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Differences between field-monitored and laboratory-measured soil moisture characteristics

机译：现场监测和实验室测量的土壤水分特征之间的差异

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A soil water retention curve (SWRC) is usually measured in a laboratory (lab SWRC), and is used to analyze in-situ soil moisture conditions. However, it is rarely verified whether and how a lab SWRC is in agreement with its equivalent relation between matric potential (h) and volumetric water content (theta) in a natural field (in-situ SWRC). In addition, most SWRCs show moisture hysteresis through which the drying process gives a larger theta at a given h than the wetting process, while an in-situ SWRC must be produced through the cycles of drying and wetting in the field. Thus, it can be hypothesized that an in-situ SWRC shows a lower value of theta than a lab SWRC for any h that the soil layer ordinarily experiences. To give experimental proofs for this hypothesis, this study aimed at quantifying seasonal behaviors of in-situ SWRCs and at comparing them with their corresponding lab SWRCs. To obtain a series of in-situ SWRCs, the h and theta were coincidently monitored at four points with three depths each in a meadow for 2.5 years using tensiometers and a capacitance-type soil moisture sensing system. As the equivalent to the in-situ SWRCs, the lab SWRCs were also measured. The in-situ SWRCs tended to have roughly 10% smaller theta than the lab SWRCs for the series of h observed in the study site, suggesting that an in-situ SWRC can hardly be reproduced by a lab SWRC only. In addition, when the driest condition in the recent 3 years was exerted on the study site, some in-situ SWRCs shifted along the theta axis on the theta(h) charts, suggesting that the most dried condition had changed the soil moisture regime of these soil layers, resulting in the reduction of monthly or annual means of soil water content in the field. Since the shifts of the in-situ SWRCs were accompanied by the increases in both the gradients 'd theta/dh' and the variation of measured h, it was implied that an extraordinary drying of a soil layer promotes the development of soil pore structure or an increase in the fraction of plant available water.
机译：土壤保水曲线（SWRC）通常在实验室（实验室SWRC）中进行测量，并用于分析原位土壤湿度条件。但是，很少能验证实验室SWRC是否以及如何与自然领域（原位SWRC）中基质势（h）和体积水含量（theta）之间的等效关系一致。此外，大多数SWRC都表现出湿滞现象，通过该过程，干燥过程在给定的h处比润湿过程具有更大的theta，而现场SWRC必须通过现场的干燥和润湿循环来生产。因此，可以假设在土壤层通常经历的任何h内，原位SWRC的theta值均比实验室SWRC的低。为了对此假设提供实验证据，本研究旨在量化原位SWRC的季节性行为，并将其与相应的实验室SWRC进行比较。为了获得一系列原位SWRC，使用张力计和电容式土壤湿度感测系统在草甸中的三个点上同时对四个点的h和θ进行了2.5年的同时监测。与现场SWRC等效，还对实验室SWRC进行了测量。对于在研究现场观察到的一系列h，原位SWRC的theta往往比实验室SWRC小约10％，这表明原位SWRC很难仅由实验室SWRC复制。此外，在研究地点施加近三年来最干旱的条件时，一些原位SWRC沿theta（h）图上的theta轴移动，这表明最干燥的条件改变了土壤的水分状况。这些土壤层，导致田间土壤水含量每月或每年减少。由于原位SWRC的位移都伴随着梯度``d theta / dh''的增加和实测h的变化，因此暗示土壤层的超常干燥会促进土壤孔隙结构或增加了植物可用水的比例。

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