Seasonal Analysis of Reference Evapotranspiration and Its Sensitivity to Meteorological Elements in IRAN

IntroductionEvapotranspiration is the combination of two separate processes, soil moisture evaporation, and plant transpiration, which amount depends on various meteorological elements. Therefore, identifying the effective factors and the amount of their impact on reference evapotranspiration (ET0) is important. This component plays an important role in various agricultural studies, including the design of irrigation and drainage systems, reservoir design, and irrigation planning (Ahmadyan et al., 2023). Accurate estimates of evaporation and transpiration play an important role in studies such as global climate change, and environmental evolution, and in various scientific fields such as hydrology, agriculture, forest and pasture management, and water resources management (Kazemi, 2020). Materials and MethodsThe research was conducted in Iran, and the data analyzed encompass various meteorological parameters, including maximum, average, and minimum temperatures, average relative humidity, wind speed, and sunshine hours. These data were collected on a daily basis from 40 synoptic stations across the country. The dataset spans from 1976 to 2020 and was sourced from the Meteorological Organization of the country (IMO, 2022).The research employed the FAO Penman-Monteith method, specifically the 56th version, to estimate seasonal ET0 (evapotranspiration) values.In this research, for statistical evaluations of ET0 and revealing the trend of time series on a seasonal scale, the non-parametric Mann-Kendall (M-K) test; (Kendall, 1948; Mann, 1945) was used. To identify the changing trend of the ET0 time series, the ITA method was used on a seasonal scale. Four meteorological stations and the 45-year time scale (1976-2020) used in the current research, it had a better performance than other interpolation methods, which was used as the superior method. To understand the possible changes of one or more meteorological variables in ET0, the sensitivity of Reference Evapotranspiration to six meteorological variables (relative humidity, hours of sunshine, average temperature, maximum temperature, minimum temperature, and wind speed) was estimated. For this purpose, Sobol's method (Sobol, 1993). Sensitivity analysis was used. Results and DiscussionAccording to the ET0 survey results, the highest amount of ET0 was observed in the spring season in the south and south-eastern parts, and the highest average value was 1050 mm/year in Zabul station. The increase of ET0 in these areas can be due to the sun's radiation and more warming of the earth's surface in the southern latitudes of the country. In summer due to the length of the day and higher temperature, we saw an increase in ET0, especially in the southern and southeastern regions of the country. In autumn, due to the decrease in the length of the day and the decrease in temperature, the amount of ET0 has also decreased significantly in the northern parts of the country. In winter, with a decrease in temperature and an increase in relative humidity, which is more noticeable in northern than southern regions.In the summer season, all stations generally showed an increasing trend in ET0. In most of the stations, the significance level was 5% and it did not follow a specific pattern. In the autumn season, an increasing trend of ET0 was observed at a significant level of 5% in Khoy and Saqez stations, and a significant decreasing trend was observed in Qazvin and Shiraz stations. In the winter season, in the western and northwestern regions, all study stations showed an increasing trend of ET0. Finally, the overall results indicate that there is a significant increasing trend of ET0 during the summer in Iran. The graphical results of the ET0 trend by the four seasons on a scale of 44 showed that, in general, there was an increasing trend in ET0 in both high and low areas in all seasons. The values of meteorological variables have been changed by the Sobol method in the range of 40% to investigate the effect of meteorological elements on ET0 in different seasons of the year. The ranking of the sensitivity coefficient of the most effective meteorological parameter on the increase of the seasonal ET0 using Sobol's method showed that, in general, in the spring season, the minimum temperature had the greatest effect on the reference evaporation and transpiration rate. Also, the ratings obtained in the summer season indicate that wind speed has the greatest effect on the ET0 amount. In the autumn season, wind speed is still the first rank in affecting the rate of evaporation and transpiration. Finally, in the winter, the maximum temperature is the most important influencing factor among the other meteorological parameters. ConclusionAccording to the results, the amount of ET0 was increasing and it has been noteworthy in the eastern half of Iran in recent years. The trend of changes in ET0 showed that most stations had a positive value. The ET0 seasonal time series analysis with the ITA method indicated that in Kerman station; ET0 increased in all seasons and these results were at Bandar Anzali station. It was also observed that the seasonal trend of ET0 was increasing. The results of the sensitivity analysis graphs showed that relative humidity generally had a negative effect, and the other parameters indicates a positive effect in increasing the ET0. Also, the results explained that in spring, summer, autumn, and winter, meteorological variables of minimum temperature, wind speed, and maximum temperature played a greater role in increasing ET0. The findings of the present research and the results of the ranking of the sensitivity of factors affecting the ET0 rate showed that in each period, different conditions prevail in terms of the influence of meteorological elements on the ET0 rate.