Spatiotemporal Trend Analysis of Reference Evapotranspiration in the Western Half of Iran during 1996–2023

Document Type : Research Paper

Authors

1 PhD student in Agricultural Meteorology, Bu-Ali Sina University, Hamedan, Iran

2 Water science and engineering department,, agriculture faculty, Bu-Ali Sina University, Hamedan,Iran

3 Assistant Professor, Department of Water Science and Engineering, Bu-Ali Sina University

10.22059/ijswr.2025.398578.669979

Abstract

Surface energy balance and water balance are determined by evapotranspiration-, a crucial part of the hydrological cycle. Using the FAO56 Penman–Monteith method, this study examines the spatiotemporal patterns of reference evapotranspiration (ET0) in Iran's semi-western region (including the western half and parts of the center) over a long period (1996–2023) and a recent decade. Trends were evaluated using Sen's slope estimator and Mann–Kendall nonparametric test. It is found that hot, arid regions in the south and southwest have the highest ET0, while humid northern and western regions have the lowest. There has been a significant increase in ET0 trends at over 70% of stations, with Kashan in the center having the largest slope at 0.20mm per year, while Ardabil and Rasht had the smallest slopes at 0.02–0.03mm per year. In all seasons, the majority of stations exhibit growing ET0 trends, with summer (78% of stations) and winter (64% of stations), and summer slopes of 0.49 mm (Kashan) being the highest. There was, however, a negative seasonal trend in ET0 in autumn only in Shiraz (-0.04 mm). Compared with a baseline period, the annual mean ET0 has increased in recent decades, with the largest increases occurring in central Iran (78%), the Zagros Mountains (48%), and the Lake Urmia basin. In light of the fact that ET0 is expected to increase in response to climate change, it is important for water resources management and agricultural planning to understand how these changes may affect the hydrological cycle.

Keywords

Main Subjects

Introduction

As the link between the water and surface energy cycles, evapotranspiration is a crucial component of the hydrological cycle. As a result of global warming, evapotranspiration has been affected significantly, accelerating the hydrological cycle, particularly in semiarid and arid areas. Consequently, understanding the changes in evapotranspiration is essential and plays a critical role in studies related to climate change impacts, hydrology, and agriculture. Therefore, this study examines spatiotemporal changes in reference evapotranspiration (ET0) across semi-western Iran (covering the western half and parts of the center) over long periods of time (1996-2023), comparing its changes with the recent period (2011-2023).

Methodology

The aim of this research was to examine spatiotemporal changes in reference evapotranspiration (ET0) in the semi-western region of Iran. Using FAO-56's Penman-Monteith method, ET0 was calculated. Daily meteorological data were gathered from 25 weather stations in semi-western and central parts of the region for the calculation of monthly and annual ET0 values, including temperature, relative humidity, wind speed, and sunshine hour. The non-parametric Mann–Kendall test was used to determine significance (at two confidence levels: 99% and 95%), and Sen's slope estimator was used to measure slope of the trend. As part of the assessment of the effects of climate change on evapotranspiration dynamics, spatial analyses were conducted at different time scales—seasonal (spring, summer, autumn, and winter) and annual—to compare spatial heterogeneity and temporal changes between long-term means and those of recent decades.

Results and Discussion

 A long-term annual and seasonal mean of ET0 (1996-2023) indicates an increase in evapotranspiration from north to south of the study area. The southern regions exhibit the highest ET0, with a peak of 2419 mm in summer, whereas the humid coastal areas along the Caspian Sea and the northern highlands show the lowest levels, with less than 150 mm in winter. The varying topography of the Zagros Mountains, the Central Iranian Desert (Kavir), and the Khuzestan Plain likely contribute to these differences. Moreover, the high humidity and lower radiation levels in the northern coastal regions significantly contribute to the reduction of ET0. Also, the monthly ET0 trend for most stations (72% of stations) shows a positive trend in most months. On an annual basis, 70% of the stations studied experienced a significant upward trend, achieving a high confidence level of 99%. The greatest proportion of stations showing a significant increase occurred in summer (71%), followed by winter (64%), and then spring (60%). There was a significant increasing trend in Kashan (central Iran) of 0.49 mm during summer. In contrast, the Shiraz station exhibited a significant negative trend in ET0, with a decrease of 0.04 mm in autumn. This decline may be attributed to increased cloud cover and reduced energy available for evapotranspiration during that season. Overall, the temporal analysis of the trend lines indicates that the increasing trends demonstrate a steeper slope compared to the decreasing trends.

There has been a significant increase in the annual average ET0 in recent years, specifically from 2011 to 2023, compared to the baseline period of 1996 to 2010. The most notable increases have occurred in central regions, which saw an increase of 78%, and in northwest regions, which experienced a rise of 48%. In contrast, the southwestern coast registered the smallest increase at just 10%. The effects of climate change may lead to greater temperature rises in higher latitudes, potentially explaining the increase in ET0 observed in the highlands. On the southwestern coast, however, the high humidity has moderated the impact of rising temperatures, resulting in only a slight change in ET0.

Conclusion

The research confirms a marked and widespread acceleration in reference evapotranspiration across semi-western Iran during the period 1996–2023. This trend is most pronounced in the central and higher elevation northwestern regions, particularly in the most recent decade. The observed increase in atmospheric water demand poses a substantial threat to regional water resources, underscoring the imperative for prompt management strategies and the implementation of advanced irrigation practices.

Author Contributions

  1. Kh:Data preparation, Software, Formal Analysis, Validation, and writing-original draft preparation. M. Kh: Methodology, Conceptualization, Results Interpretation, Writing-review and editing, Final report review. Supervision. V. V: Writing-review and editing.

Data Availability Statements

Data available on request from the authors.

 

Acknowledgment

The authors would like to express their sincere gratitude to the Iran Meteorological Organization (IRIMO) for providing the meteorological data used in this study.

Ethical Consideration

The authors avoided data falsification, fabrication, plagiarism, and misconduct.

Conflict of Interest

The author declares no conflict of interest.

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Iranian Journal of Soil and Water Research
Volume 56, Issue 11
January 2026
Pages 2969-2987

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