Vineyards in many semi-arid regions globally face limited water resources. Monitoring evapotranspiration (ET) of vineyards is critical for water resource management, but remains difficult due to the complex biophysics of the surfaces. Both measurement and modeling approaches for estimating turbulent water vapor transport rely on implicit assumptions that exchanges occur in a reasonably regular fashion over the time scales generally used for averaging. However, heterogeneous vegetation in semi-arid climates, such as many vineyards, presents inherent factors, including canopy row/row space structure and frequent periods of light wind, unstable conditions, that can create episodic transport characteristics. Eddy covariance data were collected above and within the canopy of two vineyards in the Central Valley of California during the Grape Remote sensing Atmospheric Profile & Evapotranspiration eXperiment (GRAPEX). The goal was to document and quantify the existence of intermittent turbulence transport of water vapor, and associated episodic canopy venting. These effects were found to correlate with periods light winds and highly unstable/convective conditions. Power and cross-spectra for intermittent periods documented enhancement of low-frequency water vapor exchange events compared to more steady periods, and diminished time scale correlation between humidity within the canopy and above the canopy. Analyses show that intermittent cases can necessitate longer flux-averaging periods (up to 2 h) than more steady conditions. Episodic exchange events were isolated and summed to determine their relative contribution to the overall water vapor flux. Since light wind, unstable conditions are relatively common in many arid vineyard regions, these findings have implications for mechanistic ET models that rely on time-averaged vertical gradients, which implies reasonably steady transport.

中文翻译：

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在微风和对流条件下，葡萄园水蒸气通量的间歇性

全球许多半干旱地区的葡萄园面临水资源有限的问题。监测葡萄园的蒸散量（ET）对于水资源管理至关重要，但由于地表复杂的生物物理学，监测仍然很困难。用于估计湍流水汽输送的测量和建模方法都依赖于隐含的假设，即在通常用于平均的时间尺度上以相当有规律的方式发生交换。然而，半干旱气候下的异质植被（例如许多葡萄园）存在固有因素，包括冠层行/行空间结构和频繁的微风、不稳定的条件，这些因素可能会产生间歇性的运输特征。在葡萄遥感大气剖面和蒸发蒸腾实验 (GRAPEX) 期间，在加利福尼亚州中央山谷的两个葡萄园的上方和树冠内收集了涡流协方差数据。目标是记录和量化水蒸气间歇性湍流传输的存在以及相关的间歇性冠层通风。人们发现这些影响与微风和高度不稳定/对流条件有关。间歇期的功率和交叉谱记录了与更稳定的周期相比低频水蒸气交换事件的增强，并且冠层内和冠层上方的湿度之间的时间尺度相关性减弱。分析表明，与更稳定的情况相比，间歇性情况可能需要更长的通量平均周期（最多 2 小时）。分离并总结偶发交换事件以确定它们对总水蒸气通量的相对贡献。由于微风和不稳定条件在许多干旱葡萄园地区相对常见，这些发现对依赖于时间平均垂直梯度的机械蒸散模型具有影响，这意味着相当稳定的运输。