Location: Northwest Watershed Research CenterTitle: Field test and sensitivity analysis of a sensible heat balance method to determine ice contents
|KOJIMA, YUKI - Iowa State University|
|HEITMAN, JOSHUA - North Carolina State University|
|REN, TUSHENG - China Agricultural University|
|EWING, ROBERT - Iowa State University|
|HORTON, ROBERT - Iowa State University|
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2014
Publication Date: 9/19/2014
Citation: Kojima, Y., Heitman, J., Flerchinger, G.N., Ren, T., Ewing, R.P., Horton, R. 2014. Field test and sensitivity analysis of a sensible heat balance method to determine ice contents. Vadose Zone Journal. doi: 10.2136/vzj2014.04.0036.
Interpretive Summary: Ice content of frozen soils has a large influence on water movement, infiltration, and ultimately runoff and flooding, but there are no easy and accurate methods to measure soil ice content. A new heated probe sensor to measure soil ice content was field-tested. The in situ ice contents determined by the new sensor were sometimes unrealistically large or even negative. Using numerical model simulations, it was determined that accurate knowledge of the soil thermal conductivity is critical to computing accurate soil ice content using the heated probe sensor. Although this new soil sensor may ultimately be used to measure soil ice content for better predicting the likelihood of flooding and other adverse effects associated with soil freezing, improvements in measuring soil thermal conductivity are necessary.
Technical Abstract: Soil ice content impacts winter vadose zone hydrology. It may be possible to estimate changes in soil ice content with a sensible heat balance (SHB) method, using measurements from heat pulse (HP) sensors. Feasibility of the SHB method is unknown because of difficulties in measuring soil thermal properties in partially frozen soils. The objectives of this study are i) to examine the SHB method for determining in situ ice content and ii) to evaluate the required accuracy of HP sensors for use in the SHB method. HP sensors were installed in a bare field to measure soil temperatures and thermal properties during freezing/thawing events. In situ soil ice contents were determined at 60-min intervals with SHB theory. Sensitivity of the SHB method to temperature, heat capacity, thermal conductivity, and time step size was analyzed based on numerically produced soil freezing/thawing events. The in situ ice contents determined with the SHB method were sometimes unrealistically large or even negative. Thermal conductivity accuracy and time step size were the key factors contributing to SHB errors, while temperature and heat capacity accuracy had less influence. A 15-min SHB time step estimated ice content more accurately than did a 60-min time step. Sensitivity analysis indicated that measurement errors in soil temperature and thermal conductivity should be less than ±0.05°C and ±20%, but the error in soil heat capacity could vary by ±50%. Thus, improving the accuracy of thermal conductivity measurements and using short time steps are required to accurately estimate soil ice contents with the SHB method.