Estimating transpiration as an individual component of canopy evapotranspiration using a theoretical approach is extremely useful as it eliminates the complexity involved in partitioning evapotranspiration. A model to predict transpiration based on radiation intercepted at various levels of canopy leaf area index (LAI) was developed in a controlled environment using a pasture species, tall fescue (Festuca arundinacea var. Demeter). The canopy was assumed to be a composite of two indistinct layers defined as sunlit and shaded; the proportion of which was calculated by utilizing a weighted model (W model). The radiation energy utilized by each layer was calculated from the PAR at the top of the canopy and the fraction of absorbed photosynthetically active radiation (fAPAR) corresponding to the LAI of the sunlit and shaded layers. A relationship between LAI and fAPAR was also established for this specific canopy to aid the calculation of energy interception. Canopy conductance was estimated from scaling up of stomatal conductance measured at the individual leaf level. Other environmental factors that drive transpiration were monitored accordingly for each individual layer. The Penman–Monteith and Jarvis evapotranspiration models were used as the basis to construct a modified transpiration model suitable for controlled environment conditions. Specially, constructed self-watering tubs were used to measure actual transpiration to validate the model output. The model provided good agreement of measured transpiration (actual transpiration = 0.96 × calculated transpiration, R² = 0.98; p < 0.001) with the predicted values. This was particularly so at lower LAIs. Probable reasons for the discrepancy at higher LAI are explained. Both the predicted and experimental transpiration varied from 0.21 to 0.56 mm hr⁻¹ for the range of available LAIs. The physical proportion of the shaded layer exceeded that of the sunlit layer near LAI of 3.0, however, the contribution of the sunlit layer to the total transpiration remains higher throughout the entire growing season.

中文翻译：

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叶面积指数影响下基于气孔导度和辐射截留的冠层蒸腾模型

使用理论方法将蒸腾量估算为冠层蒸散量的单个组成部分非常有用，因为它消除了分配蒸散量所涉及的复杂性。在控制环境中，使用高种羊茅（Festuca arundinacea变种 德米特（Demeter）。假定顶篷是由两个模糊的图层（定义为阳光和阴影）组成的；通过使用加权模型（W模型）计算其比例。根据顶篷顶部的PAR和对应于日照层和阴影层的LAI的吸收的光合有效辐射（fAPAR）的比例，计算各层使用的辐射能。还针对该特定冠层建立了LAI和fAPAR之间的关系，以帮助计算能量拦截。冠层电导率是通过按单个叶片水平测量气孔导度来估算的。对于每个单独的层，相应地监测了驱动蒸腾作用的其他环境因素。Penman–Monteith和Jarvis蒸散模型被用作构建适合于受控环境条件的改良蒸腾模型的基础。特别地，使用构造的自动浇水桶来测量实际的蒸腾量以验证模型输出。该模型提供了良好的蒸腾量一致性（实际蒸腾量= 0.96×计算的蒸腾量R² = 0.98；p <0.001）与预测值。在较低的LAI上尤其如此。解释了LAI较高时差异的可能原因。对于可用LAI的范围，预测蒸腾量和实验蒸腾量均在0.21至0.56 mm hr ^-1之间变化。在LAI接近3.0时，阴影层的物理比例超过了阳光照射层的比例，但是，在整个生长季节中，阳光照射层对总蒸腾量的贡献仍然较高。