Soil Temperature Distribution Research Articles

Surface water-groundwater interaction affects soil temperature distributions and variations in salt marshes

Soil temperature is an important environmental factor affecting biogeochemical processes in salt marshes and further wetland ecological functions. This study conducted field investigation and numerical simulations to examine temperature distributions and variations in a creek-marsh section subjected to estuarine and meteorological forcing conditions. The results showed that periodic tidal inundation induced water and heat exchange across sediment-water interfaces and thus modified diurnal temperature variations in shallow soil layers caused by meteorological fluctuations. The combination of periodic tidal inundation and sediment-atmosphere heat exchange led to the formation of three characteristic soil temperature zones along the creek-marsh section: a high-elevation marsh zone with diurnal temperature variations influenced by meteorological fluctuations, a near-creek zone where tide-induced porewater circulations regulated soil temperatures, and a low-elevation and sloping zone where sediment-atmosphere heat exchange decreased with the reduced marsh elevation. Sensitivity analyses demonstrated that a larger hydraulic conductivity facilitated the porewater circulation and heat transport in the near-creek zone, enhancing temperature fluctuations. An increased soil thermal conductivity intensified vertical heat exchange across sediment-water-atmosphere interfaces and increased the effect of estuarine and meteorological conditions on soil temperature. This study highlights the importance of surface water-groundwater interaction on temperature variations and helps to understand thermodynamics-dependent biogeochemical processes in salt marshes.

Earth-to-air Heat Exchanger for Cooling Applications in a Hot and Dry Climate: Numerical and Experimental Study

Shallow geothermal energy, by an earth-to-air heat exchanger (EAHE), is utilized to cool buildings with minimal energy usage. Significant parameters affecting the heat exchanger's performance must be investigated to obtain a suitable design. Shallow geothermal energy, by an earth-to-air heat exchanger (EAHE), is utilized to cool buildings with minimal energy usage. Significant parameters affecting the heat exchanger's performance must be investigated to obtain a suitable design. This article numerically and experimentally investigates the effect of pipe diameter, pipe length, inlet air temperature, soil temperature, airflow velocity, and soil thermal conductivity on the performance of the heat exchanger under hot and dry climate conditions. The soil temperature distribution was measured from the surface to a depth of 7 m in the city of Karbala (center of Iraq) in the summer season. The experimental test for EAHE was carried out in water-saturated soil and ambient air temperatures of 41 ºC, 45 ºC, and 49.5 ºC at four different velocities. The percentage drop in the EAHE outlet air temperature at 9 m/s was 28.3%, 25.5%, and 19.5%, respectively. Also, the three-dimensional model was created, and the simulation results were compared with the experimental results, which were in good agreement. An equation for the outlet air temperature was found as a function of pipe diameter and length, ambient air temperature, soil temperature around the pipe, and soil thermal conductivity. The resulted equation were compared with the current experimental results and experimental results of reported data inliterature. As a result, a very good agreement was observed. The results showed that the parameter L (length of the pipe) causes the strongest nonlinear behavior in the equation. For the cases considered, at diameters 75 and 100 mm, an approximate linear behavior for the length required to achieve a specific outlet temperature was observed. It can be concluded from the results that changing the soil type from dry one (k=0.5 W/m K) to saturated one (case of Karbala city, k=1.5 W/m K) resulted about 25% reduction in the length of the pipe. Also, the results showed that at an air velocity of 7m/s, the length required to obtain 26 ºC at the outlet of EAHE is 62.1 m which is 55% higher than the case of 29 ºC (39.9m).

