Water and heat transport in hilly red soil of southern China:I. Experiment and analysis

Studies on coupled transfer of soil moisture and heat have been widely carried out for decades. However, little work has been done on red soils, widespread in southern China. The simultaneous transfer of soil moisture and heat depends on soil physical properties and the climate conditions. Red soil is heavy clay and high content of free iron and aluminum oxide. The climate conditions are characterized by the clear four seasons and the serious seasonal drought. The great annual and diurnal air temperature differences result in significant fluctuation in soil temperature in top layer. The closed and evaporating columns experiments with red soil were conducted to simulate the coupled transfer of soil water and heat under the overlaying and opening fields’ conditions, and to analyze the effects of soil temperature gradient on the water transfer and the effects of initial soil water contents on the transfer of soil water and heat. The closed and evaporating columns were designed similarly with about 18 °C temperatures differences between the top and bottom boundary, except of the upper end closed or exposed to the air, respectively. Results showed that in the closed column, water moved towards the cold end driven by temperature gradient, while the transported water decreased with the increasing initial soil water content until the initial soil water content reached to field capacity equivalent, when almost no changes for the soil moisture profile. In the evaporating column, the net transport of soil water was simultaneously driven by evaporation and temperature gradients, and the drier soil was more influenced by temperature gradient than by evapo- ration. In drier soil, it took a longer time for the temperature to reach equilibrium, because of more net amount of transported water.

Coupled Transfer of Water and Heat in Red Soil: Experiment and Numerical Modelling

Coupled transfer of soil water and heat in closed columns of homogeneous red soil was studied under laboratory conditions. A coupled model was constructed using soil physical theory, empirical equations and experimental data to predict the coupled transfer. The results show that transport of soil water was affected by temperature gradient, and the largest net water transport was found in the soil column with initial water content of 0.148 m3 m-3. At the same time, temperature changes with the transport of soil water was in a nonlinear shape as heat parameters were function of water content, and the changes of temperature were positively correlated with the net amount of water transported. Numerical modelling results show that the predicted values of temperature distribution were close to the observed values, while the predicted values of water content exhibited limited deviation at both ends of the soil column due to the slight temperature changes at both ends. It was indicated that the model proposed here was applicable.

Water and heat transport in hilly red soil of southern China:II. Modeling and simulation

Simulation models of heat and water transport have not been rigorously tested for the red soils of southern China. Based on the theory of nonisothermal water-heat coupled transfer, a simulation model, programmed in Visual Basic 6.0, was developed to predict the coupled transfer of water and heat in hilly red soil. A series of soil column experiments for soil water and heat transfer, including soil columns with closed and evaporating top ends, were used to test the simulation model. Results showed that in the closed columns, the temporal and spatial distribution of moisture and heat could be very well predicted by the model, while in the evaporating columns, the simulated soil water contents were somewhat different from the observed ones. In the heat flow equation by Taylor and Lary (1964), the effect of soil water evaporation on the heat flow is not involved, which may be the main reason for the differences between simulated and observed results. The predicted temperatures were not in agreement with the observed one with thermal conductivities calculated by de Vries and Wierenga equations, so that it is suggested that Kh, soil heat conductivity, be multiplied by 8.0 for the first 6.5 h and by 1.2 later on. Sensitivity analysis of soil water and heat coefficients showed that the saturated hydraulic conductivity, KS, and the water diffusivity, D(θ), had great effects on soil water transport; the variation of soil porosity led to the difference of soil thermal properties, and accordingly changed temperature redistribution, which would affect water redistribution.

Different discretization method used in coupled water and heat transport mode for soil under freezing conditions

A coupled water and heat transport mode is established based on the Richards equation to study water flow and heat transport in soil during freezing process. Both the finite difference and finite element method are used in the discretization, respectively. Two different computer programs are written and used to simulate an indoor unidirectional frozen test. The freezing depth, freezing rate and temperature variation are compared among lab tests, finite difference calculation simulation and finite element calculation simulation. Result shows that： the finite difference method has a better performance in freezing depth simulation while the finite element method has a better performance in numerical stability in one-dimensional freezing simulation.

