Leaf-scale experiments reveal an important omission in the Penman-Monteith equation

Leaf-scale experiments reveal an important omission in the Penman-Monteith equation
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叶尺度实验揭示了 Penman-Monteith 方程中的一个重要遗漏

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发表时间：
2017
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通讯作者：
D. Or
D. Or
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作者：
S. Schymanski;D. Or

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Penman-Monteith (PM) 方程通常被认为是最先进的方程 基于物理的方法来计算植物的蒸腾速率 考虑气孔导度和大气驱动因素。它已被广泛 在冠层规模上进行评估，其中空气动力学和冠层阻力 水蒸气很难直接估计，导致各种经验 从叶子缩放到冠层时的更正。在这里，我们评估了 PM 使用详细的叶子能量平衡直接在叶子尺度上方程 使用受控绝缘风洞进行模型和直接测量 具有固定和预定气孔导度的人造叶子。 实验结果与详细的叶片能量平衡一致 模型；然而，结果显示与 PM 预测存在系统性偏差 通量，这指出了 PM 方程的基本问题。详细 Monteith (1965) 的推导分析 随后的修改暴露了两个错误：一是忽视了双面 平面叶片的显热交换，另一个与 下口叶的代表，在温带地区很常见 气候。两侧显热通量的遗漏导致了偏差 PM方程模拟潜热通量高达50% 一些实验中观察到的通量。此外，我们发现 忽略叶片温度和辐射能量交换之间的反馈 可能会导致潜热通量和显热通量的额外偏差。一个 修正了叶片温度和潜伏期的解析解集 给出了感热通量，并与原始 PM 进行比较 方程表明在再现实验结果方面取得了重大改进 叶规模。原始 PM 方程中的错误及其失败 在叶子尺度上再现实验结果（最初是为了 派生）传播到不准确的蒸腾敏感性和 大气条件变化的显热通量，例如 与气候变化相关（即使目前表现合理 校准后）。这里提出的新公式纠正了一些 PM 方程的缺点，可以提供更稳健的启动 冠层代表性和气候变化研究的要点。
The Penman–Monteith (PM) equation is commonly considered the most advanced physically based approach to computing transpiration rates from plants considering stomatal conductance and atmospheric drivers. It has been widely evaluated at the canopy scale, where aerodynamic and canopy resistance to water vapour are difficult to estimate directly, leading to various empirical corrections when scaling from leaf to canopy. Here, we evaluated the PM equation directly at the leaf scale, using a detailed leaf energy balance model and direct measurements in a controlled, insulated wind tunnel using artificial leaves with fixed and predefined stomatal conductance. Experimental results were consistent with a detailed leaf energy balance model; however, the results revealed systematic deviations from PM-predicted fluxes, which pointed to fundamental problems with the PM equation. Detailed analysis of the derivation by Monteith(1965) and subsequent amendments revealed two errors: one in neglecting two-sided exchange of sensible heat by a planar leaf, and the other related to the representation of hypostomatous leaves, which are very common in temperate climates. The omission of two-sided sensible heat flux led to bias in simulated latent heat flux by the PM equation, which was as high as 50 % of the observed flux in some experiments. Furthermore, we found that the neglect of feedbacks between leaf temperature and radiative energy exchange can lead to additional bias in both latent and sensible heat fluxes. A corrected set of analytical solutions for leaf temperature as well as latent and sensible heat flux is presented, and comparison with the original PM equation indicates a major improvement in reproducing experimental results at the leaf scale. The errors in the original PM equation and its failure to reproduce experimental results at the leaf scale (for which it was originally derived) propagate into inaccurate sensitivities of transpiration and sensible heat fluxes to changes in atmospheric conditions, such as those associated with climate change (even with reasonable present-day performance after calibration). The new formulation presented here rectifies some of the shortcomings of the PM equation and could provide a more robust starting point for canopy representation and climate change studies.