Effect of freeze–thaw process on heat transfer and water migration between soil water and groundwater

Understanding the mechanisms of water migration and heat transfer in frozen soil is essential for the selection of water resource management measures and determination of irrigation schemes for winters. However, the dynamics of the groundwater level and temperature caused by freezing and thawing and their impact on soil water redistribution remain unclear. In this field study, the mechanisms of water migration and heat transfer in the soil during the freeze–thaw period under the combined action of the surface temperature change (upper boundary) and groundwater level fluctuation (lower boundary) were comprehensively considered with the theory and method of heat transfer, freeze–thaw characteristic curves of soil, and water balance model. The results showed that: (1) The soil temperature distribution and groundwater temperature dynamics can be interpreted intuitively by the soil temperature gradient and accumulated heat storage. (2) The freeze–thaw characteristic curves of soil indicated that the soil temperature dynamics caused a phase transition of the soil water. This further changed the soil water potential and permeability by 1–2 magnitudes and 1–5 magnitudes, respectively, which affected the direction and quantity of water migration. (3) The water balance quantitatively revealed the controlling effect of groundwater level changes caused by freezing and thawing on soil moisture dynamics. About 20% of the ice in the frozen layer was formed by the upward recharge of the groundwater during the freezing period. In the thawing period, most of this water returned to the groundwater (84%), while the rest (16%) increased the soil water content. These results systematically explain the mechanisms of heat transfer and water migration between the soil water and the groundwater during the freeze–thaw period, providing a theoretical reference for agricultural water management in seasonally frozen regions.

Analytic model for calculation of soil temperature and heat balance of bare soil surface in solar greenhouse

This study establishes a soil temperature model of a solar greenhouse, which takes into account the soil water evaporation, solar radiation, thermal radiation and convective heat transfer on the soil surface. The model is utilized to obtain the mathematical function of soil temperature distribution according to solar radiation and air temperature in a solar greenhouse. The model is validated in two greenhouses with and without plants. Based on the model, the heat gain and loss of solar greenhouse soil are calculated. The results show that the heat accumulated by the greenhouse soil is mainly by absorbing solar radiation during the day and the heat lost by the greenhouse soil is mainly by water evaporation during the day and thermal radiation at night. Then, according to the small convective heat transfer between soil and air in the solar greenhouse, the solar-air temperature calculation equation is simplified, so that the simplified model can predict the soil temperature based on solar radiation and outdoor air temperature, which provides an important soil boundary condition for the solar greenhouse thermal model and CFD prediction. Finally, the evaporation coefficient is proposed to characterize the effect of water evaporation intensity on the soil temperature in a solar greenhouse. In this case, the average soil temperature increases by about 2℃ for every 0.1 decrease in evaporation coefficient.

Temperature Distributions in Geogrid-Reinforced Soil Retaining Walls Subjected to Seasonal Freeze–Thaw Cycles

Temperature Distributions in Geogrid-Reinforced Soil Retaining Walls Subjected to Seasonal Freeze–Thaw Cycles

Determination of the coefficient of effective thermal diffusivity of soil under natural changes in environmental conditions

An experimental setup and a data processing technique have been developed to obtain the soil temperature distribution in depth and in time. The results of measuring the soil temperature during 2021 in the city of Ramenskoye, Moscow Region at depths from 0 to 1.475 m are obtained and presented. An analytical solution of the problem of heat transfer in the soil based on the superposition principle is shown and verified. The coefficient of effective thermal diffusivity of soil a = 0.99 · 10−6 m2/s was determined indirectly in a passive experiment. Conclusions are drawn about the coincidence with the results of the experiment of the asymptotic nature of the dependence of the analytical solution for determining the amplitude of soil temperature fluctuations on depth and linear for the phase. An analytical solution is presented to obtain the temperature distribution in the soil with the known law of temperature fluctuations on the surface and the coefficient of effective thermal diffusivity.

