The objectives of this study were: (1) to submit comprehensive information about the distinctions among pan coefficient (Kp), crop coefficient (Kc), and crop-pan coefficient (Kcp); (2) to reveal the importance of using pan evaporation for irrigation scheduling. The amount of water allocated for agricultural production has been decreasing due to rapid population growth, industrialization and contamination. Therefore, to obtain maximum yield and avoid excessive water application, the water requirements of the cultivated plants should be accurately determined, and irrigation water should be applied according to the needs of the plants. Crop water consumption (ETc) varies from region to region depending on crop type, stage of growth, soil, and climate conditions. Different evapotranspiration rates may be observed even in different parts of the same region. Considering global warming and climate change in recent years, it is clear that the predicted ET values of a year can not be used safely for upcoming years. Research studies have revealed a linear correlation between crop water consumption and the amount of water evaporated from open water surface. Therefore, irrigation scheduling through using the combination of proper crop-pan coefficient and pan evaporation (Class-A pan) is very important due to both ease of adjustment to the climate changes and ease of application. Even though many studies have been conducted on this issue, there is still need to collective and detailed information about use of evaporation pan for irrigation scheduling and above mentioned coefficients.