Sensitivity analysis of temperature changes for determining thermal properties of partially frozen soil with a dual probe heat pulse sensor

Determining thermal conductivity (λ) and volumetric heat capacity (C) of partially frozen soils with a dual probe heat pulse (DPHP) sensor is challenging because an applied heat pulse melts ice surrounding the heater probe. Examining DPHP temperature changes with a commonly-used analytical solution that only accounts for heat conduction leads to inaccurate λ and C estimates for partially frozen soils at temperatures between −5°C and 0°C. In order to determine λ and C accurately and simultaneously, it is necessary to understand how various properties of partially frozen soil influence the temperature changes produced by DPHP sensors. The objective of this study is to determine the sensitivity of DPHP temperature changes to soil conditions and soil thermal properties. A numerical solution for radial heat conduction with soil freezing and thawing is developed. A series of simulations are performed, in which various errors are imposed onto a selected model parameter while other model parameters are held constant, and sensitivity coefficient values (φ) of the time of maximum probe temperature (tm) and of the maximum probe temperature rise (Tm) for each parameter are calculated. Temperature changes at the measurement probe are quite sensitive to initial soil temperature (φ values for tm and for Tm are −0.99 and 0.99, respectively), λ (φ value for tm is −0.93), and parameters determining the shape of the soil freezing characteristic (FC) curve, i.e., saturated water content θs (φ values for tm and for Tm are 0.59 and −0.73, respectively) and n (φ values for tm and for Tm are −2.7 and 2.4, respectively). Temperature changes are not very sensitive to C (φ values for tm and for Tm are 0.034 and −0.15, respectively). Although previous investigations tried to determine C by inverse analysis, this sensitivity analysis shows that the influence of C on temperature response to a heat pulse is masked by that of the FC. Thus, λ and FC parameters are the best candidate parameters to be determined by inverse analysis of DPHP data. This new result will guide further testing of DPHP sensors in partially frozen soils

A new thermo-time domain reflectometry approach to quantify soil ice content at temperatures near the freezing point

Soil ice content (θi) is an important property for many studies associated with cold regions. In situ quantification of θi with thermo-time domain reflectometry (TDR) at temperatures near the freezing point has been difficult. The objective of this study is to propose and test a new thermo-TDR approach to determine θi. First, the liquid water content (θl) of a partially frozen soil is determined from a TDR waveform. Next, a pulse of heat is applied through the thermo-TDR sensor to melt the ice in the partially frozen soil. Then, a second TDR waveform is obtained after melting to determine the θl, which is equivalent to the total water content (θt ) of the partially frozen soil. Finally, θi is calculated as the difference between θt and θl. The performance of the new approach was evaluated in sand and loam soils at a variety of θ t values. The new approach estimated θt , θl, and θi accurately. The root mean square errors (RMSE) of estimation were 0.013, 0.020, and 0.023 m3 m−3 for sand, and 0.041, 0.026, and 0.031 m3 m−3 for loam. These RMSE values are smaller than those reported in earlier thermo-TDR studies. Repeating the thermo-TDR measurements at the same location on the same soil sample caused decreased accuracy of estimated values, because of radial water transfer away from the heater tube of the thermo-TDR sensor. Further research is needed to determine if it is possible to obtain accurate repeated measurements. The use of a dielectric mixing model to convert the soil apparent dielectric constant to θl improved the accuracy of this approach. In our investigation, application of a small heat intensity until the partially frozen soil temperature became larger than about 1°C was favorable. The new method was shown to be suitable for estimating ice contents in soil at temperatures between 0°C and -2°C, and it could be combined with the volumetric heat capacity or thermal conductivity thermo-TDR based methods, which measured ice content at colder temperatures. Thus, the thermo-TDR technique could measure θi at all temperatures

Comparison of Closed Chamber and Eddy Covariance Methods to Improve the Understanding of Methane Fluxes from Rice Paddy Fields in Japan

Greenhouse gas flux monitoring in ecosystems is mostly conducted by closed chamber and eddy covariance techniques. To determine the relevance of the two methods in rice paddy fields at different growing stages, closed chamber (CC) and eddy covariance (EC) methods were used to measure the methane (CH4) fluxes in a flooded rice paddy field. Intensive monitoring using the CC method was conducted at 30, 60 and 90 days after transplanting (DAT) and after harvest (AHV). An EC tower was installed at the centre of the experimental site to provide continuous measurements during the rice cropping season. The CC method resulted in CH4 flux averages that were 58%, 81%, 94% and 57% higher than those measured by the EC method at 30, 60 and 90 DAT and after harvest (AHV), respectively. A footprint analysis showed that the area covered by the EC method in this study included non-homogeneous land use types. The different strengths and weaknesses of the CC and EC methods can complement each other, and the use of both methods together leads to a better understanding of CH4 emissions from paddy fields.Peer Reviewe

Pore-Scale Wetting Process of Capillary-Driven Flow in Unsaturated Porous Media under Micro- and Earth-Gravities

Microgravity hinders capillary-driven water flow in unsaturated porous media. Previous studies proposed pore-scale phenomena such as “air entrapment”, “particle separation”, and “interruption on widening void space” to explain gravity-dependent capillary-driven flows. Our objectives were: (1) to measure the water flux densities of the pore-scale capillary-driven flow in micro- and Earth-gravities and (2) to reveal that what makes water flow slower under microgravity than under 1 G. We found that average macroscopic water flux densities had no significant difference under micro- and Earth-gravities (p = 0.30). We did not observe “air entrapment” in the pore spaces of porous media. “Widening on a single particle” and “capillary widening” disturbed capillary-driven flow; however, “widening on a single particle” had no significant gravity dependency. “Capillary widening” may be independent of gravity, since it was observed both under microgravity and under 1 G. Water flux densities in unsaturated porous media may have gravity dependency induced by “particle separation” only when porosity is large enough to allow particles to move

Carbon sequestration and yield performances of Miscanthus × giganteus and Miscanthus sinensis

The demand for renewable energy resources, such as Miscanthus spp., has increased significantly in temperate regions. Miscanthus spp. has the potential to mitigate GHG emissions by replacing fossil fuels and sequestering carbon in soil. However, the biomass yield and C sequestration performance of Miscanthus spp. varies by climate, soil type, management practices, and land-use history. Therefore, there is a need for site-specific research on biomass yield and C sequestration of Miscanthus × giganteus (M × g). An alternative to M × g is the seed-propagated Miscanthus sinensis which could be another option as a bioenergy crop. The objective of this study was to assess the C sequestration rates and biomass yield performance of these two Miscanthus spp. The C sequestration rates were measured in side-by-side comparison plots using the 13C natural abundance technique. The results revealed C sequestration rates of M × g and M. sinensis were 1.96 ± 0.82 and 0.99 ± 0.21 Mg C ha−1 year−1 over 6 years. The average biomass yields of M × g and M. sinensis from 2010 to 2015 were 25.6 ± 0.2 and 31.2 ± 0.5 Mg ha−1 year−1, respectively. It can be concluded that M. sinensis has a higher yield while M × g has a higher potential for immediate C sequestration in cool regions such as Northern Japan