Evaluation of monthly reference evapotranspiration in Iran based on the output of CORDEX-MNA project downscaled dynamic models

Document Type : Full length article


1 Assistant Professor, Department of Humanities and Social Sciences, Farhangian University, Tehran

2 Hamzeh Ahmadi, Ph.D. in Climatology, Hakim Sabzevari University and a Trainee of Farhangian University


Extended Abstract


Evapotranspiration as one of the important meteorological variables and the relation between the land surface and the atmosphere, is important to the study of drought and the allocation of water resources. The relationship of this climatic variable with land indices as well as water supply planning reveals its importance. Due to the high relationship between this variable and air temperature, changes in air temperature patterns in global warming and climate change will have a high impact on its behavior and distribution. The relationship and entanglement of this hydro-climatic variable with climate change, especially temperature, has caused that with any fluctuation and change in the behavior of temperature patterns, its value and distribution will also change. Iran as a country located in the arid areas of the planet earth; is currently heavily involved in water stress. This water stress will be exacerbated by climate change, rising temperatures and increased evapotranspiration. Examining the impacts of climate change can be a roadmap for environmental management in this area. The aim of the present study is to reveal the behavior of this climatic parameter in the future period compared to the historical period (1980-2005) and its spatial distribution based on the data network.

Methodology (Materials and methods or Data and methods)

In the present study, using the output of downscaled dynamic models of the CORDEX-MNA project for North Africa and the Middle -East under (RCP8.5 and RCP4.5) scenarios. For the historical period (1980 – 2005), reference evapotranspiration was estimated by the Penman-Monteith, FAO method. The NOAA-GFDL-GFDL-ESM2M regional climate models (RCMs) as the dynamic model output projection was used for the future to mid-century (2025-2050).

In this study, RCP4.5 & RCP8.5 representative concentration pathway scenarios were selected, as these two scenarios are used to further investigate climate change vulnerabilities and subsequent climate change responses. The CORDEX-MNA data with 0.22 spatial resolution, RCA4 model for RCM, were used. For the future period, the ETo value is obtained directly from the output of the CORDEX-MNA coordinated project models. In fact, the reference evapotranspiration value is part of the simulation of these models. Statistical criteria are used for the correction of the models to show the similarity between the observed and modeled data in the form of statistical values.

Results and Discussion

The results showed that in the future period, based on CORDEX-MNA coordinated project simulations for North Africa and the Middle-East, downscaled RCM dynamics models; the amount of reference evapotranspiration as one of the most important components of the hydrological cycle will increase compared to the historical period (1980-2005). In terms of temporal variation, the highest changes will occur in mid-winter to early autumn. Under the RCP scenario, the ETo will increase to 34 mm in hot months such as August. Under the mid-term scenario, the ETo value will increase compared to the historical period, but this increasing will be less than the upper limit scenario. Spatially, the main foci of high reference evapotranspiration are located in the central areas around Kavir and Lut plains, Jazmourian areas and the southern half of Iran. The results showed that in the context of climate change and based on the downscaled data of the CORDEX -MNA project; the range of high potential evapotranspiration in Iran will increase. In fact, high latitudes will experience an increase in the amount of ETo due to changes in air temperature and precipitation patterns and increasing air temperature trends. These conditions will be an alarm for Iran with arid and semi-arid conditions. Any change in the amount of evapotranspiration causes drought in the environment, followed by water needs and irrigation frequency for the agricultural sector as the most important water-consuming sector in Iran. Therefore, it is important to pay serious attention to water resources management programs.


Among the models used, the output of the regional model; NOAA-GFDL-GFDL-ESM2M has more optimal conditions in simulating the effects of climate change in Iran. The ETo spatial pattern is a function of location components throughout the year. These conditions are well evident in the distribution of potential evapotranspiration. The amount of ETo will increase from west to east and from north to south of the country based on the spatial pattern obtained in the historical (1980-2005) and the future period from 2025 until 2050. The southern and central regions of Iran are the focus of areas with high ETo. The role of altitude and latitude factors in determining this spatial pattern is obvious. Comparison of ETo historical-observational period with the future decads to 2050, shows a significant increase in the amount of potential evapotranspiration in Iran. Under RCP8.5 from March to September ETo will reach the highest level. In areas with high ETo in Iran, the average amount of ETo will increase by an average of 11 mm from April to June and in the warm months by an average of 17 mm. Under RCP4.5 scenario, these conditions are also observed to be incremental with a lower value. In the cool and cold months, in the high regions, the amount of minor and small changes and in some areas without change is observed. Significant increasing in reference evapotranspiration in August as one of the hottest months of the year in Iran is significant; because at this time of year, the demand for water consumption in all sectors; in particular, the agricultural sector is reaching its highest level, which necessitates attention to water resources management. In general, reference evapotranspiration as one of the hydro-climatic components, with any change in air temperature, will face increasing conditions that result in this increase, imposing more drought on the environment and thus increasing the water demand. Especially in the agricultural sector. These conditions are important for the geography of Iran with its fragile climate. Therefore, the land of Iran is currently facing tensions and instability in the situation of water resources, these conditions will intensify in the coming decades under the conditions of climate change. The need to pay attention to risk management and increase resilience in the face of climate change due to global warming can be considered a roadmap in this area.

Keywords: Reference Evapotranspiration, Climate change, CORDEX, Penman-Monteith FAO, RCP.


Main Subjects

Articles in Press, Accepted Manuscript
Available Online from 21 March 2022
  • Receive Date: 27 October 2021
  • Revise Date: 17 February 2022
  • Accept Date: 21 March 2022
  • First Publish Date: 21 March 2022