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.

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.

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.

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.

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.

Simulating and validating the effects of slope frost heaving on canal bed saturated soil using coupled heat-moisture-deformation model

The lining canals in seasonal frozen soil areas can be severely damaged by frost heaving.The freezing and thawing contributes to the continual change of the temperature field and moisture field beneath lining canal,which will seriously affect the safe operation of the canal.In order to study the frost heaving damage mechanism of lining canal and to solve the associated engineering problems,the permafrost body was regarded as an elastomer,and a three-field,coupled,partial differential equation describing the temperature,moisture and deformation fields for a saturated two-dimensional canal bed was derived and established based on the Harlan model.The coupling equations were discretized using the finite element method in space and the finite difference method in time.The parameters were simplified appropriately based on compliance with the actual conditions and were simulated using finite element software.The results of a sample simulation showed that the simulated results and test results were basically consistent with variation laws,which proved the correctness of the numerical simulation theory and solution methods and the reliability of the calculation.The model can simulate the water,heat and deformation issues in the side slope of saturated canal bed soil in a seasonally frozen area and forecast freezing damage in the canals.

宁夏衬砌渠道渠基冻胀率水热响应试验研究

衬砌渠道周围的水分与温度场分布存在明显差异,此范围内渠基土在水热两场耦合作用下冻胀有明显不同。基于这种差异,开展渠基土在不同水分、温度条件下冻胀率试验研究。结果表明:在不同温度条件下,土体冻胀率随温度的变化过程可分为2个阶段,当温度从0℃降至-5℃左右时,在此阶段产生的冻胀率约占整个冻胀过程的50%~80%,冻胀率与温度线性关系明显。随着温度继续降低,2者非线性关系明显。在无补水情况下,相同温度条件下的土体,初始含水率越大,所产生的冻胀率越大;初始含水率25%和35%的土体产生的冻胀率均为初始含水率15%土体的1倍到2倍以上。当土体在冻胀过程中有水分补充时,其冻胀率与水分变化线性关系明显,初始含水率越大的土体产生的冻胀率增量越大,初始含水率25%、35%的土体冻胀率增量均为15%土体的2倍以上。提出的水热耦合条件与渠基土冻胀率的多元非线性关系与试验数据吻合较好。建立数值模拟模型,对衬砌渠道不同位置处冻胀作用分布与变形进行计算与分析,结果表明:温度越低、水分越大,衬砌渠道所受到的切向冻胀力与法向冻胀力越大。水热变化差异越大时,土体冻胀导致的衬砌渠道冻胀变形越显著。

跨季节土壤蓄冷-土壤源热泵系统长期运行特性实验研究

为解决土壤源热泵在长期运行条件下土壤热失衡导致的运行效率下降问题,针对夏热冬冷地区的特点,提出一种利用室外风机盘管将冬季或过渡季室外空气中的冷量存储于土壤中的跨季节蓄冷-土壤源热泵系统。利用地埋管换热系统的相似模型实验台,进行了系统运行3年的供冷、供暖、蓄冷工况实验,综合分析了埋管进出口水温、气温、土壤温度、土壤及埋管换热量、单位埋深换热量等参数,研究系统的长期运行特性。研究显示:3年的土壤平均温度分别上升0.14℃、0.22℃、0.20℃,总体来说升高幅度较小;蓄冷时的室外气温越低、土壤与室外空气的温差越大,蓄冷效果越好;埋管进出口平均温差、土壤平均温度变化、土壤换热量、埋管换热量、单位埋深换热量的大小关系一致。研究结果表明,跨季节蓄冷维持了土壤的长期热平衡,系统的长期运行效果较好。

基于土壤水热耦合运移基础理论构建水文地质数学规律模型

土壤和水是将其他形式的水进行转化和运移的纽带和重要环节,而土壤中水和热的运移是相互作用的,基于土壤水热耦合运移基础理论构建水文地质数学规律模型并进行参数反演对于研究土壤水的相变及其他相关问题的研究具有重要意义。本文通过分析皖西丘陵地区的水热生态平衡系统,运用水热运移机理,构建水文地质数学规律模型并进行参数反演,结合皖西地区水文监测站数十年的蒸发和降水数据,进一步论证水文地质数学规律,为维持皖西丘陵地区生态平衡提供了理论依据。

