Trends in pan evaporation and climate variables in Iran
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ORIGINAL PAPER
Trends in pan evaporation and climate variables in Iran Fatemeh Soroush 1 & Farshad Fathian 1
&
Farideh Sadat Hasheminasab Khabisi 2 & Ercan Kahya 3
Received: 24 September 2019 / Accepted: 4 May 2020 # Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract Contrary to expectations that global warming will be accompanied by an increase in terrestrial evaporation, pan evaporation (PE) has decreased in several parts of the world during the last decades. This opposing relationship is known as the pan evaporation paradox phenomenon that can be affected by climate change. For this purpose, this study identifies spatial and temporal trends in PE and its associated climate variables (Cvars) in Iran at seasonal and annual time scales. Hence, PE and Cvars data from 68 meteorological stations distributed over Iran during the period 1987–2016 were selected. The analyses of the temporal and spatial trends in PE, Cvars, and relationships among them were examined using the statistical techniques to identify the causes of PE trends. Results indicated that stations located in the subtropical region (latitude below 32°), which is mostly located in the southern half of Iran, showed significant negative PE trends (about 20–30% of stations) in spite of an increasing trend in temperatures indicating the “pan paradox” phenomenon. However, stations with significant positive PE trends (about 40% of stations), which are mostly located in the northern half of Iran, are placed at latitudes above 32°. Trend analysis of other Cvars indicated significant increases in temperatures, sunshine duration, and wind speed in more than half of the stations on all time scales. In addition, minimum, maximum, and mean temperatures, and sunshine duration were the most dominant variables affecting PE in annual, spring, autumn, and summer, respectively, in Iran.
1 Introduction Evaporation is one of the most important complex nonlinear components in the climate system as well as a nexus of the water, energy, and carbon cycles (Jung et al. 2010). Almost half of the total solar energy adsorbed by land surfaces is consumed to evaporate water, which returns about two-
* Farshad Fathian [email protected] Fatemeh Soroush [email protected] Farideh Sadat Hasheminasab Khabisi [email protected] Ercan Kahya [email protected] 1
Department of Water Science & Engineering, Faculty of Agriculture, Vali-e-Asr University of Rafsanjan, P.O. Box 77188-97111, Rafsanjan, Iran
2
Department of Water Engineering, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran
3
Civil Engineering Department, Water Resources Division, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
thirds of the average 700 mm/year precipitation over the land (Chahine 1992; Wang et al. 2017). Since actual evaporation is difficult to measure directly, measurement of pan evaporation (PE) is commonly used to estimate the atmospheric evaporation in many countries due to its simplicity and low cost (Li et al. 2013). In hydrology, PE
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