| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Department of Agronomy, Kansas State University, Manhattan, KS 66506
jayham{at}ksu.edu
The comments of Phillips et al. (2005) make an interesting point that the heat capacity of water (CpH2O) may increase at low soil water contents and may explain why dual-probe heat capacity (DPHC) sensors disagree with calculated values of soil heat capacity under dry conditions. Several laboratory and greenhouse studies have shown that DPHC sensors tend to increasingly overestimate soil heat capacity as the soil dries (Tarara and Ham, 1997; Song et al., 1998). Ham and Benson (2004) showed that DPHC sensors calibrated in water-saturated glass beads or agar overestimated the heat capacity of dry glass beads. In all of these studies, DPHC measurements were compared with calculated values of soil heat capacity based on the mixing model approach of DeVries (1963), and have assumed CpH2O was constant and equal to that of free water (4.18 MJ m–3 K–1). If CpH2O in the soil increases at lower water contents as Phillips et al. (2005) suggest, then indeed the DPHC readings in the previously mentioned papers may have been accurate and underestimates of CpH2O used in the mixing model is what caused the discrepancy. Variations in CpH2O with soil water content would not affect the utility of the DPHC technique as a thermal properties instrument because the sensor makes a direct reading of soil heat capacity. However, volumetric soil water content, 
v, is estimated from a DPHC measurement of soil heat capacity, 
Cpsoil, as
 |
Cpm and Cpo are the volumetric heat capacities of the mineral and organic fractions and 
m and o are their fractions in the bulk soil, respectively (Tarara and Ham, 1997). Thus, knowledge of CpH2O at the measurement site is required to estimate v. Clearly, calculation of v from DPHC measurements would become difficult if CpH2O itself were dependent on v; a numerical solution would probably be required even if the functional form of CpH2O vs. v was known. While the hypothesis of Phillips et al. (2005) is very interesting, there are data in the literature that would seem to discount the idea that CpH2O is dependent on soil water content. Several studies have compared DPHC sensor estimates of soil water content with gravimetric data and have not observed decreased accuracy at lower water contents (Noborio et al., 1996; Heitman et al., 2003; Bremer, 2003). Also, as mentioned earlier, Ham and Benson (2004) found that DPHC sensors calibrated in water-saturated media and agar, overestimated heat capacity of dry glass beads. Because the media was dry (only glass and air), errors in the estimate of CpH2O could not be responsible for the discrepancy. One could argue that the heat capacity of the glass beads was in error, but the thermal properties of the beads had been measured several times by differential scanning calorimetry. Ham and Benson (2004) argue that the discrepancy in sensor performance in wet and dry media could be caused by measurement error or from using the incorrect heat flow model. A simple infinite-line source model has been used by most investigators; it does not account for the effect the sensor body or the finite length and cylindrical shape of the heater and temperature probes.
In summary, Phillips et al. (2005) make a very interesting point that deserves further study. If the thermal properties of soil water are a function of bulk water content, then it is important that these relationships be explored to improve our understanding of the field thermal regime. However, data suggest that factors other than or in addition to changes in CpH2O are affecting DPHC sensor performance at low water contents.
ACKNOWLEDGMENTS
The insightful suggestions of Dr. G.J. Kluitenberg are greatly appreciated.
REFERENCES
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