Effects of biodegradable mulch on soil water and heat conditions,yield and quality of processing tomatoes by drip irrigation

To combat the problem of residual film pollution and ensure the sustainable development of agriculture in oasis areas,a field experiment was carried out in 2019 at the Wuyi Farm Corps Irrigation Center Test Station in Urumqi,Northwest China.Four types of biodegradable mulches,traditional plastic mulchs and a control group(bare land;referred to as CK)were compared,including a total of six different treatments.Effects of mulching on soil water and heat conditions as well as the yield and quality of processing tomatoes under drip irrigation were examined.In addition,a comparative analysis of economic benefits of biodegradable mulches was performed.Principal component analysis and gray correlation analysis were used to evaluate suitable mulching varieties for planting processing tomatoes under drip irrigation.Our results show that,compared with CK,biodegradable mulches and traditional plastic mulch have a similar effect on retaining soil moisture at the seedling stage but significantly increase soil moisture by 0.5%-1.5%and 1.5%-3.0%in the middle and late growth periods(P<0.050),respectively.The difference in the thermal insulation effect between biodegradable mulch and plastic mulch gradually reduces as the crop grows.Compared with plastic mulch,the average soil temperature at 5-20 cm depth under biodegradable mulches is significantly lowered by 2.04°C-3.52°C and 0.52°C-0.88°C(P<0.050)at the seedling stage and the full growth period,respectively,and the water use efficiency,average fruit yield,and production-investment ratio under biodegradable mulches were reduced by 0.89%-6.63%,3.39%-8.69%,and 0.51%-6.33%(P<0.050),respectively.The comprehensive evaluation analysis suggests that the black oxidized biological double-degradation ecological mulch made from eco-benign plastic is the optimal film type under the study condition.Therefore,from the perspective of sustainable development,biodegradable mulch is a competitive alternative to plastic mulch for large-scale tomato production under drip irrigation in the oasis.

Influence of CO_2 Doubling on Water Transport Process at Root/Soil Interface of Pinus sylvestris var. sylvestriformis Seedlings

Water transport at the root/soil interface of 1 year old Pinus sylvestris Linn. var. sylvestriformis (Takenouchi) Cheng et C. D. Chu seedlings under CO 2 doubling was studied by measuring soil electric conductance to survey soil water profiles and comparing it with root distribution surveyed by soil coring and root harvesting in Changbai Mountain in 1999. The results were: (1) The profiles of soil water content were adjusted by root activity. The water content of the soil layer with abundant roots was higher. (2) When CO 2 concentration was doubled, water transport was more active at the root/soil interface and the roots were distributed into deeper layer. It was shown in this work that the method of measuring electric conductance is an inexpensive, non_destructive and relatively sensitive way for underground water transport process.

The analysis of heat and water fluxes in frozen silty soil

In this paper, based on the basic equations of water flow and heat transfer, the hydrothermal coupling model is established.The numerical model was realized in COMSOL Multiphysics software, and simulation results are compared with the experimental results of Watanabe and Wake(2008) to verify the effectiveness of the model. Through the calculation, we can obtain the dynamic changes of heat and water fluxes, thermal and hydrological properties, matric potential and temperature gradient in unsaturated freezing soil; and these variables are unmeasurable in practice.

A Water-Heat-Force Coupled Framework for the Preparation of Soils for Application in Frozen Soil Model Test

The freezing of soil containing a liquid is a complex transient heat conduction problem involving phase change and release or absorption of latent heat.Existing efforts have essentially focused on theoretical research and numerical simulations.In the present study,the problem is approached from an experimental point of view using the so-called“freezing model test”method.In particular,in order to establish a precise relationship between the model and the prototype,a temperature similarity criterion is derived using the condition of an equal number of Kosovitch.Similarity is also established with respect to other aspects.A similarity criterion for the water field is determined on the basis of relevant partial differential equations.Analogous criteria for the stress field and load are derived using an elastic model.The validity of this approach is experimentally verified.The research results provide a practical and reasonable method for calculating the parameters for preparing model soils.They also constitute a theoretical basis and a technical support for the design and implementation of a water-heat-force similarity coupled framework.

Soil water and salt distribution under furrow irrigation of saline water with plastic mulch on ridge

