西北地区额济纳绿洲非饱和带水分和盐分分布

在西北地区黑河流域下游的额济纳绿洲选择3个土壤剖面采集不同深度土壤样品和潜水样品进行测试,并在野外观测非饱和带不同深度的水土势变化。结果表明,SO4^2-是非饱和带土壤和地下水中的主要离子成分,土壤和地下水呈偏碱性。土壤易溶离子含量随深度大体上呈减少趋势。绿洲非饱和带发育有收敛型和发散型零通量面,自夏季至冬季,零通量面变得不清晰直至消失。在非饱和带中,土壤易溶盐含量、含水量和总水土势随深度的变化趋势基本相同,植物根系的吸收对水分和盐分的垂直分布有着重要的影响。     

浅层包气带地温与含水量昼夜动态的实验研究

西北荒漠化地区,包气带中的水分除来自大气降水外,还来自凝结水。凝结水对维持荒漠地区的植被生态环境起到至关重要的作用,而凝结水的形成机制又反映在包气带地温与含水量的昼夜动态过程中。文中报告了室外沙坑浅层包气带地温与含水量观测的实验结果。土壤含水量变化采用原位测试的方法观测,避免了传统称重法产生的干扰和不确定性。实验中对深度0~30cm范围的土壤温度进行了高密度观测。结果表明,温度梯度对水汽的运移起到主控作用,温度梯度方向向下,土壤含水量增加,反之,含水量减少。通过热传导方程对土壤中的传热过程进行分析,得到傅立叶级数表示的温度波方程,用于预测不同深度土壤响应地表条件而产生的温度变化。实验中还对近地表微气象以及土壤负压等因素进行了观测。     

Transfer of moisture through the unsaturated zone in the tropical forest using the neutron probe

Knowledge of moisture content is crucial in assessing spatial and temporal movement of water through the unsaturated zone. Moisture storage is also important for monitoring the soil water balance and for validation of water balance models. The purpose of this work was to determine and analyse moisture content profiles at point locations in the unsaturated zone of a lateritic soil around Nsimi, south of Cameroon. Neutron probe has been connected to a set of tensiometers in an area of 60 ha. A comparative study between a site covered with vegetation and a site uncovered was conducted to assess the influence of vegetation in the process of moisture transfers. The results showed that the spatial distribution of moisture profiles varied according to the site and the texture of the soil, with in general increasing of moisture from the surface horizon toward the deeper layers. The mean values of moisture varied fromθ_m = 0,397 cm³/cm³ on barren site, against 0 = 0,429 cm³/cm³ in vegetation. Values of suction were generally strong in surface and at depth, but weak in the intermediate layers.     

Coupled heat and mass transfer in unsaturated soil—a potential-based solution

An analysis of coupled heat and moisture movement in unsaturated soil in terms of the fundamental potentials for flow is examined. The approach adopted is based on the assumption that the total potential for liquid flow consists of two components, the elevation and the capillary potential. The fundamental potentials employed in the work are, therefore, temperature and capillary potential. The full theoretical formulation of the problem is presented, together with full details of the solution algorithm employed. Spatial discretization is achieved via the use of the finite element method, with the time-varying behaviour described by a finite difference technique. Soil parameter variations as functions of both temperature and moisture content are included in a one-dimensional approach. The work is applied to a practical engineering problem, namely heat and mass transfer in the upper layers of a soil stratum. This problem is of importance to the utilities, since many services are buried in this zone. Material parameters obtained from an associated programme of experimental work are employed. Moisture content and temperature profiles indicating the extent and rate of penetration of drying and heating fronts are produced.     

云南滇池流域强烈灌溉条件下西芹大棚土壤水分动态特征

利用TRIME-T3型TDR仪土壤含水量测量系统和WM-1型负压计系统现场监测了强烈灌溉条件下整个作物生长期西芹大棚土壤水分、土壤负压(基质势)的动态变化;研究表明,不同深度土壤含水量受强烈灌溉的影响程度不同,0.3 m深度处受影响最大,1.0,2.0 m深度处受影响最小;受灌溉的影响,0.3,0.6,1.0 m深度处负压有所波动,其中0.3 m深度处负压的波动幅度最大;2.0 m深度处的土壤一直处于"饱和-非饱和"两种状态的转换之中;在地下水埋深为2.0 m左右,且上部土层的渗透性较差(饱和渗透系数仅为0.04 m/d)的情况下,地下水水位受灌溉的影响极小.     

