9-, and 12-month SPI data between 1962–2022 in the Southeastern Anatolia Region, there was an increase in drought conditions in the Siirt and Şanlıurfa provinces.
Mediterranean Region
In the drought analysis study by Karabulut (2015) using 1- and 12-month SPI values from 1975–2010 around the Hatay and Kahramanmaraş provinces, Karabulut emphasized that the frequency of drought increased, especially in the north of the region. In addition, Karabulut (2015) determined a trend toward arid values in the Burdur and Isparta provinces. In this study, while no significant trend was found in the MK test, there was a similar decreasing trend in the moderately wet class according to the ITA test results. In a study by Nazmi et al. (2016) using the 3-, 6-, 12-, and 24-month SPI classification method between 1970–2004, they stated that drought increased in the Antalya province. In this study, there was a decrease in the moderately wet class and an increase in the near normal class for the same province according to the MK test, and an increase was observed in the moderately dry drought class and the near normal class according to the ITA results. In another study conducted by Bacanli and Akşan (2019) using SPEI data from 1-, 3-, 6-, 9-, and 12-month periods between 1970–2018 in the Mediterranean Region, they emphasized that near normal and mild drought classes were stationary. The present study observed an increase in the near normal drought class according to the MK test. In contrast, a larger-scale areal increase in drought was observed in both the northwestern and central parts of the region according to the ITA.
Central Anatolia Region
In Yildiz's (2007) study, in which monthly SPI values between 1956–2004 were used, Yıldız stated that the Central Anatolia Region was under the influence of high-frequency drought. However, in Yeğnidemir's (2005) study using 1-, 3-, 6-, 12-, 24-, and 48-month SPI values from 1953–2003, Yeğnildemir emphasized that the change in drought severity within the region was low. In the current study, according to the MK and ITA tests, the near normal drought class had an increasing trend, while the moderately dry class had a decreasing trend.
Eastern Anatolia Region
In the study conducted by Çeli̇k et al. (2018) in the Eastern Anatolia Region using SPI values from 1967–2017, they stated that while the humidity trend increased in the provinces of Bingöl, Bitlis, Hakkari, and Iğdır, serious drought trends were observed in the provinces of Malatya, Elazığ, Tunceli, Van, and Kars. However, according to the MK and ITA tests in the present study, while there was a decrease in almost all drought classes in most provinces, there was an increase in near normal drought classes. A transition from humid to dry classes was observed only in the Erzurum province.