Furrow irrigation when combined with plastic mulch on ridge is one of the current uppermost wa- ter-saving irrigation technologies for arid regions. The present paper studies the dynamics of soil water-salt trans- portation and its spatial distribution characteristics under irrigation with saline water in a maize field experiment. The mathematical relationships for soil salinity, irrigation amount and water salinity are also established to evaluate the contribution of the irrigation amount and the salinity of saline water to soil salt accumulation. The result showed that irrigation with water of high salinity could effectively increase soil water content, but the increment is limited com- paring with the influence from irrigation amount. The soil water content in furrows was higher than that in ridges at the same soil layers, with increments of 12.87% and 13.70% for MMF9 （the treatment with the highest water salinity and the largest amount of irrigation water） and MMF1 （the treatment with the lowest water salinity and the least amount of irrigation water） on 27 June, respectively. The increment for MMF9 was gradually reduced while that for MMF1 increased along with growth stages, the values for 17 August being 2.40% and 19.92%, respectively. Soil water content in the ridge for MMF9 reduced gradually from the surface layer to deeper layers while the surface soil water content for MMF1 was smaller than the contents below 20 cm at the early growing stage. Soil salinities for the treatments with the same amount of irrigation water but different water salinity increased with the water salinity. When water salinity was 6.04 dS/m, the less water resulted in more salt accumulation in topsoil and less in deep layers. When water salinity was 2.89 dS/m, however, the less water resulted in less salt accumulation in topsoil and salinity remained basically stable in deep layers. The salt accumulation in the ridge surface was much smaller than that in the furrow bottom under this technology, which was quite different from traditional furrow irrigation. The soil salinities for MMF7, MMF8 and MMF9 in the ridge surface were 0.191, 0.355 and 0.427 dS/m, respectively, whereas those in the furrow bottom were 0.316, 0.521 and 0.631 dS/m, respectively. The result of correlation analysis indicated that compared with irrigation amount, the irrigation water salinity was still the main factor influ- encing soil salinity in furrow irrigation with plastic mulch on ridge.

Effects of Terracing and Agroforestry on Soil and Water Loss in Hilly Areas of the Sichuan Basin, China

Soil erosion in hilly areas of the Sichuan Basin is a serious concern over sustainable crop production and sound ecosystem. A 3-year experiment was conducted using the method of runoff plots to examine the effects of terracing and agroforestry in farmland systems on soil and water conservation of slope fields in the hilly areas in Jianyang County, Sichuan Province, Southwestern China. A power function (Y = aXb) can statistically describe the relationship between water runoff (Y) and rainfall (X). The regression equation for the treatment of sloping terraces with crops (Plot 2) is remarkably different from that for the treatment of sloping terraces with grasses and trees (Plot 1) and the conventional up- and down-slope crop system (Plot 3) regarding equation coefficients, while regression equations are similar between Plot 1 and Plot 3. Water runoff amount and runoff coefficient of slope fields increased by 21.5～41.0 % and 27.5～69.7 % respectively, compared to those of sloping terraces, suggesting that terracing notably reduced the water runoff in the field. In the case of sloping terraces, lower amount of water runoff was observed on sloping terraces with crops than on sloping terraces with grasses and trees. Sediment yields on the slope fields in the normal year of rainfall distribution were notably higher (34.41～331.67 % and 37.06～403.44 % for Plot 1 and Plot 2, respectively) than those on sloping terraces, implying that terracing also plays a significant role in the reduction in soil erosion. It is suggested that terracing with crops is significantly effective for soil and water conservation in cultivatedfarmland, while the conventional practice of up- and down- slope cultivation creates high rates of water runoff and soil sediment transport. Terracing with grasses and fruit trees shows a less reduction in water runoff than terracing with crops, which was observed in the 3-year experiments.

Analysis of Coupling between Soil and Water Conservation and Economic-social Development

The coupling relation exists in water and soil conser-vation and economic-social development. The article analyses the relation of soil and water conservation and economic-social development stages as well as the coupling analytical method. Then calculates the expecting income by dispersing Markov decision and calculates the correlation coefficient and the re-lationship degree. The article obtains the relationship of soil and water conservation investments and all kinds of incomes. Finally, it analyzes the important meaning in socio-economic development of water and soil conservation.

Effect of Crop Root on Soil Water Retentivity and Movement

The objective of this study was to clarify the effect of crop root on soil water retentivity and movement to improve the crop growth environment and irrigation efficiency. To simulate soil water movement considering the crop root effect on the physical properties of soil, a numerical model describing the soil water and heat transfers was introduced. Cultivation experiments were conducted to clarify the effect of the crop root on soil water retentivity and verify the accuracy of the numerical model. The relationship between soil water retentivity and the root content of soil samples was clarified by soil water retention curves. The soil water content displayed a high value with increasing crop root content in the high volumetric water content zone. The experimental results indicated that the saturated water content increased with the crop root content because of the porosity formed by the crop root. The differences of the soil water retentivity became smaller when the value of the matric potential was over pF 1.5. To verify the accuracy of the numerical model, an observation using acrylic slit pot was also conduced. The temporal and spatial changes of the volumetric water content and soil temperature were measured. Soil water and heat transfers, which considered the effect of the crop root on the soil water retentivity clarified by the soil water retention curves, were simulated. Simulated volumetric water content and temperature of soil agreed with observed data. This indicated that the numerical model used to simulate the soil water and heat transfer considering the crop root effect on soil water retentivity was satisfactory. Using this model, spatial and temporal changes of soil water content were simulated. The soil water condition of the root zone was relatively high compared with the initial conditions. This indicated that the volumetric water condition of the root zone increased with the soil water extraction and high soil water conditions was maintained because the soil water retentivity of root zone increased with the root effect.