Nitrate in groundwater and the unsaturated zone in (semi)arid northern China: baseline and factors controlling its transport and fate

Knowledge of the baseline of groundwater nitrate is essential for water quality management. As large-scale anthropogenic activities, especially utilization of chemical fertilizers began from the 1950s in most countries, such as China, the baseline of groundwater nitrate can be determined from pre-modern water using tritium and statistical analysis. In the (semi)arid northern China, the median values of nitrate baseline for the three large regions (Tarim river basin, TRB; Loess Plateau of China, LPC; North China Plain, NCP) range from 2 to 9 mg/L (as NO₃). Several main factors control nitrate content in the unsaturated zone moisture and in groundwater, e.g., nitrate input, sediment moisture movement (direction and rate), and depth of water table at the macroscopic scale in (semi)arid areas, where nitrate loss by denitrification can be limited. Sixteen unsaturated zone profiles (638 sediment samples in total) with depths ranging from 5 to 18.25 m were sampled to demonstrate how those factors affect groundwater nitrate. As sediment moisture moves upward from the water table in the TRB case, a large inventory of nitrate in the unsaturated zone with evapo-transpired origin would never enter groundwater and groundwater nitrate contents remain at the baseline level. On the contrary, in the LPC and NCP, nitrate from fertilizers may pass through the unsaturated zone and eventually reach the water table to pollute groundwater. It is also noticed that there is a time lag between land-use change and groundwater quality response, due to the buffering capacity of the thick unsaturated zone, to which attention should be paid regarding water quality management.     

Time-domain reflectometry probing systems for the monitoring of hydrological processes in the unsaturated zone

Precise measurements of the downward movement of precipitation through the unsaturated zone, as well as return flow of moisture to the atmosphere via evaporation, have always been challenging in regard to in-situ monitoring techniques. This study investigates the profile of volumetric water-content fluctuations within the unsaturated zone through a combination of field techniques, including in-situ measurements of the volumetric porewater content at different depths using specially designed time-domain reflectometry (TDR) probes. The probes are installed through direct-push vibro-coring methods, at significant depths within the unsaturated zone, providing continuous readings of the soil-moisture content throughout the unsaturated column. The measured waveform is analyzed by using the inverse modeling approach resulting in an apparent relative dielectric permittivity profile of the surrounding medium along the TDR probe length. The approach sufficiently analyzes the mechanisms of water fluxes through significant depths within the unsaturated zone, which in turn can be used to quantify groundwater recharge at areas where the unsaturated zone hydrology plays a key role in the recharge of the underlying aquifers (such as arid and hydrologically sensitive areas). The approach was applied at an experimental field site in the Upper Rhine Valley, Germany.     

土壤包气带中气体对入渗水流运动影响的实验研究

利用专门的实验系统,研究了降雨入渗过程中土壤包气中气压势形成、发展和消亡的规律及其对下渗水流运动的影响。结合成因分析,探讨了气压势形成的条件;审视和解释了气压势与土壤水总势能和地下水位的相互关系。实验结果表明,在地下水位埋深较浅的条件下(<2m),积水入渗时所形成的气压势将使下渗量减少1/3。     

