872 SSSAJ: Volume 71: Number 3 • May–June 2007 Soil Sci. Soc. Am. J. 71:872–880 doi:10.2136/sssaj2006.0327 Received 15 Sept. 2006. *Corresponding author (jheitman@iastate.edu). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA 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. T hermal gradients drive soil heat and water transfers. Heat and water transfers, in turn, create transient temperature, water content, and thermal conductivity distributions. Heat and water transfer is a coupled process important for unsatu- rated, near-surface conditions. Yet our understanding of this process has been limited by a lack of thorough experimental testing. To date, laboratory data on temperature distributions for coupled heat and water transfer have been collected, but undesired two-dimensional distributions of both temperature and water often occur, which limits comparison and analysis (Prunty and Horton, 1994). Laboratory data on θ has most commonly been obtained through destructive sampling (cf. Nassar and Horton, 1989), which prevents measurement of transient conditions and precludes the possibility of applying more than one set of boundary conditions to a given sample. These limitations restrict testing and refinement of coupled heat and water transfer theory. Evaluation of the dominant transfer theories has been limited primarily to model calibration against steady-state moisture and temperature distributions. Attempts at validating the calibrated model or at describing transient boundary conditions are sorely lacking. There are a few reports of in situ measurement of θ using time domain reflectometry (TDR) to study coupled heat and water transfer (cf. Cahill and Parlange, 1998); however, TDR does not provide measurement of soil thermal properties or temperature. Thus, existing measurement approaches can lead to difficulty in interpretation of experimental results or prevent measurement of transient temperature, moisture redistribu- tion, and thermal properties. Recent improvements in tem- perature control (Zhou et al., 2006) and in situ measurement of both θ and soil thermal properties (Ren et al., 2005) can overcome these limitations and provide new opportunities for investigation of coupled heat and moisture transfer in labora- tory experiments. Prunty and Horton (1994) recognized that laboratory experi- ments aimed at one-dimensional thermal and moisture redistribu- tion were often affected by ambient temperature conditions. Two- dimensional distributions of both moisture and temperature result from the combined effect of imposed temperature conditions and ambient temperature interference. These two-dimensional condi- tions may provide linear or even convex (i.e., steepest temperature gradients near the cool end) temperature distributions between boundaries, which differ significantly from theoretical description and modeling efforts (e.g., Bach, 1992). When this interference is removed, temperature distributions typically become concave, because one-dimensional moisture redistribution results in non- An Improved Approach for Measurement of Coupled Heat and Water Transfer in Soil Cells J. L. Heitman* R. Horton Dep. of Agronomy Iowa State Univ. Ames, IA 50011 T. Ren Dep. of Soil and Water China Agricultural Univ. Beijing 100094 China T. E. Ochsner USDA-ARS Univ. of Minnesota 1991 Buford Cir. St. Paul, MN 55108 Laboratory experiments on coupled heat and water transfer in soil have been limited in their measurement approaches. Inadequate temperature control creates undesired two-dimen- sional distributions of both temperature and moisture. Destructive sampling to determine soil volumetric water content (θ) prevents measurement of transient θ distributions and pro- vides no direct information on soil thermal properties. The objectives of this work were to: (i) develop an instrumented closed soil cell that provides one-dimensional conditions and permits in situ measurement of temperature, θ, and thermal conductivity (λ) under transient boundary conditions, and (ii) test this cell in a series of experiments using four soil type–ini- tial θ combinations and 10 transient boundary conditions. Experiments were conducted using soil-insulated cells instrumented with thermo-time domain reflectometry (T-TDR) sensors. Temperature distributions measured in the experiments show nonlinearity, which is consistent with nonuniform thermal properties provided by thermal moisture distribution but differs from previous studies lacking one-dimensional temperature control. The T-TDR measurements of θ based on dielectric permittivity, volumetric heat capacity, and change in volumetric heat capacity agreed well with post-experiment sampling, providing r 2 values of 0.87, 0.93, and 0.95, respectively. Measurements of θ and λ were also consistent with the shapes of the observed temperature distributions. Techniques implemented in these experi- ments allowed observation of transient temperature, θ, and λ distributions on the same soil sample for 10 sequentially imposed boundary conditions, including periods of rapid redistri- bution. This work demonstrates that, through improved measurement techniques, the study of heat and water transfer processes can be expanded in ways previously unavailable. Abbreviations: TDR, time domain reflectometry; T-TDR, thermo-time domain reflectometry. SOIL PHYSICS Published online May 16, 2007