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Reproducing Field-Scale Active Layer Thaw in the Laboratory Aaron A Technical Note Reproducing Field-Scale Active Layer Thaw in the Laboratory Aaron A. Mohammed,* Robert A. Schincariol, Ranjeet M. Nagare, and William L. Quinton A method to simulate freeze–thaw and permafrost conditions on a large peat-soil column, housed in a biome, was developed. The design limits ambient temperature interference and maintains one-dimensional freezing and thawing. An air circulation system, in a cavity surrounding the active layer, allows manipulation of the lateral temperature boundary by actively maintaining an air temperature matching the average temperature of the A method is presented to maintain soil column. Replicating realistic thermal boundary conditions enabled field-scale rates of active-layer thaw. Radial temperature gradients were one-dimensional heat transport in small and temperature profiles mimicked those for similar field conditions. variably saturated soil cells over The design allows complete control of key hydrologic processes related to freezing and thawing tempera- heat and water movement in permafrost terrains without scaling require- tures. It applies transient thermal ment; and presents a path forward for the large-scale experimental study boundary conditions and repli- of frozen ground processes. Because subarctic ecosystems are very vulner- able to climate and anthropogenic disturbances, the ability to simulate cates field-scale ground thaw. The perturbations to natural systems in the laboratory is particularly important. method allows the isolated study of coupled heat and transport in Abbreviations: ATI, ambient temperature interference. permafrost environments. Permafrost and active-layer processes occur over approximately 25% of the Earth’s surface (Williams and Smith, 1989) and play a vital role in the hydrologic func- tioning of northern watersheds (Woo and Winter, 1993). Layered peat deposits underlain by permafrost are geographically extensive and pervade large areas of arctic, subarctic, A.A. Mohammed, Dep. of Earth and boreal environments (Woo and Winter, 1993). High-latitude warming trends and Sciences, Univ. of Western Ontario, increased anthropogenic activities stress the need for a comprehensive understanding of 1151 Richmond Street N, London, ON, N6A 5B7 Canada, currently at Dep. the mechanisms responsible for permafrost sustainability in these sensitive ecosystems. of Geoscience, Univ. of Calgary, 2500 Because peat hydraulic conductivity generally decreases by several orders of magnitude University Drive NW, Calgary, AB, T2N 1N4 Canada; R.A. Schincariol, Dep. with depth (Quinton et al., 2000), active-layer thicknesses strongly influence the drainage of Earth Sciences, Univ.of Western characteristics of these northern landscapes (Wright et al., 2009). For example, Zhang Ontario, 1151 Richmond Street N, London, ON, N6A 5B7 Canada; R.M. et al. (2010) concluded, in a study of infiltration into peat-covered permafrost soils, that Nagare, Water Business Unit, Worley the single most important factor controlling infiltration is ground thaw depth. Active- Parsons Canada, 4445 Calgary Trail, Edmonton, AB, T6H 5R7 Canada; and layer thickness is controlled by heat and water transfer between the ground surface and W.L. Quinton, Cold Regions Research the underlying permafrost table (Hayashi et al., 2007). The freezing and thawing of soils Centre, Wilfred Laurier Univ., 75 Ave- induces both water and heat movement (Harlan, 1973). These processes are not indepen- nue West, Waterloo, ON, N2L 3C5 Canada. *Corresponding author dent as water flows under matric and thermal gradients, while moving water transports ([email protected]). energy and affects soil thermal properties (Hoekstra, 1966). In variably saturated media, because frozen soil retains water, freezing temperatures perturb the soil pressure–satura- Vadose Zone J. tion equilibrium, resulting in steep gradients that redistribute water from warmer to colder doi:10.2136/vzj2014.01.0008 Received 15 Jan. 2014. zones (Dirksen and Miller, 1966). As a result of this tight coupling and nonlinear nature, problems related to water and heat transport are often difficult to study in the field and can benefit from controlled laboratory work where the important climatic and physical properties can be isolated and investigated (Zhou et al., 2006). © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. The primary mechanism controlling ground heat flux through the shallow subsurface to All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by the frost table is often heat conduction (Hayashi et al., 2007). In a natural terrestrial any means, electronic or mechanical, including photocopying, recording, or any information sto- environment, any arbitrary soil column