Factors controlling deep-profile soil organic carbon and water storage following Robinia pseudoacacia afforestation of the Loess Plateau in China

Background:Afforestation is a common and effective approach used for the restoration of degraded ecosystems worldwide.In China,Robinia pseudoacacia(RP)is among the main non-native tree species and has been widely planted in revegetation of the Loess Plateau.However,owing to uncertainties regarding soil water consumption and carbon sequestration,it is necessary to assess the suitability and sustainability of R.pseudoacacia in restoration.In this study,we aimed to analyse the dynamic effects of R.pseudoacacia forest on soil carbon storage(SCS)and soil water storage(SWS).Specifically,we investigated the association between soil water content(SWC)and soil organic carbon(SOC)and underlying factors in the 0-500-cm profile of a 10-to 50-year-old chronosequence.Results:The results obtained indicated that the dynamics of SWS and SCS on this time scale could be divided into an initial reduction phase(the initial 20 years after afforestation)and subsequent recovery(20-50 years after afforestation).Compared with in the abandoned land(AL),the net accumulation of SCS in R.pseudoacacia forest was 2.51 Mg·ha^(-1)at 50 years after afforestation,whereas there was a 398.76-mm deficit in SWS.Additionally,the natural succession of R.pseudoacacia forest has contributed to the continuous change in stand structure(e.g.vegetation cover(VC),understory vegetation coverage(UVC),and litter biomass(LB)).Conclusions:These findings indicate that vegetation restoration increases carbon sequestration while causing soil water deficit.Furthermore,stand density(SD)was established to make a predominant contribution to the dynamics of SWS and SCS via its effects in altering vegetation,soil,and litter characteristics.Therefore,high-density plantation forests in the Loess Plateau area should be appropriately thinned to reduce the density of forest stands on the basis of soil erosion control and wind and sand fixation,so as to increase carbon sink with lower water consumption,thus realizing the synergistic development of soil carbon sequestration and water connotation.

Numerical simulation and experimental validation of moisture-heat coupling for saturated frozen soils

In seasonally frozen regions,freezing-and-thawing action is the main cause responsible for the destruction of canals,which is closely linked to the temperature gradient and water transport.To investigate the behaviour of soils under freezing-and-thawing actions,many numerical models have been established that consider the important coupling of moisture transport and temperature evolution;but they contain excessive parameters,some of which are rather difficult to determine.Based on the well-known Harlan's theory,a simple moisture-heat coupling model was recently proposed to quantify the coupled moisture-heat transport performance of soils in terms of the central temperature and porosity.The mathematical module of COMSOL Multiphysics was further employed to solve the governing equations numerically.To validate our model,a thorough experimental scheme was carried out in our lab.The measured temperature distribution was found to be consistent with the predicted results.

Evaluation of Soil Water Management Difference in Mango Orchards between Thailand and Japan

The objective of this study is to evaluate the difference of the soil water management in mango orchards between the varieties of “Irwin” in Japanand “Nam Dok Mai” inThailand. Field observations were conducted in mango orchards in Okinawa, Japan and Phrao, Thailand to clarify the water management practices. Measurement of the hourly soil water content in Phrao indicated that the irrigation was scarce and the volumetric water content in the soil was maintained almost constant. in the flowering season. This can be the farmers’ practice for flower induction. After the flowering season, irrigation was frequent in order to produce the large fruit. In the harvest season, the soil water content was relatively high because of frequent irrigation and rainfall. In Okinawa, the volumetric water content was maintained at the same level in a relatively deep layer. The result at the5 cmdepth indicated that the farmer carefully controlled the soil water content. In the flowering season, the soil water content was relatively low. While the orchard was managed empirically, the volumetric water content near the soil surface was maintained over 25% during the harvest season. This result indicates that the farmer performed the good soil water management to enhance mango fruit quality even without technical measurement. A numerical model describing the soil water and heat transfers was introduced to predict the farmer’s empirical soil water management in Okinawa. Using the meteorological data in March 2010, the irrigation regime was predicted using the simulated soil water content. In the flowering season, the farmer irrigated when the soil surface water content reached 14%. Based on this criterion for the empirical soil water management, the simulation result indicated that the farmer irrigated four times in this period. The numerical model presented here can be useful for evaluating the differences in water management practices of local farmers.

