The response of shallow geotechnical structures is affected by interaction with the atmosphere. Since the ground surface is very often vegetated, plant transpiration plays a major role in such an interaction. Transpiration in geotechnical applications is generally modelled by way of a transpiration reduction function (e.g. the Feddes function). However, its parameters are generally borrowed from the agricultural literature, where the focus is on crop species and often loosely compacted organic agricultural soils. For the non-crop species in denser soils typically encountered in geotechnical applications, monitoring of the flow taking place in the soil through the xylem up to the leaves can potentially be exploited to characterise the transpiration reduction function. The main challenge is the measurement of the water pressure in the xylem. Techniques currently used include the pressure chamber and thermocouple psychrometer. The pressure chamber is destructive and thus not suitable for continuous monitoring and/or where a relatively small number of leaves is available (as often occurs in laboratory experiments). The thermocouple psychrometer is not accurate at low water tension, is affected by the presence of solutes in the xylem water and is significantly sensitive to temperature. This paper explores a novel application of the high-capacity tensiometer (HCT), initially developed for pore-water pressure measurement in soils. The HCT was installed on the stem or branch of different trees and its measurement validated against pressure chamber measurements over a range of xylem water pressure down to −1300 kPa. In addition, its measurement was used to investigate the response of the soil–plant continuum. Results show that the HCT is a viable and convenient instrument to use for xylem water pressure measurement and can provide field-based data for the modelling of plant transpiration. Installing HCTs on stems and branches is quite straightforward and this will help achieve a step change in testing and modelling the effect of plant transpiration on the soil water regime in the vadose zone.

中文翻译：

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使用大容量张力计测量植物木质部水压及其对土壤-大气相互作用建模的意义

浅层岩土结构的响应受大气相互作用的影响。由于地表经常是植被，因此植物的蒸腾作用在这种相互作用中起主要作用。岩土工程应用中的蒸腾作用通常通过减少蒸腾作用的函数（例如Feddes函数）来建模。但是，它的参数通常是从农业文献中借来的，这些文献的重点是农作物物种，通常是松散压实的有机农业土壤。对于土力工程应用中通常遇到的稠密土壤中的非农作物物种，可以潜在地利用对通过木质部直至叶的土壤中流动的监测来表征蒸腾作用。主要挑战是木质部中水压的测量。当前使用的技术包括压力室和热电偶干湿计。压力室具有破坏性，因此不适合进行连续监控和/或可用相对较少的叶片（在实验室实验中经常发生）。热电偶干湿计在低水张力下不准确，受木质部水中溶质的存在影响，并且对温度非常敏感。本文探讨了大容量张力计（HCT）的新应用，该系统最初是为测量土壤中的孔隙水压力而开发的。HCT安装在不同树木的茎或树枝上，并且在木质素水压低至-1300 kPa的范围内，其测量值已针对压力室测量结果进行了验证。此外，它的测量被用来调查土壤-植物连续体的响应。结果表明，HCT是用于木质部水压测量的可行且方便的仪器，可为植物蒸腾建模提供基于现场的数据。在茎和枝上安装HCT非常简单，这将有助于在测试和模拟植物蒸腾作用对渗流区土壤水分状况的影响方面实现步骤变化。