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Trends in reference evapotranspiration in Turkey: 1975–2006

This study examines the trends in reference evapotranspiration (ETo) in Turkey by analysing data from 77 weather stations for a 32‐year period (1975–2006). ETo values were calculated using the Penman–Monteith method using air temperature, wind speed, relative humidity, and sunshine hours data. Trends in annual and monthly ETo were determined using the Mann–Kendall trend test with the trend‐free prewhitening procedure. The magnitude of trends was estimated by calculating the Sen's slope. The collective or field significance of the trends was evaluated using Walker test. The possible causes of changes in ETo were discussed by analysing the trends in air temperature, wind speed, relative humidity, and solar radiation data collected at the same stations. The implications of ETo trends for crop water requirements were evaluated. The analyses showed that the majority of stations (88%) in Turkey had annual ETo between 750 and 1200 mm during the 32‐year period and ETo decreased gradually from south to north. From 1975 to 2006, 58% of stations had upward trends in annual ETo. Upward trends were statistically significant at the 0.05 level for 32% of stations. The rates of changes in annual ETo were on average 1.20 mm year−2. The trends detected in monthly ETo were mostly upward with an average magnitude between −0.01 and 0.14 mm month year−1. Trends detected at the annual timescale and for the majority of the months provided the field significance at the 0.05 level. Analysis of other climatic data showed that upward trends in air temperatures, downward trends in wind speeds, and downward trends in relative humidity were widespread over Turkey for the same time period. Changes in these three parameters could explain the majority of the changes in ETo rates. The ETo changes affect crop water requirements and increase the demand for irrigation.