Abstract
Evapotranspiration is one of the most important components of the hydrological cycle. It is essential in all of hydro climatological studies, irrigation and drainage calculations, water balance, and crop water requirements. The aim of this study is to investigate temporal trends of reference evapotranspiration (ETo) in the Northwest part of Iran. For this purpose, the meteorological data from 20 synoptic stations over a 22-year time period (1986–2007) were used. After the calculation of ETo using the Penman-Monteith FAO-56 method, the non-parametric Mann-Kendall test was used to investigate the temporal trends on monthly, seasonal and annual scales. The Sen’s estimator method was used and to calculate the slope of the trend line. The results indicated that the ETo trend was increasing (positive) in various months, except for the Sarab station in December. On a monthly scale, the Maragheh station showed the highest positive slope in August, and the Bijar station showed the highest negative slope in May. 43.34% of the stations showed a significant trend, and 56.66% did not show a significant trend. In seasonal and annual time periods there was not a significant decreasing trend in any of the stations. The analysis of the results shows that in the spring 20%, in the summer 55%, in the fall 70%, in the winter 75%, and on an annual scale 60% of the stations under study showed an increasing trend. For modeling these changes, the spatial distribution of ETo trends (on monthly, seasonal, and annual scales) were mapped in ArcGIS. Final models determine that most stations in this study show an increasing trend on monthly, seasonal and annual scales.
Highlights
Evapotranspiration, following precipitation, is the most important component of the hydrological cycle
This study showed that 53 percent of the stations in different seasons or on an annual scale had a significant trend, and 47 percent were insignificant
The analysis of the results shows that in the Spring 20%, in the Summer 55%, in the Fall 70%, in the Winter 75%, and on an annual scale 60% of the stations under study showed an increasing trend, The spatial scattering of the stations having a significant trend, separated annually and seasonally, are shown in Figures 4 and 5
Summary
Evapotranspiration, following precipitation, is the most important component of the hydrological cycle. It is one of the main components of water balance, and proper irrigation scheduling programs in any location. It is essential to have sufficient information about the changes in evapotranspiration for accurate planning of agricultural irrigation and water resource management in any region. The issue of climate change and its impacts has created many challenges in international communities. Such that all nations attempt to determine the change in the components of climate and its process and direction. In addition to elements such as temperature and precipitation, which are often studied, recently other elements such as evapotranspiration have been of interest to researchers
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