Introduction:
The increasing scarcity of water resources in arid and semi-arid regions, driven by population growth, climate change and extended droughts, highlights the necessity for sustainable agricultural practices. Greenhouse cultivation offers a practical solution for such challenging environments, particularly in countries like Iran, which experience severe water shortages. Developing greenhouse cultivation and implementing smart water management systems are essential strategies for enhancing water-use efficiency. Accurate estimation of crop water requirement in greenhouses is essential for optimizing irrigation schedules and improving productivity. This study reviews various methods for estimating evapotranspiration (ET) in greenhouses, evaluates their effectiveness and provides recommendations. Greenhouse environments differ from open-field conditions, due to controlled factors such as air temperature, humidity and wind speed. In addition to the controlled variables, uncontrollable factors like altitude (which affects atmospheric pressure) and solar radiation can influence the performance of ET estimation methods. Understanding the strengths and limitations of different approaches is essential for developing reliable models tailored to greenhouse conditions.
 Review of Research Studies:
Several methods have been proposed for estimating greenhouse evapotranspiration. Some methods, such as Turc, Jensen-Haise, Hargreaves-Samani and Blaney-Criddle, exhibit poor accuracy and are unsuitable for greenhouse applications. These methods oversimplify complex environmental interactions or rely on limited meteorological data, resulting in significant deviations from actual ET values. Other methods, including FAO-Radiation, Priestley-Taylor and FAO-Penman, provide relatively acceptable results but still show considerable errors under certain conditions. While they incorporate more variables than simpler methods, their reliance on assumptions about energy balance and climatic relationships limits their precision. The Penman-Monteith equation is a methods that widely used and recommended by many researchers. This approach is recognized by the Food and Agriculture Organization (FAO) as the standard for estimating reference evapotranspiration (ET₀) and integrates key meteorological parameters such as air temperature, humidity, wind speed and solar radiation into a comprehensive energy balance framework. Its adaptability to diverse climatic conditions makes it particularly suitable for greenhouse environments where microclimatic factors are precisely controlled. Another promising method is the Stanghellini equation, specifically designed for greenhouse conditions. Although fewer studies have evaluated this approach, initial findings suggest that Stanghellini equation provides highly accurate estimates when applied in technologically advanced greenhouses. In contrast, the FAO-Penman-Monteith variant shows weaker performance, especially in low-tech greenhouse settings, highlighting the importance of aligning methodology selection with greenhouse infrastructure. The diversity in the performance among some of these methods across various regions can be attributed to uncontrollable parameters such as altitude, which affects air pressure. Also, the variations in the amount of the important parameter such as solar radiation, despite the possibility of controlling it in greenhouses, can be important and lead to differ the results among the methods in distinct regions, due to the difference in its control methods. The choice of input parameters in the equations also influences accuracy. Therefore, careful calibration and validation based on local conditions are imperative. Alternative approaches, such as lysimeters and evaporation pans, offer high accuracy. Lysimeters, while providing direct measurements of plant water consumption, are time-consuming and impractical for large-scale applications. Similarly, evaporation pans require precise depth measurements and coefficient adjustments, making them less suitable for automated greenhouse management systems.
 Conclusion:
The literature review showed that no single method can be universally recommended for all greenhouse conditions. However, the findings suggest that the Penman-Monteith, FAO-Penman-Monteith and Stanglini methods are more reliable and exhibit less error in estimating greenhouse evapotranspiration compared with other methods. Their ability to utilize multiple meteorological parameters, and adapt to controlled greenhouse environments has demonstrated greater accuracy than alternative methods. Additionally, the literature review revealed that for low-tech greenhouses, the Penman-Monteith and FAO-Penman-Monteith methods are preferred due to their ease of application; while in technologically advanced greenhouses, the Stanghellini equation is more beneficial, provided that careful calibration is performed. These methods enable smart irrigation systems, facilitating real-time adjustments and optimization of water use efficiency. Consequently, while lysimeters represent the most accurate method for direct measurement of evapotranspiration, their limitations make them unsuitable for widespread use in greenhouses. Similarly, evaporation pans lack the precision required for modern greenhouse management. Therefore, well-calibrated empirical equations such as Penman-Monteith and Stanghellini, can enhance water efficiency in greenhouses. It is recommended that future research concentrate on refining and calibrating these methods through extensive field testing and using technological advances such as sensors to improve data collection and model accuracy.