设施灌溉条件下不同次生盐渍化土壤盐分离子迁移特征

通过在设施灌溉条件下的室内土柱模拟试验,研究了灌水对不同次生盐渍化水平设施土壤盐分离子迁移特征的影响。结果表明：Ca2＋与NO3-在灌水后5~15 d表现出随时间的推移剖面分布呈＂双峰＂特性,随着次生盐渍化水平的升高,Ca2＋与NO3-淋洗速度变慢同时其返盐时间也较迟。地下水对K＋迁移影响不显著,使得K＋在灌水后剖面分布呈＂单峰＂特性,SO42-在所有次生盐渍化水平下一次性灌水很难将其淋洗出耕层。从环境因素看,Ca2＋在土层的表层和底层以及灌水后期都表现出与水分和温度较好的耦合效应,并且两者对Ca2＋运移具有协同作用。K＋在0~20 cm土层运移主要受水分的影响,而30~50 cm土层主要受温度的影响,从灌水时间看,在灌水后5~15 d内主要受温度影响,而灌水20 d后主要受水分影响。NO3-和SO42-不论从土层还是时间看主要受水分的影响较大。

覆膜不同开孔程度蒸发条件下土壤水热变化动态研究

覆膜灌溉播前铺膜通常无法覆盖全田面 ,而播种后需为作物破土预留出苗孔 ,覆膜灌溉时还需增设灌水孔 ,对上述各类孔的问题 ,可采用覆膜开孔率来统一表征。到目前为止 ,尚未见国内外关于覆膜开孔蒸发的水热耦合迁移方面的报道。在研制了室内蒸发水热动态观测实验系统的基础上 ,本文根据剖面初始含水率均匀的覆膜开孔蒸发土壤水分和温度的动态观测资料 ,分析了土壤温度变化规律及剖面水分的降低趋势 ,并建立了覆膜开孔蒸发的累积蒸发量、相对累积蒸发量及单位膜孔面积累积蒸发量随时间和不同覆膜开孔率变化的函数关系。分析结果表明 ,不同覆膜开孔率控制下的累积蒸发量与时间平方根之间均符合Gardner提出的裸土蒸发理论关系。该结果发展了传统的Gardner方程 ,拓展了该关系的应用途径 ,对今后进一步研究覆膜开孔蒸发的水热运动规律有重要意义。

浅层包气带水汽昼夜运移规律及其数值模拟研究

西北干旱、半干旱地区，浅层包气带水分通量主要由水汽组成，而水汽在运移过程中产生的能量转换和质量迁移是地表质能平衡计算不可缺少的重要源汇项。在野外进行一个沙坑实验，发现土壤水在中午（12：00～15：00）达到最大值（10cm深度，5．9～6．1cm^3／cm^3；30cm深度，11．9～13．1cm^3／cm^3），而在凌晨（02：00～05：00）出现最小值（10cm深度，4．4～4．5cm^3／cm^3；30cm深度，10．4～10．8cm^3／cm^3）。为进一步验证该实验条件下的土壤水运移及分布规律，考虑了土壤水、汽、热耦合运移的HYDRUS-ID模型被用来对实验过程进行模拟，模拟结果与实测结果吻合较好。为描述土壤水分昼夜运移模式，笔者将土壤水耦合运移的时间信息和空间信息进行同步分析；并根据土壤水运移的不同驱动力，分别对温度梯度、基质势梯度作用下的液态水及汽态水通量进行了分析。

土壤水热盐运动模型的建立与初步验证

以土壤物理基本定律为基础，建立了结冻、未结冻、饱和、非饱和土壤的水热盐耦合运动通用模型。该通用模型可简化为土壤水或热运动的单一模型。通过二维有限元数值解、一维模拟程序的编制和计算预测，与内蒙河套长胜渠杨树壕试验点在１９８８年秋～１９８９年春期间耕层积盐过程实测值对照，模型得到初步验证及应用。

