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[AGE05-04] Quantifying soil ice content with a heat pulse probe for an entire range of temperature during soil freezing and thawing
★Invited papers
Soil freezing and thawing is important for winter hydrology. Despite its importance, measuring in-situ soil ice content θI has been difficult. Volumetric heat capacity measurement with a heat pulse probe (HPP) has been used to quantify θI (hereafter, VHC method). The VHC method determines θI only when soil temperature is below -5°C. In this study, we propose a new method to determine θI from HPP by considering sensible heat balance in soils (hereafter, SHB method). We tested both VHC and SHB methods for θI determination.
A HPP measures soil temperature T, volumetric heat capacity C, and thermal conductivity λ. For the VHC method, only C is used to determine θI. For the SHB method, a HPP is inserted into soil such that each needle is located at a different depth. When the heat balance of a thin soil layer which has boundaries at the middle of each HPP needle is considered, there is conductive heat flux at the first boundary H1, conductive heat flux at the second boundary H2, change in sensible heat storage ΔS, and latent heat flux L, i.e., H1-H2-ΔS=L. H1, H2 and ΔS can be estimated from HPP measurements and equations, thus, L can be calculated. When T is < 0°C, L is associated with soil freezing and thawing. Thus, change in θI can be determined by dividing L by latent heat for water freezing Lf. θI can be determined by integrating ΔθI with respect to time once T drops below 0 °C.
Soil was packed into 0.3 m long PVC columns with 0.28 m3 m-3 water content. A HPP was inserted through the column wall. Additional columns were prepared for destructive sampling to determine total soil water content after soil freezing. Upper boundary temperature was initially 5°C, and then it was decreased to -15°C gradually within 24 hours. After 6 days, the temperature was increased to 5°C within 24 hours. The temperature for the lower boundary was maintained at 5°C. Transient θI was estimated with VHC and SHB methods.
θI determined by sampling was around 0.20 m3 m-3. θI estimated with the VHC method was close to 0.20 m3 m3 when T was < -5 °C. The SHB method could additionally estimate transient θI when T was between 0 and -5 °C but failed at T < -5°C. Thus, we measured θI for a whole T range by using the SHB method with T between 0 and -5°C and using the VHC method with T < -5°C.
A combination of SHB and VHC methods allowed determination of transient θI for the entire range of temperature during freezing. Accordingly, a HPP can be a useful sensor for monitoring θI under freezing and thawing conditions.
A HPP measures soil temperature T, volumetric heat capacity C, and thermal conductivity λ. For the VHC method, only C is used to determine θI. For the SHB method, a HPP is inserted into soil such that each needle is located at a different depth. When the heat balance of a thin soil layer which has boundaries at the middle of each HPP needle is considered, there is conductive heat flux at the first boundary H1, conductive heat flux at the second boundary H2, change in sensible heat storage ΔS, and latent heat flux L, i.e., H1-H2-ΔS=L. H1, H2 and ΔS can be estimated from HPP measurements and equations, thus, L can be calculated. When T is < 0°C, L is associated with soil freezing and thawing. Thus, change in θI can be determined by dividing L by latent heat for water freezing Lf. θI can be determined by integrating ΔθI with respect to time once T drops below 0 °C.
Soil was packed into 0.3 m long PVC columns with 0.28 m3 m-3 water content. A HPP was inserted through the column wall. Additional columns were prepared for destructive sampling to determine total soil water content after soil freezing. Upper boundary temperature was initially 5°C, and then it was decreased to -15°C gradually within 24 hours. After 6 days, the temperature was increased to 5°C within 24 hours. The temperature for the lower boundary was maintained at 5°C. Transient θI was estimated with VHC and SHB methods.
θI determined by sampling was around 0.20 m3 m-3. θI estimated with the VHC method was close to 0.20 m3 m3 when T was < -5 °C. The SHB method could additionally estimate transient θI when T was between 0 and -5 °C but failed at T < -5°C. Thus, we measured θI for a whole T range by using the SHB method with T between 0 and -5°C and using the VHC method with T < -5°C.
A combination of SHB and VHC methods allowed determination of transient θI for the entire range of temperature during freezing. Accordingly, a HPP can be a useful sensor for monitoring θI under freezing and thawing conditions.