Advances in the Coupled Soil Water and Groundwater Models

Models simulating the reciprocal transformation between the soil water and groundwater are of great practical importance to the development and utilization of water resources and prevention and remedy of water pollution. In this paper,popular coupled models of soil water and groundwater will be analyzed. Besides,advantages and disadvantages of different models will be summarized as a reference for the numerical model of soil water and groundwater.

AHFO-based soil water content sensing technology considering soil-sensor thermal contact resistance

The actively heated fiber-optic(AHFO)technology has become emerged as a research focus due to its advantages of distributed,real-time measurement and good durability.These attributes have led to the gradual application of AHFO technology to the water content measurement of in situ soil.However,all existing in situ applications of AHFO technology fail to consider the effect of soilesensor contact quality on water content measurements,limiting potential for the wider application of AHFO technology.To address this issue,the authors propose a method for determining the soilesensor thermal contact resistance based on the principle of an infinite cylindrical heat source.This is then used to establish an AHFO water content measurement technology that considers the thermal contact resistance.The reliability and validity of the new measurement technology are explored through a laboratory test and a field case study,and the spatial-temporal evolution of the soil water content in the case is revealed.The results demonstrate that method for determining the soilesensor thermal contact resistance is highly effective and applicable to all types of soils.This method requires only the moisture content,dry density,and thermal response of the in situ soil to be obtained.In the field case,the measurement error of soil water content between the AHFO method,which takes into account the thermal contact resistance,and the neutron scattering method is only 0.011.The water content of in situ soil exhibits a seasonal variation,with an increase in spring and autumn and a decrease in summer and winter.Furthermore,the response of shallow soils to precipitation and evaporation is significant.These findings contribute to the enhancement of the accuracy of the AHFO technology in the measurement of the water content of in situ soils,thereby facilitating the dissemination and utilization of this technology.

A modified soil water content measurement technique using actively heated fiber optic sensor

Soil water content measurement is critical in practical engineering.The actively heated fiber Bragg grating optic sensor(FBGS)has great potential of multi-point measurement for soil water content measurement in field.In this study,the effect of heating time on the measurement accuracy is discussed,and modifications are made for actively heated fiber optic(AHFO)sensors.The results demonstrate that if an integration data analysis method is used,the accuracy and reliability of soil water content measurement with AHFO sensors will be improved.Both a short fiber length and a short-term heating pattern are effective and can help to reduce soil disturbance.With the proposed integration method,a short heating time is guaranteed for measuring the soil water content.Such improvements will reduce the thermal disturbance to soil sample and improve the reliability of measurement.

Considering temperature dependence of thermo-physical properties of sandy soils in two scenarios of oil pollution

We analyzed the heat conductivity and volumetric heat capacity of sandy soil contaminated in two scenarios of oil pollution, and also determined the temperature dependencies of these changed thermophysical properties. In the first pollution scenario, the oil product was introduced into wet river sand, and in the second case, dry sand was contaminated by the oil product and was then moistened with water. By considering these two scenarios as multicomponent dispersion systems with varying degrees of contamination and humidity, and by using a polystructural granular model with pore spaces and closed inclusions, we calculated that the heat conductivity of the sandy soil increased under the first pollution scenario and decreased under the second, but the change in the volumetric heat capacity of the sandy soil was proportional only to the amount of oil pollution, not the manner in which it was introduced. We also determined the temperature dependencies of these two thermophysical properties of sandy soil when polluted by oil, of which information will be useful for future containment and remediation of oil-contaminated soil.

Solute transport characteristics of a deep soil profile in the Loess Plateau,China

Understanding solute transport behaviors of deep soil profile in the Loess Plateau is helpful for ecological construction and agricultural production improvement. In this study, solute transport processes of a deep soil profile were measured by a conservative tracer experiment using 25 undisturbed soil cores （20 cm long and 7 cm diameter for each） continuously sampled from the surface downward to the depth of 500 cm in the Loess Plateau of China. The solute transport breakthrough curves （BTCs） were analyzed in terms of the convection-dispersion equation （CDE） and the mobile-immobile model （MIM）. Average pore-water velocity and dispersion coefficient （or effective dispersion coefficient） were calculated using the CDE and MIM. Basic soil properties and water infiltration parameters were also determined to explore their influence on the solute transport parameters. Both pore-water velocity and dispersion coefficient （or effective dispersion coefficient） generally decreased with increasing depth, and the dispersivity fluctuated along the soil profile. There was a good linear correlation between log-transformed pore-water velocity and dispersion coefficient, with a slope of about 1.0 and an average dispersivity of 0.25 for the entire soil profile. Generally speaking, the soil was more homogeneous along the soil profile. Our results also show that hydrodynamic dispersion is the dominant mechanism of solute transport of loess soils in the study area.