Sensitivity of Reference Evapotranspiration to Global Warming in the Caspian Region, North of Iran

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The goal of this study was to estimate the sensitivity of reference evapotranspiration (ET0) to changes in meteorological parameters in the Caspian region. Long-term meteorological data (1961-2008) were obtained from three synoptic meteorological stations. The region was primarily classified into three climatic zones, based on De Martonne climate classification. The Penman–Monteith equation was applied to calculate the ET0 and the sensitivity of ET0was studied in terms of changes in air temperature, solar radiation, wind speed and vapor pressure within a possible range of ±20% from the normal long-term meteorological parameters. The results indicated that the Caspian region could be classified into the three climatic classes: Mediterranean, humid, and extremely humid. During the last half century, meteorological parameters and, in particular, temperature of the Caspian region has changed dramatically and the ET0 has increased as a result of climate change. The study suggests that an increase in temperature by 20% (approximately 3.3 ºC) will result in ET0 demand increase by 16%. Changes in vapor pressure (20%) represented the highest inverse effect on annual ET0throughout the Caspian region (-19% in the Mediterranean, and -30% in other climates). The Mediterranean and extremely humid climates of the Caspian region showed an increase of 2 and 5% of the total ET0, respectively, in response to 20% change in the wind speed. It is quite essential for managers to take into consideration the expected change in evapotranspiration owing to global warming while planning for development of artificial and natural ecosystems in the Caspian region.