MULTI-TIMESCALE ASSESSMENT OF METEOROLOGICAL DROUGHT USING SPI AND SPEI IN ÇANAKKALE, TÜRKİYE

20TH INTERNATIONAL İSTANBUL SCIENTIFIC RESEARCH CONGRESS, İstanbul, Türkiye, 28 - 30 Eylül 2026, ss.1323-1332, (Tam Metin Bildiri)

Drought assessments based only on precipitation may underrepresent changes in atmospheric

evaporative demand, particularly in Mediterranean-climate regions experiencing long-term warming.

This study compared the Standardized Precipitation Index (SPI) with a non-parametric implementation

of the Standardized Precipitation Evapotranspiration Index (SPEI) to characterize meteorological

drought in Çanakkale, northwestern Türkiye, during 1929-2025. Monthly precipitation and mean

temperature records from Turkish State Meteorological Service station 17112 were analyzed at 1-, 3-,

6-, and 12-month accumulation periods. SPI was fitted using a gamma distribution with correction for

zero precipitation. SPEI was derived from the climatic water balance between precipitation and Blaney-

Criddle reference evapotranspiration and standardized separately for each calendar month using an

empirical cumulative distribution. Drought was defined at index values <= -1.0; frequency, duration,

severity, and intensity were quantified, and long-term tendencies were tested using the Mann-Kendall

test and Sen's slope. Overall drought-month frequencies were similar for SPI (15.06-16.48%) and SPEI

(15.46-15.61%), and corresponding series were strongly correlated (r = 0.946-0.957). Nevertheless, their

long-term behavior differed. Annual mean SPI showed no significant trend, whereas annual mean SPEI

decreased significantly at 1-, 3-, and 6-month scales. At the 12-month scale, drought frequency increased

from 10.32% in 1929-1958 to 25.83% in 1996-2025 for SPEI, compared with 12.89% to 19.72% for

SPI. The fully observed 2024-2025 event was also longer and more severe in SPEI at several timescales.

The results show that precipitation and evaporative-demand information are complementary and that

including the atmospheric-demand component can reveal recent drought intensification that is less

apparent in precipitation-only monitoring.