Modelling Soil Water, Salt and Heat Dynamics under Partially Mulched Conditions with Drip Irrigation, Using HYDRUS-2D

Drip irrigation under mulch is a widely used technique in the arid region of northwest China. The partially mulched soil and the bare strips between mulched areas may complicate the migration of water, salt, and heat in soils, and cause lateral salt accumulation on bare soil surfaces. For investigating hydrothermal dynamics and salt distribution patterns under such circumstances, tank experiments with drip irrigation under plastic film on partially mulched soil were conducted under two intensities of drip irrigation (i.e., low (W1) and high (W2)) with the same total irrigation amount. The spatial distributions of soil water, temperature, and electrical conductivity were monitored accordingly. The two-dimensional (2D) model of soil water, salt, and heat transport under drip irrigation and partially mulched soil conditions was established using HYDRUS-2D, and kinetic adsorption during salt migration was considered. The results of the experiments showed that the uneven distribution of the hydrothermal state led to the accumulation of salt on the un-mulched soil surface. Water migrated from where the dripper was located, and heat accumulated mainly in the mulched soil. HYDRUS-2D matched reasonably well with the observed data, with an R2 higher than 0.54. Under the partially mulched conditions, lower intensity of drip irrigation (W1) show higher desalination efficiency in root zones, with less even lateral salt distribution. Scenario simulations further demonstrated that a larger total irrigation amount would result in a larger desalination zone, and drip irrigations with appropriate incremental intensity could improve salt leaching in the root zone with increased lateral migration of water.

Effects of plastic film mulching on the spatiotemporal distribution of soil water, temperature, and photosynthetic active radiation in a cotton field.

Plastic film mulching (PFM) affects the spatiotemporal distribution of soil moisture and temperature, which in turn affects cotton growth and the spatiotemporal distribution of canopy photosynthetically active radiation (PAR). Due to the spatial heterogeneity of soil moisture, temperature and limited monitoring methods, the issues such as relatively few sampling points and long sampling intervals in most existing studies prevent the accurate quantification of spatiotemporal changes in moisture and temperature along soil profile. To investigate the effects of PFM on spatiotemporal changes in soil moisture, temperature, and canopy PAR in cotton fields, two field trials of plastic film-mulched (M) and nonmulched (NM) cultivations were performed in 2018 and 2019. The grid method was used for the soil information continuous monitoring and multiple-time fixed-site canopy PAR monitoring during the duration of cotton growth. Two-year field trial data showed that, M cultivation increased soil moisture by approximately 13.6%–25% and increased temperature by 2–4 °C in the 0–50 cm soil layer before the first irrigation (June 20) and by 1–2 °C in the 70–110 cm soil layer, compared with NM cultivation. In addition, the temperature difference between the two treatments gradually decreased with the increase in irrigation and air temperature. The M treatment reached the peak PAR interception rate 10 days earlier than the NM treatment. In 2018 and 2019, the PAR peak value under the M treatment was 4.62% and 1.8% higher than that under the NM treatment, respectively, but the PAR interception rate was decreased rapidly in the late growth stage. Overall, PFM had an effect on soil moisture retention during the whole growth period and greatly increased the soil temperature before budding stage, thus promoted the early growth of cotton. Considering this, we suggest that the irrigation quota and frequency could be appropriately decreased in the case of plastic film mulching cultivation. For nonmulching cultivation, the irrigation quota and frequency should be increased, and it is necessary to take measures to improve the soil temperature before middle July.

Heat transfer of a large-scale water pit heat storage under transient operations

An accurate and less time demanding model is required when integrating pit thermal energy storage (PTES) into solar heating systems. Multi-node (1D) models are commonly used, but these models face challenges when calculating PTES thermal stratification and heat loss. Therefore, a full-scale computational fluid dynamics (CFD) model of PTES inclusive water and soil regions is developed using FLUENT to improve the accuracy of heat transfer calculation of a multi-node model. The CFD model is validated against the Dronninglund PTES measurements regarding PTES thermal stratification, inlet/outlet energy flow, and soil temperature distribution. The model corresponds well to the measurements in three aspects: (i) a maximum temperature difference of 1 K in the water region; (ii) a maximum temperature difference of 2 K in the soil region; (iii) a maximum outlet temperature difference of 3 K. An indicator RΔT/δ defined as the ratio between the thermocline temperature difference and the thermocline thickness is proposed to assess suitable grid size for PTES models, and the quantitative relationship between RΔT/δ and grid size is recommended. Investigations with a range of grid sizes show that by using the recommended grid size, the prediction accuracy of the multi-node model TRNSYS Type 343 is significantly improved. The root mean square deviations of the predicted MIX number are decreased by 11–43 % for different years, and the relative differences of the monthly charge/discharge energy from the measurement are within 5 %. The findings of this study provide guidance for selecting appropriate grid sizes to achieve better calculation accuracy for large-scale PTES.

