Recent Rainfall Trends in the Aegean Region of Turkey

Omer Levend Asikoglu Civil Engineering Department, Ege University, Izmir, Turkey

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Dogangun Ciftlik State Hydraulic Works, Aydin, Turkey

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Abstract

This study is intended to determine potential trends in annual rainfall series with the parametric Student’s t test and the nonparametric Mann–Kendall, Spearman’s rho, and Sneyers tests. The study includes a trend analysis of annual rainfall data from 47 rain gauges, mostly located at rural sites, in the Aegean region of Turkey. The chi-square and Kolmogorov–Smirnov tests showed that the null hypothesis of normality for the majority of the data (45 out of 47 stations) is acceptable. Moreover, the serial independence assumption, based on the lag-1 sample autocorrelations, is rejected in 14 datasets. The parametric Student’s t test detected significant downward trends at 15 rain gauge stations; 14 of them were also confirmed by the three nonparametric tests and by the trend-free version of the Mann–Kendall test. The results of the Sneyers test revealed that the approximate start years of significant downward trends were in the early 1970s and, sometimes, in the early 1980s. Moreover, the testing found that the normalized slopes of linear downward trends were significantly dependent on the station’s longitude, which meant that the farther the station was located from the coast, the smaller the decreasing trend in annual rainfall was. Additional studies carried out on the normalized regional data and on the 5-yr running means showed that a considerable portion of the detected downward trends were mainly due to interactions of the particular start and end times of a large number of stations with interdecadal fluctuations and the dry conditions over the Mediterranean region during the last 25–30 years.

Corresponding author address: O. L. Asikoglu, Civil Engineering Department, Ege University, Bornova, 35100 Izmir, Turkey. E-mail: omer.asikoglu@ege.edu.tr

Abstract

This study is intended to determine potential trends in annual rainfall series with the parametric Student’s t test and the nonparametric Mann–Kendall, Spearman’s rho, and Sneyers tests. The study includes a trend analysis of annual rainfall data from 47 rain gauges, mostly located at rural sites, in the Aegean region of Turkey. The chi-square and Kolmogorov–Smirnov tests showed that the null hypothesis of normality for the majority of the data (45 out of 47 stations) is acceptable. Moreover, the serial independence assumption, based on the lag-1 sample autocorrelations, is rejected in 14 datasets. The parametric Student’s t test detected significant downward trends at 15 rain gauge stations; 14 of them were also confirmed by the three nonparametric tests and by the trend-free version of the Mann–Kendall test. The results of the Sneyers test revealed that the approximate start years of significant downward trends were in the early 1970s and, sometimes, in the early 1980s. Moreover, the testing found that the normalized slopes of linear downward trends were significantly dependent on the station’s longitude, which meant that the farther the station was located from the coast, the smaller the decreasing trend in annual rainfall was. Additional studies carried out on the normalized regional data and on the 5-yr running means showed that a considerable portion of the detected downward trends were mainly due to interactions of the particular start and end times of a large number of stations with interdecadal fluctuations and the dry conditions over the Mediterranean region during the last 25–30 years.

Corresponding author address: O. L. Asikoglu, Civil Engineering Department, Ege University, Bornova, 35100 Izmir, Turkey. E-mail: omer.asikoglu@ege.edu.tr
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