冻融和非冻融条件下包气带土壤墒情垂向变化的试验与分析

冻结层的存在使得寒区有着与非寒区差别明显的水文循环过程,土壤冻融规律、水热盐运移、融雪水入渗等已成为众多学者的研究对象. 寒区低温条件下冻融土壤持水性质与非冻融土壤不同,其包气带冻结层往往具有弱透水性、蓄水保墒和隔热减渗的作用,使得寒区春季冻结层土壤的墒情较高. 以冻融土壤和非冻融土壤墒情对比监测为基础,选取地表以下100 cm的土壤为研究对象,在黑龙江大学呼兰校区设置冻融和非冻融对比监测试验场,同时段、同频率、同埋深（间隔 20 cm土层）进行土壤结构、水热及环境参数监测. 通过对比分析了不同埋深不同冻融阶段的墒情参数,量化了低温冻融条件下土壤墒情较非冻融土壤的高出部分,最后对冻土保墒的机理进行探讨与分析. 结果表明：冻结条件下土壤水分重新分布,在土水势的作用下由非冻结区向冻结区迁移. 初冻期地表土壤墒情达到最大,冻结期土壤最大墒情值随冻结锋面迁移分别在20、40、60 cm处达到最大,稳定冻结期和融化初期在80 cm处达到最大;土壤最大墒情值一般在冻结锋面前沿的10～20 cm处,较好地保持了土壤水分. 无论是从空间（不同埋深）还是时间（不同冻融阶段）角度分析,冻融土壤含水率均大于非冻融土壤,二者含水率的差值随埋深和冻融阶段的推移而加大,在稳定冻结期80 cm处达到最大,差值量可达6.4%～7.8%.     

非饱和路基土水分运移的室内试验研究

模拟路基填土的实际情况,研究设计了不同表面辐射温度和降雨强度的非饱和土水分运移的室内试验方法。通过测定土样在相应边界条件下土体不同深度位置的含水率随时间的变化,揭示了温度、压实度、降雨强度3个因素对非饱和土水分运移影响的规律,据此探讨了路基填土的水分运移规律,认为提高压实度能显著减少水分运移变化。     

喀斯特溶槽岩-土界面优势流及其对土壤水分动态的影响

通过对两个典型溶槽剖面的野外入渗实验,结合对入渗锋面的红外成像和染色示踪,分析溶槽中土壤湿润锋运移受岩—土界面优势流、初始水分状态（干湿程度和不均匀分布）及植被根系的影响程度。其结果表明:岩—土界面优势流增加入渗锋面到达处的土壤含水率是溶槽中部的1.1～14.5倍,但岩—土界面优势流形成的侧向水势梯度可降低优势流锋面下移;初始入渗中表土湿润锋运移主要受植被根系大孔隙优势流的影响,岩—土界面优势流作用不显著,但随着入渗深度的加大,岩—土界面优势流侧向弥散对湿润锋运移和土壤水分的影响加强。      

甘肃黑河中下游影响绿洲植被发育的某些因素

黑河下游额济纳绿洲区和中下游沿黑河干流河床附近的潜水位埋深较浅,是维系地表植被生长的重要因素。中游高台附近潜水位存在每年双峰值的变化,较之下游额济纳绿洲潜水位每年单峰值的变化更有利于植被的生长。在包气带剖面中,同一时刻水土势、含水量、含盐量随深度的变化趋势基本相同。采样点包气带土壤中易溶离子含量相对较低,全盐量小于5%,中游采样点土壤中以HCO3-为主,下游以SO42-为主,由SO42-、Cl-和Na+、Ca2+组成的盐类构成了包气带土壤中的主要盐分。土壤中易溶离子含量自地表至潜水面之间随深度总体上呈减小趋势。植被的根系分布对包气带土壤含水量、水土势和易溶离子含量随深度的分布有着重要的影响,在植物根系发育带的土壤中含水量、水土势和易溶离子含量升高。     

