Seasonally frozen soil in alpine and subalpine zones in the mountains of Qinghai-Tibetan Plateau is particularly sensitive to global climate change. Therefore, a better understanding of the thermal properties of frozen soil is crucial for predicting the responses of frozen soils to soil warming. In this study, thermal properties of frozen soil with different moisture contents under subzero temperature (0 °C - 20 °C ) in an alpine forest in western Sichuan were analyzed by KD2 Pro in its cooling and heating processes, respectively. Our results reveal that the soil apparent volumetric specific heat capacity ( Cv ) and apparent thermal conductivity (K) under the same water content show similar response patterns to changing temperature lower than - 2 °C in both heating and cooling processes. Moreover, ice content of frozen soils can be well predicted by Logistic model in cooling and heating processes. The Cv and K tend to increase along with increasing soil moisture contents. Remarkably, asymptotic characters of the value of Cv and K are at the vicinity of the initial temperature of phase transitions, indicating that both Cv and K are particularly sensitive to changing soil temperature at the range of - 2 °C to 0 °C . Therefore, the widely distributed frozen soil layers with temperature above - 2 °C in alpine and subalpine zones over Qinghai-Tibetan Plateau are susceptible to the observed climate warming during cold season.
Approximately 50% of the land surface in high altitudes and latitudes of Northern Hemisphere is characterized by seasonally frozen ground, with more than 25% estimated to be covered by permafrost [
Phase changes of soil water below 0˚C lead to specific heat capacity change and latent heat transformation, hence total soil water, including both ice and liquid water, has profound influences on the soil hydraulic, thermal and mechanical properties [
In fact, soil thermal property is originated by a complex combination of conductive processes and intra-porous convection processes, besides they are related to soil water phase change. We then prefer to be more concerned with the apparent soil thermal properties and discuss the ice content of frozen soils. Recently, a heat pulse probe (HPP) method was developed and proven to be a promising technology for measuring soil apparent thermal properties, water and ice contents and snow density [
Permafrost and seasonally frozen soil in Qinghai-Tibetan Plateau and high mountains in the western China occupy the largest area of the high altitudinal lands of the Northern Hemisphere [
Soil sampling site is located at Mt. Mengbi (31˚33.25'N, 102˚24.92'E, elevation 3250 m ), which is located between the north section of Qionglai Mountains and Daxue Mountains in the northwest of Sichuan Province. The sampling site is dominated by the mountainous monsoon climate with a cool-rainy summer and cold-dry winter. The mean annual temperature and total annual precipitation are 8.6˚C and 760 mm , respectively. The alpine coniferous forest and mixed forest with the dominant species of Abies faxoniana and Picea balfouriana are distributed from elevation of 3000 m to 4300 m. The sampled soil is organic loamy sand and classified as mountain brown coniferous forest soils [
Forest topsoil (0 - 20 cm layer) is collected, homogenized, and sieved through a 2 mm mash screen to remove gravel, litter materials and fine roots. The sieved homogenized soil is repacked into cylindrical PVC columns with 5 cm in diameter and 20 cm in height keeping its original bulk density. The repacked soil columns are pre-incubated in situ under the natural conditions of alpine forest for three years. Soil samples were collected in July 2014. Before measurement, soil columns packed in PVC container were collected and placed into coolers and transported to laboratory within 24 hours. Soil columns for thermal property measurement were sampled and examined with an oven-dry bulk density of 0.91 g∙cm−3 and soil organic carbon content of 139.09 g∙kg−1.
Soil apparent thermal properties are measured by KD2 Pro Thermal Properties Analyzer (Decagon Devices, Inc., Pullman, WA). The KD2 Pro Analyzer takes measurements using the transient line heat source method [
Moisture contents of soil columns in PVC container were adjusted and balanced for 2 days, and determined accurately by calculating the weight and volume of soil columns after every measurement, and then wrapped up tightly and carefully by transparent soft film. Wrapped soil column with a certain moisture content was incubated in a thermostatic incubator. The incubation temperature was adjusted and balanced completely before measuring the soil thermal properties. Measurements were performed by inserting the KD2 Pro probe horizontally into the middle of soil samples with different moisture contents, soil volumetric specific heat capacity and thermal conductivity were recorded during the cooling process (from 0˚C to −20˚C) and subsequently a heating process(from −20˚C to 0˚C) for each soil sample. Three repeated measurements were conducted for each sample during both heating and cooling processes.
Using SPSS statistical software to analyze data.