Simulations and Experiments of Soil Temperature Distribution after 2.45 GHz Long-Term Microwave Treatment

Soil disinfection is an important agronomic measure to prevent soil-borne diseases, insects, weeds and other hazards. Based on the premise of being environmentally friendly, microwave soil disinfection can improve crop yield and quality in a pollution-free, residue-free and green way. The law of microwave soil heating is the theoretical basis of microwave soil disinfection. Therefore, in this paper, loess soil of North China and black soil of Northeast China are used as test materials to explore the law of soil heating under the action of microwaves. First, COMSOL Multiphysics software was used to simulate the temperature field change in the microwave-irradiated soil, and a simulation model of the temperature field of the microwave-irradiated soil was constructed to analyze the effective working range and temperature distribution characteristics of the microwave-irradiated soil. Second, using the 2.45 GHz microwave treatment, the following conditions were tested: soil moisture conditions of 10%, 15%, 20% and 23% (within the natural moisture content range). The loess and black soil were treated by microwave irradiation for 1~12 min, respectively (1 min/time increment). A single-factor experiment was designed to explore the influence of these factors on the soil heating law. The results show that the two soil surface temperatures are positively correlated with the soil moisture content, both of which satisfy Tsurface23% > Tsurface20% > Tsurface15% > Tsurface10%, and the surface temperature of black soil is higher than that of loess. According to the experimental results of the internal temperature distribution of loess and black soil irradiated by microwaves, the surface equations of “irradiation time–soil depth-soil temperature” and “irradiation time–soil moisture content–soil temperature” were constructed by surface fitting. When the irradiation time and moisture conditions are the same, the average temperature inside the irradiation area satisfies T¯α black soil > T¯α loess. The results of long-term microwave soil heating preliminarily confirmed the feasibility of microwave soil disinfection and the optimal conditions of microwave irradiation in loess of North China and Northeast black soil, which provides a certain reference for the study of soil-borne diseases inactivation at high temperature.

Numerical Simulation of Soil Temperature With Sand Mulching During the Growing Season of Spring Wheat

The thermal conditions of soil are important in practical agricultural production. The characteristics of heat flux, moisture content, thermal conductivity, and other soil parameters vary with temperature. This study uses VADOSE/W to create a model of heat transmission between soil and atmosphere, simulating daily changes in soil temperature using sand mulching. By using the published data to verify the model, the results show that the fitting effect is good and the reliability of the model is verified. We also used this model to determine the temporal and spatial distributions of soil temperature, temperature differences, and a temperature gradient for sand mulching and bare soil during the growing season of spring wheat. These results indicated that the sand mulching preserved heat at night. The difference in temperature at each depth and the temperature gradient became negative with the gradual increase in solar altitude. The difference in soil temperature at each depth and the temperature gradient tended to become positive by 22:00. These results indicated that the sand mulch effectively maintained the soil temperature in the morning and night during the growing season and impeded the transfer of heat at mid-day. This study provides a new method for determining the transfer of heat in sand-mulched soil, which can guide the effective regulation of soil temperature.

The Study of Soil Temperature Distribution for Very Low-Temperature Geothermal Energy Applications in Selected Locations of Temperate and Subtropical Climate

The publication presents the results of research on soil temperature distribution at a depth of 0.25–3 m in three measurement locations. Two boreholes were located in Białystok in the temperate climatic zone and one measuring well was installed in Belmez in the subtropical climatic zone. Measurements were made in homogeneous soil layers in sand (Białystok) and in clay (Białystok and Belmez). Based on the results of the measurements, a simplified model of temperature distributions as a function of depth and the number of days in a year was developed. The presented model can be used as a boundary condition to determine heat losses of district heating pipes located in the ground and to estimate the thermal efficiency of horizontal heat exchangers in very low-temperature geothermal energy applications.

