The precise determination of evapotranspiration rate is challenging because it is a quantity that is difficult to measure and to parametrize. Direct estimates include the determination of the change of mass of a volume of soil and vegetation that evapotranspirates using lysimeters, or direct measurements of turbulent water vapour fluxes by eddy-covariance systems. Parametrized estimates that make use of the Monin–Obukhov similarity theory use vertical gradient measurements of temperature and moisture at one point, and line or area averages by means of scintillometers operating at high frequency. A relation for the evapotranspiration from well-watered surfaces was initially developed by Penman and later expanded for vegetated surfaces and for heterogeneous croplands. A popular simplified expression was obtained by Priestley and Taylor. The current challenge is to find expressions for the evapotranspiration in non-saturated conditions, which are common in arid and semi-arid climates, and for heterogeneous terrain. In numerical models, the estimated actual evapotranspiration over land is obtained as the result of the explicit representation of the different involved sub-processes taking place in the soil and the canopy, using so-called land-surface models. Usually these mechanisms are described in a simplified manner and rely on a number of adjustable parameters. The improvement of such descriptions relies in the availability of experimental measurements to make the physical models more complete and robust.

中文翻译：

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从边界层气象学角度看陆地蒸散

蒸散率的精确测定具有挑战性，因为它是一个难以测量和参数化的量。直接估计包括使用蒸渗仪确定蒸发蒸发的土壤和植被体积的质量变化，或通过涡流协方差系统直接测量湍流水蒸气通量。利用 Monin-Obukhov 相似性理论的参数化估计使用温度和湿度在某一点的垂直梯度测量，以及通过高频操作的闪烁计的线或面积平均值。Penman 最初开发了灌溉充足表面蒸散量的关系，后来扩展到植被表面和异质农田。Priestley 和 Taylor 获得了一个流行的简化表达式。当前的挑战是找到非饱和条件下蒸散量的表达式，这在干旱和半干旱气候以及异质地形中很常见。在数值模型中，使用所谓的陆地表面模型，通过显式表示土壤和冠层中发生的不同相关子过程，获得估计的陆地实际蒸散量。通常这些机制以简化的方式描述，并依赖于许多可调整的参数。这种描述的改进依赖于实验测量的可用性，以使物理模型更加完整和稳健。在数值模型中，使用所谓的陆地表面模型，通过显式表示土壤和冠层中发生的不同相关子过程，获得估计的陆地实际蒸散量。通常这些机制以简化的方式描述，并依赖于许多可调整的参数。这种描述的改进依赖于实验测量的可用性，以使物理模型更加完整和稳健。在数值模型中，使用所谓的陆地表面模型，通过显式表示土壤和冠层中发生的不同相关子过程，获得估计的陆地实际蒸散量。通常这些机制以简化的方式描述并依赖于许多可调整的参数。这种描述的改进依赖于实验测量的可用性，以使物理模型更加完整和稳健。