Soil apparent volumetric specific heat capacity (
We observed a significant change for
We fitted an inverse function for the relationship between
where A and B are constants, t is soil temperature ranging from −20 to 0˚C, and
Ice, water and soil grain are three main phase diagrams impacting thermal properties of freezing soil [
Generally, the contribution of air and organic matter can be ignored [
where
ww | A | B | R2 | |
---|---|---|---|---|
Heating process | 0.551 | 2.103 | −3.246 | 0.961** |
0.514 | 2.311 | −2.211 | 0.936** | |
0.431 | 0.578 | −4.199 | 0.951** | |
0.378 | 1.331 | −1.736 | 0.962** | |
0.266 | 1.656 | −0.907 | 0.602** | |
Cooling process | 0.551 | 1.872 | −0.91 | 0.484** |
0.514 | 2.792 | −1.465 | 0.945** | |
0.431 | 0.712 | −3.34 | 0.997** | |
0.378 | −0.274 | −6.259 | 0.896** | |
0.266 | 0.656 | −3.303 | 0.935** |
Note: Ww is initial soil qualitywater contents, A and B are constants in Equation (1). * and ** indicate the variation explained by each model is significant at P < 0.05 level, and significant at P < 0.01 level.
Equation (2), when divided by the total mass of the soil sample, yield
where Ws, Ww and Wi are percentage quality of soil particles, water and ice, respectively.
Since the apparent volumetric specific heat capacity
where
So, Equation (3) can be rewritten in the following equation:
Then the ice content of frozen soils can be written as
When T < −10˚C, the vast majority of the unfrozen water is frozen within a few degrees of 0˚C. The unfrozen water content changes very slightly with the initial water content within the range of 1% when the temperature is lower than −10˚C [
Then
Generally, the values of
where
All of soils were regressed by Equation (9) for ice content prediction and the values of constants were presented in
Soil ecosystems in high altitude and latitude are regarded as higher temperature sensitivity to climate change [
Soil apparent thermal conductivity (K) decreases with the decrease of temperature and soil moisture contents below 0˚C in both cooling and heating processes (
We found that there is also a dramatic change of soil apparent thermal conductivity in the study region from −2˚C to 0˚C whether heating or cooling process (
ww | a | b | c | R2 | Sum of residual squares | |
---|---|---|---|---|---|---|
Heating process | 0.571 | 1.056 | 17.692 | 0.629 | 0.974 | 0.007 |
0.514 | 1.008 | 232.132 | 1.524 | 0.986 | 0.002 | |
0.431 | 1.114 | 990.085 | 1.978 | 0.941 | 0.006 | |
0.378 | 1.069 | 45.495 | 0.637 | 0.973 | 0.002 | |
0.266 | 1.024 | 114.479 | 0.481 | 0.962 | 0.002 | |
Cooling process | 0.571 | 1.224 | 22.172 | 1.267 | 0.939 | 0.008 |
0.551 | 1.086 | 5.011 | 0.345 | 0.848 | 0.032 | |
0.431 | 1.170 | 140.458 | 1.183 | 0.985 | 0.003 | |
0.378 | 1.089 | 5.968 | 0.281 | 0.999 | 0.000 | |
0.266 | 0.998 | 7.544 | 0.210 | 0.973 | 0.001 |
Note: ww is initial soil quality moisture contents; a, b and c are constants in Equation (9).
below 0˚C with the line heat source, but thermal conductivity data changed adversely in freezing process.
Seigo [
where A and B are constant, T is absolute temperature in the study region ranging from 100 kelvin units to 270 kelvin units, K stands for apparent thermal conductivity in freezing soil. In our study, however, the relationship between K and T is presented by the following equation,
where A and B are constant, T is temperature in centigrade (˚C) in the region ranging from −20˚C to 0˚C.
Values of constant A and B with different moisture contents in our study are presented in
considered most likely that constant A is the mean value of soil thermal conductivity when soil temperature is below −10˚C, and we also found that values of A decrease as soil initial water content decrease in cooling process.
ww | A | B | R2 | |
---|---|---|---|---|
Heating process | 0.551 | 1.179 | −0.254 | 0.830** |
0.514 | 0.905 | −0.174 | 0.678** | |
0.431 | 0.295 | −0.543 | 0.984** | |
0.378 | 0.326 | −0.140 | 0.914** | |
0.266 | 0.054 | −0.798 | 0.939** | |
Cooling process | 0.551 | 1.072 | −0.623 | 0.955** |
0.514 | 0.943 | −0.217 | 0.917** | |
0.431 | 0.359 | −0.256 | 0.998** | |
0.378 | 0.226 | −0.171 | 0.854** | |
0.266 | 0.136 | −0.330 | 0.792** |
Note: Ww is initial soil water contents, A and B are constants in Equation (11). * and ** indicate the variation explained by each model is significant at P < 0.05 level, and significant at P < 0.01 level.
We analyzed the relationship between the apparent volumetric specific heat capacity (
This study was funded by National Natural Science Foundation of China (NSFC, No. 41271094 and No. 40871124). We appreciate the anonymous reviewers for their excellent comments that benefit the improvement of this paper.
Hui Sun,Shuai Liu,Jihong Qin, (2016) Characterizing Subzero-Temperature Thermal Properties of Seasonally Frozen Soil in Alpine Forest in the Western Sichuan Province, China. Journal of Water Resource and Protection,08,583-593. doi: 10.4236/jwarp.2016.85048