Soil Thermal Response to Temperature Cycles and End Boundary Conditions of Energy Piles

This paper explores the influence of building cover and pile toe boundary conditions on ground temperature distributions surrounding energy piles. Experimental and numerical studies were conducted on two isolated cast-in-place energy piles installed in dense unsaturated sand, one exposed to the atmosphere at the ground surface (diameter=0.6 m and length=16.1 m) and the other installed under a six-story building (diameter=0.6 m and length=10 m). Investigations were conducted for monotonic heating and daily cyclic temperature changes of the piles ranging between 10°C and 35°C. The changes in ground temperature decreased as radial distance increased from the edge of both piles. Cyclic temperatures in both piles induced lower ground temperature changes and decreased the radial thermal influence zone compared with monotonic heating. However, the radial thermal zone in cyclic operating mode can be influenced by different ratios of heating to cooling times and hence should be selected carefully to avoid unexpected ground temperature changes. Atmospheric effects were observed up to a depth of 2 m for the energy pile exposed to the atmosphere. The insulation provided by the building footprint slightly decreased the impacts of ground-atmosphere interaction on the soil temperature distribution with depth near the surface compared to the energy pile exposed to the atmosphere. The ground temperature variations were dominant along the length of the heat exchanger loops for both piles. However, they were negligible near the pile toe below the heat exchanger loops for both piles.

An inversion method to estimate the thermal properties of heterogeneous soil for a large-scale borehole thermal energy storage system

Borehole thermal energy storage (BTES) is a technology in which the thermal energy generated over summer may be collected and stored in soil for extraction in winter. The difficulty of evaluating the thermal conductivity and volumetric heat capacity of heterogeneous soil introduces considerable uncertainty in BTES system design and operation. This study proposes a method to estimate the thermal properties of heterogeneous soil in a BTES system based on the modified duct ground heat storage model (DST) model. The inversion estimation method was verified using long-term monitoring data of soil temperature distribution and fluid temperatures from a large-scale BTES project in Chifeng, China. Results show that the layered thermal properties of heterogeneous soil led to the irregular distribution of soil temperature at different depths. The cumulative heat storage prediction error decreased from 27.8% to 3.7% when utilizing the proposed method relative to a thermal response test (TRT) - based parameter estimation. The influence of inaccurate estimates of soil thermal properties on the scaled expansion of BTES systems was also examined, and with results emphasizing the importance of correctly estimating soil thermal properties for BTES system design.

Thermal properties of Cambisols in mountain regions under different vegetation covers

Soil thermal properties regulate the thermal and water balance and influence the soil temperature distribution. The aim of the current study is to present data on the changes in the thermal properties of Cambisols at different ratios between the water content and the air in the pore space under different vegetation covers in mountain regions. The undisturbed soil samples were taken from the surface soil layers under grassland, deciduous and coniferous forests in three experimental stations of the Forest Research Institute – Gabra in Lozen Mountain, Govedartsi in Rila Mountain and Igralishte in Maleshevska Mountain. The soil thermal conductivity (λ), the thermal diffusivity (α) and the volumetric heat capacity (C<sub>v</sub>) were measured with the SH-1 sensor of a KD2Pro device at different matric potentials in laboratory conditions. The thermal conductivity of the investigated soils was also measured with the TR-1 sensor of a KD2Pro device at the transitory soil moisture in field conditions. An increase in the thermal properties with the soil water content was best pronounced for λ and depended inversely on the total porosity. As the total porosity increased with the soil organic carbon content and decreased with the skeleton content, the lowest value of λ was established in the surface horizons of Dystric Cambisols (Humic) in the experimental station in Govedartsi. The soil thermal conductivity increased with the depth under the deciduous forest (Gabra and Igralishte) due to the lower soil organic carbon content (SOC) and the total porosity. There were no such changes in the subsurface horizon under the grassed associations. The increase in the heat capacity with the water content depended on the SOC to less extent. In the horizons with a SOC of less than 1.5%, the changes in the thermal diffusivity over the whole range of wetness were 1.7 times higher than those with a higher SOC.

