The paper by C. H. B. Priestley and R. J. Taylor discusses the assessment of surface heat flux and evaporation using large-scale parameters. The authors emphasize that the large-scale parameterization of surface fluxes, such as sensible and latent heat, should be expressed in terms of energetic considerations over land, while bulk aerodynamic formulas are more suitable for the sea. They propose a general framework for understanding the relationship between sensible and latent heat fluxes, particularly over saturated land and open water surfaces. Key findings include: 1. **Saturated Land Surfaces**: The evaporation rate is found to follow a linear decrease after a certain amount of water is evaporated, eventually reaching zero. This rate is influenced by the surface temperature and the conductive capacity of the soil. 2. **Open Water Surfaces**: The fluxes over the sea are primarily governed by net radiation and the thermal balance between the water and the atmosphere. The authors suggest that the ratio of latent to sensible heat fluxes (α) can be used to estimate the main space variations in daily latent and sensible heat fluxes. 3. **Data Analysis**: The authors analyze data from various lysimeter and fluxatron measurements to determine the value of α. They find that α is around 1.26 for saturated surfaces, which can be used to estimate potential evaporation rates. 4. **Unsaturated Land Surfaces**: The ratio of actual to potential evaporation rates is found to decrease linearly with increasing accumulated actual evaporation, approaching zero when the potential evaporation rate is about 5 cm more than the initial saturated state. 5. **Conclusion**: The paper emphasizes the importance of an energy approach to understanding heat flux and evaporation over land surfaces, highlighting the need for accurate maps of net radiation and other boundary conditions. The authors also discuss the limitations and potential explanations for the observed values of α, suggesting that it may be influenced by the physical properties of the atmosphere and the surface. Overall, the paper provides a comprehensive framework for the assessment of surface heat flux and evaporation, with practical implications for large-scale dynamical computations and meteorological modeling.The paper by C. H. B. Priestley and R. J. Taylor discusses the assessment of surface heat flux and evaporation using large-scale parameters. The authors emphasize that the large-scale parameterization of surface fluxes, such as sensible and latent heat, should be expressed in terms of energetic considerations over land, while bulk aerodynamic formulas are more suitable for the sea. They propose a general framework for understanding the relationship between sensible and latent heat fluxes, particularly over saturated land and open water surfaces. Key findings include: 1. **Saturated Land Surfaces**: The evaporation rate is found to follow a linear decrease after a certain amount of water is evaporated, eventually reaching zero. This rate is influenced by the surface temperature and the conductive capacity of the soil. 2. **Open Water Surfaces**: The fluxes over the sea are primarily governed by net radiation and the thermal balance between the water and the atmosphere. The authors suggest that the ratio of latent to sensible heat fluxes (α) can be used to estimate the main space variations in daily latent and sensible heat fluxes. 3. **Data Analysis**: The authors analyze data from various lysimeter and fluxatron measurements to determine the value of α. They find that α is around 1.26 for saturated surfaces, which can be used to estimate potential evaporation rates. 4. **Unsaturated Land Surfaces**: The ratio of actual to potential evaporation rates is found to decrease linearly with increasing accumulated actual evaporation, approaching zero when the potential evaporation rate is about 5 cm more than the initial saturated state. 5. **Conclusion**: The paper emphasizes the importance of an energy approach to understanding heat flux and evaporation over land surfaces, highlighting the need for accurate maps of net radiation and other boundary conditions. The authors also discuss the limitations and potential explanations for the observed values of α, suggesting that it may be influenced by the physical properties of the atmosphere and the surface. Overall, the paper provides a comprehensive framework for the assessment of surface heat flux and evaporation, with practical implications for large-scale dynamical computations and meteorological modeling.