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

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.