Published online 25 August 2005
Published in Soil Sci Soc Am J 69:1666 (2005)
DOI: 10.2136/sssaj2005.0140le
© 2005 Soil Science Society of America
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Comments and Letters to the Editor
Comments on "On the Construction and Calibration of Dual-Probe Heat Capacity Sensors"
Nathan G. Phillips*,a,
Guido D. Salvucci
,b and
Justin C. Pettijohn
,c
a Geography Department, Boston University, Boston, MA 02215
b Geography and Earth Sciences Departments, Boston University, Boston, MA 02215
c Earth Sciences Department, Boston University, Boston, MA 02215
Ham and Benson (2004) found that dual-probe heat capacity (DPHC) sensors produce apparently biased estimates of soil heat capacity, consistent with previous studies (Tarara and Ham, 1997; Song et al., 1998; Basinger et al., 2003). At low water contents, DPHC sensors appear to systematically overestimate total heat capacity of soils (solids plus liquid water). Other calorimetric observations also show higher soil heat capacities than predicted (e.g., Abu-Hamdeh, 2003). Although the magnitudes of these errors are small and may be of minor consequence for many studies, resolving this unknown source of error would improve confidence in the use of the DPHC sensor and soil heat storage studies in general. Here we suggest that increased heat capacity of liquid water in porous media at low water contents may resolve this issue.
Both theory (e.g., Poole et al., 1994; Rebelo et al., 1998) and experimental data (e.g., Oguni and Angell, 1980) demonstrate that liquid water may have different thermophysical properties than in its bulk state. Studies on plant tissues show increases in heat capacity of liquid water at water contents similar to those in which errors are observed in soil heat capacity using DPHC sensors (Buitink et al., 1996; Simpson and TenWolde, 1999; Walters, 2004). To the extent that plant tissues and soils behave similarly with respect to matric interactions with water, the magnitude of increase of heat capacity of liquid water with dehydration—up to more than double the value for bulk water at room temperature—is sufficient to account for the apparent overestimate of heat capacity in dry soils using DPHC sensors. A simple mixing model of heat capacity for the solids and liquid water in soils, applied to data from Tarara and Ham (1997) demonstrates this:
where Cpsoil, CpH2O, and Cpsolids are the specific heats of the solid–liquid water mixture, bulk water (4.182 J g–1 °C), and heat capacity of soil solids (0.76 J g °C), respectively, and MH2O, Msolids, and Msoil are the masses of the corresponding components. Using 10% gravimetric water content (which is approximately the same as volumetric water content assuming soil bulk densities given in Tarara and Ham [1997]), and doubling CpH2O based on results from Buitink et al. (1996), results in over 40% larger estimates of Cpsoil, which is more than enough to account for the apparent overestimate of soil heat capacity from DPHC sensors. Thus, the apparent overestimates of soil heat capacity noted by Ham and Benson (2004) may in fact reflect true variability of soil heat capacity as a function of water content, taking into account variable heat capacity of liquid water in partially dehydrated porous materials. To further investigate this possibility, direct calorimetric studies on dehydrated soils should be performed, without constraining CpH2O as a constant.
NOTES
* nathan{at}bu.edu 
gdsalvuc{at}bu.edu
geocory{at}bu.edu
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