黄土中水分迁移规律现场试验研究

黄土地区降雨诱发滑坡是不争的事实,但黄土地区地下水位很深,降雨入渗地面后如何运移,与地下水有无直接联系,目前还不是很清楚。为此设计了人工滴水试验模拟天然降雨条件,通过在一深度为10 m的探井井壁上埋设土壤水分计,观测人工滴水入渗过程中不同深度土体含水率随时间的变化情况,以确定其入渗影响深度。监测结果显示：降雨量为3.82 mm/d (小雨)时,0.5 m内土体含水率变化明显,0.5 m以下土层含水率几乎没有变化;降雨量为10.31 mm/d(中雨)时,1 m内土体含水率有所增加;降雨量达25.21 mm/d(大雨)时,1 m内土体含水率增长明显,1.0～1.6 m范围内有微弱增长;随着深度增加,土体含水率变化逐渐滞后,增幅逐渐减小。这说明在干旱的黄土地区,若无明显入水通道,短期内降雨的入渗深度有限,很难到达地下水位;但深部古土壤层的观测结果表明,即使在其上部黄土中含水率变化极其微弱的情况下,古土壤中的含水率上升明显,表明黄土中非饱和渗流或水汽迁移是存在的。通过试验还表明,陇东黄土高原地区土壤中水分循环主要发生在浅层0.7 m以内的蒸发带。降雨入渗到蒸发带以内,若无后续降雨补给,则向上蒸发排泄;若入渗至蒸发带以下,则不受蒸发影响,得以继续向下迁移;当遇到不透水面时,会在层面附近富集,有限元模拟也较好地反映了这一规律。     

Transport of BTEX Vapours Through Granular Soils with Different Moisture Contents in the Vadose Zone

The single-ring aromatic hydrocarbons benzene, toluene, ethylbenzene and m-xylene (BTEX) are common and dangerous pollutants in subsurface environments. The diffusive transport of BTEX vapours through the unsaturated zone of the ground is a potential health hazard to humans, living in the vicinity of petroleum fuel contaminated sites. Past studies have shown that gas transport through the vadose zone can be influenced by moisture content due to variations in gaseous permeability, phase partitioning and aerobic biodegradation. In this particular study laboratory soil column experiments were employed to compare the diffusive transport of BTEX vapours through a sand layer of high moisture content, where biodegradation of BTEX compounds occurred, with diffusion through air-dried sand. The presence of a thin soil layer of high moisture content reduced the gaseous concentrations of benzene and toluene and stopped the migration of ethylbenzene and m-xylene vapours, demonstrating its efficiency as a barrier on the diffusive transport of BTEX vapours in unsaturated soil.     

Spatial distribution of soil organic carbon and its influencing factors at different soil depths in a semiarid region of China

Studying spatial variability of soil organic carbon (SOC) is crucial for understanding the largest active carbon pool in terrestrial ecosystems. The main objectives ofves of this study were (1) to analyze the spatial pattern of SOC at different depths and (2) to define the soil properties affecting the spatial patterns of SOC at different depths. A total of 910 soil samples were collected at different locations and soil layers in a semiarid zone (3800 km²) of northeastern China. A geostatistical approach was used to predict and map SOC at different depths while the soil properties affecting the spatial patterns of SOC were analyzed by using canonical correspondence analysis. As expected, SOC content decreased gradually with depth and such a SOC decrease depended on soil type, which resulted the main factor affecting horizontal and vertical distributions of SOC content. Slope and bulk density exhibited a negative correlation with SOC content, whereas vegetation index, soil moisture content, elevation, and aspect showed a positive correlation. Sensitivity of SOC to influencing factors differed with increasing soil depth. SOC was significantly correlated with vegetation and bulk density on topsoil, while soil moisture and aspect played an important role in controlling SOC with increasing depth.     

A study of the water and energy transfer at the soil surface in cropped field on the Loess Plateau of northwestern China

In this study, we tried to model the processes of moisture and heat transfers in the soil–vegetation–atmosphere system in an integrated comprehensive way. The purpose of the study is to simulate profiles of soil water content and temperature at root active zone (i.e., 0–50 cm), taking the root water uptake, soil evaporation, and canopy transpiration into account. The water and heat transfer equations are solved by an iterative Newton–Raphson technique and a finite difference method is used to solve the governing equations. Soil water content and soil temperature dynamics could be simulated rather accurately in a cropped field on Loess Plateau area. The water and heat transfer flux predicted by the classical theory of Philip and de Vries (Tans Am Geophys Union 38:222–232, 1957) slightly overestimated near the surface and underestimated at the deeper depths, as a result of the overestimated soil evaporation at the top soil layer (0–10 cm) and underestimated crop canopy transpiration at the deeper depths (10–50 cm). Water content tended to be underestimated for the entire profile at the soil surface (from 0 to 50 cm). Soil temperatures during the simulated period was slightly overestimated in the nighttimes and underestimated in the daytimes, as a result of the underestimated soil water content at the top soil layer (0–10 cm) and overestimated at the deeper depths (10–50 cm). Soil temperatures tended to be underestimated for the entire profile at the soil surface (from 0 to 50 cm). While the sum of the water and heat regimes yielded a much better match with the soil water content and soil temperature obtained from the field observations. The results obtained show that the model coupled water and heat transfer is able to capture the dynamics of soil water content.     