Experimental study on the thermal imbalance and soil temperature recovery performance of horizontal stainless-steel ground heat exchanger

Soil thermal imbalance has become a common problem for ground source heat pump (GSHP) system, leading to the fluctuation of ground temperature and decline of heat transfer efficiency. However, compared with vertical ground heat exchanger (GHE), there is a lack of research and evaluation on the soil thermal imbalance for horizontal GHE. Therefore, in this paper, to investigate the extent of soil thermal imbalance and its effects on performance of GSHP system, a sandbox experiment embedded with horizontal stainless-steel GHE was implemented. Various operation conditions were designed to explore effects of intermittent time, water content (rainfall) and buried depth of pipes on soil temperature distribution and restoration performance. The experimental data was validated by a numerical simulation. The results indicated that 5-day intermittent operation of GSHP leads to an increase of 6.41 ℃ in soil temperature. After 2-day natural recovery, soil temperature recovery rate can only reach 0.72. Increasing soil moisture content and intermittent control are effective solutions to mitigate thermal accumulation and improve thermal performance. The heat transfer performance of GHE with an intermittent ratio of 6:18 which GSHP system runs for 6 h and stops for 18 h during a day is 33% higher than that with the intermittent ratio of 14:10 and 10:14. The higher the water content, the lower the soil temperature fluctuation and the stronger the soil temperature recovery capacity. This can be attributed to the increase of soil thermal conductivity and capacity, while the decrease of thermal diffusivity.

Thermal performance of ground heat exchanger under winter conditions

The use of buried pipe ground source heat pump technology can provide a green and environmentally friendly solution for the winter bridge snow melting project. However, the factors affecting the heat transfer efficiency of buried pipes in winter conditions are still unclear. For the single U-bend buried pipe heat exchanger, numerical calculations were preformed to study the effects of different inlet temperatures, backfill material ratio, hole depth, and spacing on the heat extraction capacity and the surrounding soil temperature distribution. Combined with an actual bridge heating project, three single heat exchanger models with different buried depths and three heat exchanger group models with different spacing were established. The results show that when the system runs for 48 hours, the heat transfer power at 2 ℃ will increase by about 82% compared with that at 8 ℃. Reducing the content of bentonite in the backfill is beneficial to heat transfer. As the heat exchanger depth increases, the heat extraction capacity per linear meter decreases. The expansion of the heat exchanger spacing can reduce the thermal interference effect between each other. When running 8 hours, the heat transfer power at an interval of 4 m is increased by 5.4% compared with that at the interval of 3 m, and the heat transfer power at an interval of 5 m is increased by 1.9% compared with that at the interval of 4 m.

Statistical understanding for snow cover effects on near-surface ground temperature at the margin of maritime Antarctica, King George Island

Snow cover plays an important role in water supply through melting of snow/ice in polar ecosystem and environments, in particular, Antarctica. Although a site access to Antarctica is high-priced, measurements of ground surface temperature (GST) using small self-recording temperature sensors (iButtons) can provide a powerful and relatively inexpensive approach to trace the spatial and temporal distributions of soil temperature and in addition, absence/presence of snow cover. In this study, principal component analysis (PCA) was utilized to explain major patterns of GST from 128 sites at King George Island in maritime Antarctica. Variations of GST were monitored between December 2011 and January 2013. The iButtons were initially installed in snow free areas in the austral summer of 2011. Principal components 1 and 2 were associated with air temperature and snow cover, respectively. Both PCs showed good correlations with the mean GST of JJA (June to August), not with that of DJF (December to January). Based on the results excluding an outlier, PCA divided the 127 GST observations into three groups effectively: absence of snow cover (Group 1), intermittent snow cover (Group 2), and presence of snow cover (Group 3). Snow cover can supply water to the ecosystem and the GST pattern of Group 2 may imply an abundance and high productivity of mosses in the study area. Using approaches suggested by previous studies, snow cover showed up nine days (days 317–326), and the melt-out date was day 326 (KG125 in Group 2). The GST data and statistical approaches used in this study can be useful in other GST studies, particularly, for both polar regions.

