There is increased interest in the potential of tree planting to help mitigate flooding using nature-based solutions or natural flood management. However, many publications based upon catchment studies conclude that, as flood magnitude increases, benefit from forest cover declines and is insignificant for extreme flood events. These conclusions conflict with estimates of evaporation loss from forest plot observations of gross rainfall, through fall and stem flow. This study explores data from existing studies to assess the magnitudes of evaporation and attempts to identify the meteorological conditions under which they would be supported. This is achieved using rainfall event data collated from publications and data archives from studies undertaken in temperate environments around the world. The meteorological conditions required to drive the observed evaporation losses are explored theoretically using the Penman–Monteith equation. The results of this theoretical analysis are compared with the prevailing meteorological conditions during large and extreme rainfall events in mountainous regions of the United Kingdom to assess the likely significance of wet canopy evaporation loss. The collated dataset showed that event Ewc losses between approximately 2 and 38% of gross rainfall (1.5 to 39.4 mm day−1) have been observed during large rainfall events (up to 118 mm day−1) and that there are few data for extreme events (>150 mm day−1). Event data greater than 150 mm (reported separately) included similarly high percentage evaporation losses. Theoretical estimates of wet-canopy evaporation indicated that, to reproduce the losses towards the high end of these observations, relative humidity and the aerodynamic resistance for vapour transport needed to be lower than approximately 97.5% and 0.5 to 2 s m−1 respectively. Surface meteorological data during large and extreme rainfall events in the United Kingdom suggest that conditions favourable for high wet-canopy evaporation are not uncommon and indicate that significant evaporation losses during large and extreme events are possible but not for all events and not at all locations. Thus the disparity with the results from catchment studies remains.

中文翻译：

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评估极端降雨事件期间森林湿冠层蒸发对英国山区缓解洪水的重要性

人们对植树利用基于自然的解决方案或自然洪水管理来帮助减轻洪水的潜力越来越感兴趣。然而，许多基于集水区研究的出版物得出的结论是，随着洪水规模的增加，森林覆盖率的下降对极端洪水事件的影响微不足道。这些结论与通过秋季和茎流对总降雨量的森林地块观测得出的蒸发损失估计值相矛盾。本研究探索了现有研究的数据，以评估蒸发量，并尝试确定支持蒸发量的气象条件。这是使用从出版物和数据档案中整理的降雨事件数据来实现的，这些数据来自在世界各地温带环境中进行的研究。使用 Penman-Monteith 方程从理论上探讨了驱动观察到的蒸发损失所需的气象条件。将此理论分析的结果与英国山区大雨和极端降雨事件期间的主要气象条件进行比较，以评估湿冠层蒸发损失的可能重要性。整理的数据集显示，在大降雨事件（高达 118 mm day-1）期间观察到事件 Ewc 损失约为总降雨量的 2% 到 38%（1.5 到 39.4 mm day-1），并且几乎没有极端数据事件（>150 mm day−1）。大于 150 毫米的事件数据（单独报告）包括类似的高百分比蒸发损失。湿冠层蒸发的理论估计表明，为了重现这些观察结果的高端损失，相对湿度和蒸汽传输的空气动力学阻力需要分别低于约 97.5% 和 0.5 到 2 sm-1。英国大型和极端降雨事件期间的地表气象数据表明，有利于高湿冠层蒸发的条件并不少见，并表明在大型极端事件期间可能会出现大量蒸发损失，但并非针对所有事件，也不是在所有地点。因此，与流域研究结果的差异仍然存在。英国大型和极端降雨事件期间的地表气象数据表明，有利于高湿冠层蒸发的条件并不少见，并表明在大型极端事件期间可能会出现大量蒸发损失，但并非针对所有事件，也不是在所有地点。因此，与流域研究结果的差异仍然存在。英国大型和极端降雨事件期间的地表气象数据表明，有利于高湿冠层蒸发的条件并不少见，并表明在大型极端事件期间可能会出现大量蒸发损失，但并非针对所有事件，也不是在所有地点。因此，与流域研究结果的差异仍然存在。