冻融循环下初始含水率对非饱和膨胀土剪切特性试验

为探究冻融循环条件下初始含水率对膨胀土偏应力-应变关系和剪切特性的影响,本文对不同初始含水率的非饱和膨胀土进行了不固结不排水三轴剪切试验。结果表明：1）非饱和膨胀土的偏应力-应变曲线的形式随含水率增大由应变软化逐渐转变为应变硬化,偏应力峰值随含水率增大而降低;2）非饱和膨胀土的抗剪强度随初始含水率的增加呈线性下降趋势;3）非饱和膨胀土在第1次冻融循环后偏应力及抗剪强度大幅度下降,在3~7次冻融循环后达到稳定。初始含水率是影响非饱和膨胀土力学性质的主要原因。     

黄土高原丘陵沟壑区包气带土壤水运移过程

包气带土壤水运移过程是黄土高原生态修复中亟需回答的关键科学问题。环境同位素方法可获取其他方法难以获取的水文过程信息。通过对黄土高原丘陵沟壑区羊圈沟小流域降水、包气带0~150 cm土壤水和绣线菊（Spiraea salicifolia）木质部水等样品的同位素δD和δ18O进行测定。结果表明:羊圈沟小流域降水同位素组成受蒸发作用影响较大,呈现明显分馏效应。包气带土壤水、降水与木质部水同位素组成存在明显月份变化特征。降水是土壤水的主要补给来源,灌丛的水分利用来源主要为降水和土壤水,符合降水-土壤水-植被水的运移特征。灌丛木质部水和20~40 cm土壤水δD和δ18O最为接近,20~40 cm土壤水是灌丛水分利用的主要来源。研究揭示了包气带土壤水运移过程及植物水分利用来源,为土壤水运移过程、模型结构与参数识别等提供科学依据。     

沙漠包气带水分运移机理研究

针对沙漠包气带的特点,运用中子仪—负压计—地温计联合观测法,对试验场典型地段水位埋深分别为lm,2m,3m的包气带含水量、水分势能和温度场进行了长期野外实地观测,并总结出相应的日变化、季节变化规律,分析了降水、蒸发等气象因素对包气带水分的影响过程,从而得出沙漠包气带水分运移机理及运移规律。为红砂岗矿区水源地水资源评价提供了科学依据。      

水势理论在四水转化研究中应用时代特征与趋势:纪念ZFP方法引进我国30周年

20世纪80年代零通量面方法在我国应用中解决了"四水"转化研究中参数不确定性带来的问题,发现在降水入渗补给地下水过程中土壤总水势梯度大于1.0cm H2O/cm,且逐渐降低,流入、流出被监测土层的水量相等时土壤总水势梯度趋近于1.0cm H2O/cm。20世纪90年代,利用土壤水势与含水量之间量化关系,指导了农业节水灌溉,提出在灌溉过程中土壤水势梯度等于1.0cm H2O/cm的时间持续愈长,表明过剩灌溉而浪费的水量愈多的认识。进入21世纪以来,水势理论较广泛地用来解决土壤水盐分运移数值模拟与入渗模型中水文地质参数问题和降水入渗土壤水势运移微观机理研究,并发现表聚型、中聚型和底聚型土壤盐分剖面的水动力学特征。通过土壤水动力场调控改变土壤水盐(养分或污染物)运移是未来重要研究方向。