Hydra Probe and Twelve‐Wire Probe Comparisons in Fluids and Soil Cores

Abstract

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Hydra Probe and Twelve-Wire Probe Comparisons in Fluids and Soil Cores Soil Physics A ccurate soil water content data are needed for evaluating the effi ciency of soil water use and for understanding soil water fl ow in the landscape. Automated soil water sensors infer the temporal dynamics of water content in the soil as part of the soil-plant-atmosphere continuum (McCoy et al., 2006). Many of these sensors are based on a relationship between permittivity and water content (θ). Th e assumption for using dielectric sensors to determine θ is based on known properties for free water, minerals, and air. Most sensors have a measurement frequency (f) in the megahertz to low gigahertz range, below the permittivity relaxation frequency (f rel) for free water (~17 GHz); therefore, the assumption is oft en made that the permittivity of pure water is relatively constant at the low-f value (76-80 depending on temperature). Permittivity is complex (ε*), with real (ε') and imaginary (ε") components. Th e ε" is aff ected by direct current electrical conductivity (σ dc) as well as a relaxation component (ε rel "), whereas ε' is only aff ected by a relaxation component:

Estimation of Soil Clay Content from Hygroscopic Water Content Measurements

“…Diff erent authors have used diff erent values, Banin and Amiel (1970) used air-dry samples, while Petersen et al (1996) used a pressure of 1.5 MPa. Logsdon et al (2010) determined hygroscopic water content of air-dry soils in a vapor-tight container over distilled water at ~99% RH. In an eff ort to standardize a method, Lebron et al (2009), used a hygroscopic water content of 41% to determine gypsum content in soils.…”

“…In the studies of Banin and Amiel (1970) and Dirksen and Dasberg (1993), hygroscopic water content had a strong linear correlation (r 2 = 0.936) with soil-specifi c surface area. Recently, Logsdon et al (2010) determined hygroscopic water content of soils in a vapor-tight container over distilled water at ~99% RH and concluded that higher hygroscopic water content is associated with high soil-specifi c surface area. To come to an agreement about a specifi c RH level at which hygroscopic water content ought to be determined, in-depth knowledge of the water release characteristics of diff erent clay minerals is required.…”

Measurement of Soil Water Content with Dielectric Dispersion Frequency

“…Heimovaara et al (1996) compared TDR and FDR measurements of dielectric permittivity in the 0 to 1 GHz range. Skierucha and Wilczek (2010) measured soil permittivity in the 10 to 500 MHz range (Delta-T Devices, 1999; Kelleners et al, 2005;Logsdon et al, 2010;Seyfried et al, 2005).…”

Soil Water Characteristic Curves from Measures of Electrical Permittivity

“…For comparison purposes, the equation relating real permittivity and volumetric water content suggested by Logsdon et al (2010) is also fitted through the data in Figure 5. It appears that the trendline developed for the current study agrees well with the previously published expression.…”

“…Previous researchers (Topp et al, 1980;Seyfried and Murdock, 2004;Logsdon et al, 2010) have shown that volumetric water content, θ , can be estimated reasonably well using the dielectric constant of the soil, which is the real component of the complex permittivity. Also, recent research (Rao and Singh, 2010) has shown that matric suction, ψ , for a particular soil may be correlated with measures of voltage for that soil.…”