will be radially insulated by the soil around it, so rage and retrieval system, without permission in writing from the publisher. temperature forcing is primarily downward from ground surface temperatures. This means Vadose Zone Journal that thermal gradients and heat conduction are effectively vertical period of time (Carey and Woo, 2005). Once the active layer is and one-dimensional (Hayashi et al., 2007). Creating an experi- completely frozen, there is less temperature variation in the verti- mental setup that can simulate one-dimensional vertical heat flow cal direction in the soil profile as it changes gradually with depth. is critical to being able to adequately replicate field conditions. For Thawing, however, occurs only from the top down, and this creates example, both Zhou et al. (2006) and Nagare et al. (2012a) stressed a stark temperature difference between the thawing front and the the importance of minimizing ambient temperature interference soil profile above it (Quinton and Hayashi, 2008). Establishing (ATI) in their coupled heat and water movement laboratory stud- realistic ground thaw conditions in a controlled environment can ies. This interference creates lateral temperature gradients that give insight into parameters controlling the thermal regime of the make maintaining one-dimensional thermal gradients challeng- active layer during thawing. ing (Prunty and Horton, 1994). This often results in studies using sample volumes that may be too small to capture field-scale pro- The goal of the design was to limit ATI by placing the soil column cesses and limits the use of field instrumentation within the sample. concentrically within another larger container and radially Work done by others to reduce ATI have used passive insulation insulating the soil by circulating air within the cavity between the methods, the most effective of which was presented by Zhou et two containers. Our hypothesis was that while the column was al. (2006) and utilized by Gieselman et al. (2008) and Heitman exposed to transient temperature boundary conditions at its ends, et al. (2007) for experiments with and without freezing condi- we could actively manipulate the temperature within the cavity to tions, respectively. These studies used insulated soil cells with a reflect the average temperature in the active-layer portion of the soil concentric layer of the same soil at similar moisture conditions. column and reduce lateral thermal gradients. The air space and the This method is very effective for experiments of relatively short outer volume of soil were expected to buffer the 30-cm-diameter duration (e.g., 46 d for Gieselman et al., 2008) but fails for longer core, where the instrumentation is housed. This setup enables ATI periods of time because no insulation can be perfect. Ambient to be minimized while the soil column is exposed to a range of temperature interference will eventually cause lateral temperature transient temperatures associated with freezing and thawing. We gradients; this is especially true for conditions where very steep provide data highlighting the column’s ability to maintain one- lateral thermal gradients between the sample and its surroundings dimensional freezing and thawing front movement. exist. Thus, passive insulation may not be suitable for experiments where ATI needs to be minimized for a longer time period than 6 can be achieved with passive methods. For studies investigating Materials and Methods the field-scale hydrologic implications of freezing and thawing, Climate Chamber larger samples that allow the establishment of realistic thermal and moisture gradients for longer periods of time (i.e., many months) The climate chamber used was that presented by Nagare et al. are required. Such experiments therefore require careful instru- (2012a), a unique two-level biome capable of simulating large-scale mentation and alternative approaches to reduce ATI and validate subsurface environments (Fig. 1). The chamber consists of an upper field-scale conditions (Henry, 2007; Lewis and Sjöstrom, 2010). and lower level, allowing soil monoliths to be placed between the two levels separated by an adjustable insulated floor, as shown We have developed an experimental method that reduces radial in Fig. 2. Both levels of the chamber are capable of simulating a temperature gradients at a scale, and under boundary conditions, temperature range from 40 to −40°C. Additional climatic con- that replicate coupled heat and water movement in permafrost trols in the upper level include light intensity, relative humidity, environments. We made use of the two-level climate chamber rain, and CO2 concentrations. Situating part of a monolith in the presented by Nagare et al. (2012b) designed for large-scale study lower
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