冻融期地下水位变化情况下土壤水分运动的初步研究

建立了地下水浅埋条件下土壤冻融期水热耦合迁移模型，并采用混合型Ｒｉｃｈａｒｄｓ方程对冻融过程中的土壤饱和—非饱和水分流动问题进行了初步研究。模拟结果与试验资料的对比表明所采用的模型是可靠的。根据模拟结果，冻结过程中潜水位最大降深与初始潜水埋深间的关系可用反Ｌｏｇｉｓｔｉｃ公式表示。对于内蒙古河套灌区，考虑融化期春小麦需水要求的土壤封冻前的适宜地下水位约为１．５ｍ～２．０ｍ。

冻融土壤水热迁移与作用机理研究

季节性冻融土壤的水热变异及空间迁移过程是一个复杂的动力学系统,冻融土壤水热系统作为自然界能量循环的重要环节,在水资源、环境、能源以及人类工程等方面占有极其重要的地位,详细探求冻融土壤的水热扩散机理,准确地掌握土壤的水热迁移状况,对于科学合理的制定春播制度具有深远的影响。以高效利用冻土资源、提高农业经济、改善生态环境为宗旨,在总结国内外研究成果的基础上,阐述了土壤冻融交替的物理过程、冻层水分入渗、热量传导、土壤溶质的转化过程以及工程冻害防治,概括了冻融土壤的水热分布对于地表环境改变以及气候因子差异的响应机制,进而从机理上对冻融土壤特征参数、水热耦合动态模拟及预测模型进行了论述。同时,提出了冻融土壤结构多尺度结合、土壤覆被联合体间反演、统计模型与随机模型并行的研究思路,旨在为寒旱区季节性冻土水热循环、水热平衡研究以及科学合理的指导农业生产提供借鉴。

水肥耦合对玉米田间土壤水分运移的影响

以干旱区绿洲农田玉米水肥耦合试验为基础,利用含有根系吸水项的一维土壤水动力学模型,模拟了不同水肥条件下玉米蒸散、根系吸水、土体贮水量变化以及田间土壤水量平衡。结果表明:在玉米生长初期以棵间蒸发为主,其后则是以植株蒸腾为主;玉米在30～40cm土层吸水速率达到最大值;玉米灌水量为田间持水量的70%与85%对土壤0～80cm土层贮水量的贡献是相等的,并且高肥力在一定程度上可增强根系的吸水能力,高灌水可增强玉米根系对养分的吸收利用,但对于提高水肥利用率来说,理想的处理为中肥中水。

叶尔羌河流域裸地潜水蒸发的数值模拟研究

利用地表能量平衡原理、微气象学以及土壤水热迁移理论建立了潜水蒸发模型，并采用全隐式有限差分格式进行求解。在求解过程中，对土壤热运动方程的上边界采用了第三类边界条件，同时采用自动调节步长等方法，增加了数值计算的稳定性，提高了计算速度。用本计算模型模拟了１９９４年５月份新疆叶尔羌河流域地下水均衡场的裸地潜水蒸发过程，并与当地的试验资料进行了比较分析，结果表明，采用的计算模型和方法可以进行较长时间内裸地的潜水蒸发估算。

冬灌对越冬期土壤水分状况影响的数值模拟

根据土壤水动力学、地表能量平衡、空气动力学理论建立了越冬期土壤冻融条件下土壤—大气连续体的水热传输模型，包括冻融条件下土壤水热迁移子模型、地气间水热交换子模型。在该模型的基础上，模拟了北京地区冬灌对越冬期土壤水分状况的影响，从理论上得到了冬灌具有贮水作用的结论，并分析了合理的冬灌定额。

蒸发条件下冬小麦田土壤水热耦合运移模拟

综合考虑了根区土壤水分动态、根系吸水、蒸发蒸腾、地表能量分配和土壤中温度分布 5个子系统 ,建立了冬小麦蒸发条件下土壤水热耦合运移模拟模型 ,经实测资料检验其精度较高。该模型可用于田间水分动态、根系吸水、蒸发蒸腾。

求解SPAC系统水热输移的耦合迭代计算方法

根据描述ＳＰＡＣ系统蒸腾蒸发及土壤水热状况的物理模型，导出了反映群丛蒸腾蒸发与土壤水热状况互为作用的冠层湿温方程，提出了能定量描述二者耦合关系，并能同时求解二者的耦合迭代法．经检验，该方法模拟结果可靠．