Impact of Climate Change on Reference Crop Evapotranspiration Trends in the west rejoin of Iran.

A better understanding of trends in reference crop evapotranspiration (ET0) is crucial, in scientific management of water resources, in arid and semiarid regions. According to the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC), the global warming trend, which is mainly caused by the increasing amount of greenhouse gas emissions, will be continued. Climate change is known as a global environmental challenge facing humanity with implications for food production, natural ecosystem, and fresh water supply. The expected climate change is thought to have a severe impact on different elements of natural systems such as agriculture, hydrology, and ecosystem. reference crop evapotranspiration is one of the main elements of hydrological cycle affected by climate change. reference crop evapotranspiration can be considered as a measure of atmospheric evaporative demand. It is independent of crop type, crop development, and management practices. It provides a measure of the integrated effect of several meteorological parameters such as radiation, wind speed, air temperature, and humidity. reference crop evapotranspiration variability in a single station is affected by these climatic parameters. Estimating reference crop evapotranspiration is one of the important steps for calculating crop water requirements that has a special economic importance in the rationalization of water consumption in the agricultural field under current and future climate conditions. Moreover, this parameter plays an important role in the energy balance of basin ecosystems. Several hydrological processes, such as soil moisture dynamics, groundwater recharge, and runoff generation affected by ET0 fluctuation. 

The data of the 18 synoptic stations located in the west and Northwest of Iran were obtained from the Islamic Republic of Iran Meteorological Organization (IRIMO). In this study, the stations which had at least 20 years of daily data (up to 2016) were chosen for analysis. The gathered data including the meteorological parameters namely maximum air temperature (Tmax), minimum air temperature (Tmin), mean air temperature (Tmean), wind speed in 10 m height (U), maximum relative humidity (RHmax), minimum relative humidity (RHmin), sunshine hours duration (n), and actual vapor pressure (ea). Each of the mentioned data was checked for quality, separately. These data were used to estimate the daily values of ET0 in selected stations using the FAO-56 PM method. The trends of ET0 were detected by using the Mann–Kendall test. The slopes of the trend lines were computed by using the Sen’s slope estimator. And, to investigate the spatial and temporal variability the IDW interpolation method was used

The results showed significant increasing as well as decreasing trends in the annual and monthly ET0. Although, the increasing trends in ET0 was more pronounced than the decreasing trends. In the monthly scale, during the warmer months of the year, the observed increasing trends were more than the decreasing trends of the ET0. In the annual scale, the stronger positive trend in ET0 magnitude was found at Kermanshah stations (Z=5.79), and the strongest negative trend was found at Khodabandeh station (Z=-1.78). Also, in the monthly scale and in the warm season, the strongest positive trend magnitude was found in August at Kermanshah station, (Z= 5.43), and the monthly strongest negative trend magnitude was found in August at Khodabandeh station, (Z=-3.47). In general, it is possible to conclude that, in the recent decades the required water of the plants is increased in the studied area.

Reference crop evapotranspiration is one of the main elements of hydrologic systems. Climate change in different parts of the Earth impacted natural systems in a different way. This study examined the trends of ET0 in 18 weather stations selected in west and northwest of Iran. The FAO-56 PM method used to calculate the ET0. The MK method was used to detect the trends in monthly and annual ET0 time series.The Sen’s estimator was used to estimate the magnitude of the trends. Results indicated that most of the monthly ET0 time series had upward trends. In annual time scale, Most of the stations showed increasing annual ET0 trends, which were significant at 10 percent level. Therefore, it can be concluded that in all water-related activities, especially in agriculture, fresh water should be used scientifically. Otherwise, all of the water-dependent activities might be adversely affected in future.