Assessment of Drought Indices Based on Effective Precipitation: A Case Study from Çanakkale, a Humid Region in Türkiye

This study investigates the influence of different effective precipitation (Pe) estimation methods on drought index performance in a humid region of Türkiye. The standard precipitation index (SPI) and the reconnaissance drought index (RDI) were compared with their effective precipitation-based counterparts, Agricultural Standardized Precipitation Index (aSPI) and Effective Reconnaissance Drought Index (eRDI), using four Pe estimation methods: USBR (U.S. Bureau of Reclamation), USDA-(Simplified and CROPWAT) (U.S. Department of Agriculture), and FAO (Food and Agriculture Organization). Data from three closely located meteorological stations (Çanakkale, Bozcaada, and Gökçeada) were analyzed across multiple time scales (1-, 3-, 6-, 12-month, and annual). Statistical metrics—coefficient of determination (R2), root mean square error (RMSE), and Nash–Sutcliffe efficiency (NSE)—were used to assess the indices, and trend analyses were conducted using the Mann–Kendall and Sen’s Slope tests. The USDA-Simplified method consistently showed the highest accuracy across all stations and time scales (R2 ≈ 0.99; lowest RMSE ≈ 0.09; NSE > 0.95), while the FAO method performed poorly, particularly at the 1-month scale. Drought frequency and severity were found to increase with time scale, contrary to trends observed in arid regions. Trend analysis revealed no significant changes at short time scales, but statistically significantly increasing drought severity was detected in longer scales, especially in Çanakkale, with slopes reaching up to –0.018 per year. The findings highlight the importance of selecting appropriate Pe estimation methods for accurate drought assessment, even in humid climates, and support the use of aSPI and eRDI with the USDA-Simplified method.