Simulation of maize crop growth using an improved crop model considering the disintegrated area of biodegradable film

• An improved crop growth model (FCM) under biodegradable film mulching was developed. • FCM model performed well in capturing the temporal trends in crop growth dynamics. • Biodegradable film mulching results in a slight decrease of crop yield. • An irrigation depth of 22.5 mm as optimal treatment was recommended in arid region. • Disintegrated rate should not exceed 63 % and 38 % in 2018 and 2019, respectively. Biodegradable film is recognized as a promising alternative to plastic film within crop mulching. This is because biodegradable film disintegrates into water and carbon dioxide through soil microorganisms-facilitated catalytic decomposition during the crop growth season. However, biodegradable film mulching (BFM) also has some disadvantages, such as the water and heat conservation being poor during nighttime period. Crop growth and yield is reduced by a suboptimal distribution of soil water and temperature. Therefore, there is value in assessing the effects of BFM on crop growth during daytime and nighttime periods, and particularly under different irrigation levels. This study developed a crop growth model (FCM) considering the disintegrated area of biodegradable film mulching to capture crop growth dynamics under different soil water contents and mulching areas. The disintegrated area of biodegradable film was determined by an image method. The model was firstly calibrated and validated using observed data collected over two years (2018 and 2019) under three mulching treatments [plastic film mulching (PFM), biodegradable film mulching (BFM), and no film mulching (NFM)] and three irrigation levels (irrigation depths of 30, 22.5, and 15 mm). The model was then used to evaluate crop growth rate ( k ), crop yield ( CY ), and water use efficiency (WUE) during daytime and nighttime periods. The results showed that the simulation accuracy of the FCM model was sufficiently high under different treatments, with a root mean square error ( RMSE) of 284.6 kg ha –1 . The benefits of BFM for crop growth were similar to that of PFM under a low range of effective accumulated soil temperature (EAST). However, differences in crop growth variables between PFM and BFM increased with an increase in EAST. Average leaf area index ( LAI ) and aboveground dry biomass ( AB ) during both years increased by 16.9 % and 39.2 % under PFM compared to that under BFM, respectively. In general, the harvest index ( HI ), CY , and water use efficiency (WUE) under BFM were lower than that under PFM. There were comparable variations in crop growth under different irrigation levels. The highest WUE occurred in the BFM 22.5 treatment at 44.5 kg ha –1 mm –1 . Therefore, this study recommends an irrigation depth of 22.5 mm as an optimal irrigation treatment in an arid region.

Monitoring of soil moisture and temperature distributions in seasonally frozen ground with fiber optic sensors

The evolution of moisture and temperature fields plays an important role in governing soil behavior, which heavily influences the performance of infrastructures built on the ground soil. However, conventional geotechnical instrumentation presents many limitations in the monitoring of seasonally frozen soils in cold regions. This study investigates the feasibility of actively heated fiber Bragg grating (AH-FBG) in measuring the temperature, water content, and ice content of seasonally frozen soil through laboratory tests and field monitoring. The working principle of the AH-FBG monitoring system is introduced in detail. A series of laboratory calibration tests were performed on soil specimens to explore its performance. Long-term in-situ monitoring was carried out on the Loess Plateau in northwestern China. Results show that the AH-FBG technique can be applied to measure the total water content of soils using the Côté and Konrad model. In the field monitoring, the freezing-thawing process of frozen soil was captured by measuring soil temperatures and ice contents during field monitoring, and the maximum freezing depth was deduced. The AH-FBG technique is verified to be a powerful tool in estimating the actual behavior of in-situ ground soil and provides rich information on multi-field interactions in cold regions.