Characteristics of In-Situ Soil Water Hysteresis Observed through Multiple-Years Monitoring

A soil water retention curve (SWRC) is an essential soil physical property for analyzing transport and retention of water in a soil layer. A SWRC is often described as a single-valued function that relates the soil water potential ψ to volumetric water content θ of the soil. However, an in-situ ψ − θ relation should show soil water hysteresis, though this fact is often neglected in analyses of field soil water regimes while long-term in-situ soil water hysteresis is not well characterized. This study aimed at probing and characterizing in-situ ψ − θ relations. The developments of large hysteresis in the in-situ ψ − θ relations were observed only a few times during the study period of 82 months. Any of the large hysteretic behaviors in the ψ − θ relations began with an unusually strong continual reduction in ψ. The completion of a hysteresis loop required a recorded maximum rainfall. Because the study field had very small chances to meet such strong rainfall events, it took multiple years to restore the fraction of soil water depleted by the unusually strong continual reduction in ψ. While wetting-drying cycles had occurred within a certain domain of ψ, hysteretic behaviors tended to be so small that the in-situ ψ − θ relation can be approximated as a single-valued function of θ(ψ). These observed patterns of the in-situ ψ − θ relations were characterized by kinds of difference in dθ/dψ between a drying process and a wetting process at a given ψ. Thus, more amounts of experimental facts about wetting SWRCs in parallel with drying SWRCs should be needed for correct modelling, analyzing, and predicting soil water regimes in fields. It is also necessary to increase our understandings about the long-term trends of occurrences of extreme weather conditions associated with possible change in climate.

Rapid testing and prediction of soil–water characteristic curve of subgrade soils considering stress state and degree of compaction

The subgrade soil is generally in saturated or unsaturated condition. To analyze complex thermo-hydro-mechanical-chemical (THMC) behaviors of subgrade, it is essential to determine the soil–water characteristic curve (SWCC) that represents the relationship between matric suction and moisture content. In this study, a full-automatic rapid stress-dependent SWCC pressure-plate extractor was developed. Then, the influences of overburden stress and degree of compaction on the SWCC of subgrade soil such as high liquid limit silt (MH) and low liquid limit clay (CL) were analyzed. Accordingly, a new model taking into account the influences of overburden stress and degree of compaction based on the well-known Van Genuchten (VG) SWCC fitting model was presented and validated. The results show that with the increase of the degree of compaction and overburden stress, the saturated moisture content of subgrade soil decreases, while the air-entry value increases and the transition section curve becomes flat. The influences of the degree of compaction and overburden stress on the SWCC of MH is greater than that of CL. Meanwhile, there was a satisfactory agreement between the prediction and measurement, indicating a good performance of the new model for predicting the SWCC.

Unraveling the hydraulic properties of loess for landslide prediction:A study on variations in loess landslides in Lanzhou,Dingxi,and Tianshui,China

Loess has distinctive characteristics,leading to frequent landslide disasters and posing serious threats to the lives and properties of local re sidents.The involvement of water repre sents a critical factor in inducing loess landslides.This study focuses on three neighboring cities sequentially situated on the Loess Plateau along the direction of aeolian deposition of loess,namely Lanzhou,Dingxi,and Tianshui,which are densely populated and prone to landslide disasters.The variations in hydraulic properties,including water retention capacity and permeability,are investigated through Soil Water Characteristic Curve(SWCC)test and hydraulic conductivity test.The experimental findings revealed that Tianshui loess exhibited the highest water retention capacity,followed by Dingxi loess,while Lanzhou loess demonstrated the lowest water retention capacity.Contrastingly,the results for the saturated permeability coefficient were found to be the opposite:Tianshui loess showed the lowest permeability,whereas Lanzhou loess displayed the highest permeability.These results are supported and analyzed by scanning electron microscopy(SEM)observation.In addition,the water retention capacity is mathematically expressed using the van Genuchten model and extended to predict unsaturated hydraulic properties of loess.The experimental results exhibit a strong accordance with one another and align with the regional distribution patterns of disasters.

Temperature Effects on Unsaturated Hydraulic Property of Bentonite-Sand Buffer Backfilling Mixtures

The influence of temperature on the engineered properties of bentonite-sand mixtures （B/S） is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. We experimentally studied the influence of temperature on soil unsaturated hydraulic properties related to water holding capacity and permeability of GMZ B/S in China. The vapor equilibrium method and water infiltration apparatus were used to measure the soil water characteristic curve （SWCC） and unsaturated hydraulic conductivity （k）. The results show that the SWCC under different temperatures from 20℃ to 60 ℃ tends to be the same. Temperature influence on unsaturated permeability is more relevant at low suctions, no clear effect is detected below a degree of saturation of 74%, and experimental data show that temperature dependence on unsaturated permeability is small.

Soil physical quality as influenced by long-term fertilizer management under an intensive cropping system

In China’s major rice(Oryza sativa L.)production regions,the traditional fertilization modes are challenged by the continued decrease in manure and increase in mineral fertilizer.However,limited information exists on the influences of long-term fertilizer management on soil organic carbon(SOC)and soil physical properties under the intensive rice production system in southern China.The objective of this study was to characterize the changes of soil physical properties as related to mineral fertilizer,crop residues,and manure application based on a long-term field experiment.The experiment,initiated in 1986,has five treatments:unfertilized,mineral fertilizer alone,rice residues plus mineral fertilizer,low manure rate plus mineral fertilizer,and high manure rate plus mineral fertilizer.The cropping system consists of barley(Hordaum vulgare L.),early rice,and late rice,three crops in a year.In May 2006,after barley harvest,soil samples were collected from the 0~10 cm and 10~20 cm layers to determine SOC concentration,aggregate size distribution,bulk density(rb),saturated hydraulic conductivity(Ks),and soil water characteristic curves(SWCC).The results indicated that manure significantly reducedrb,increased SOC concentration,soil aggregation,Ks,transmission and storage porosity,as well as water retention capacity.Combined application of crop residue and mineral fertilizer also improved soil physical properties,but the improvement by mineral fertilizer alone was limited.Correlation analysis demonstrated that S,the slope of the SWCC at its inflection point,was closely associated with the selected physical parameters,suggesting S was an effective parameter for soil physical quality evaluation.Nevertheless,in applying the S-theory,a unified approach to define the residual water content should be considered.

Evaluating mechanism and inconsistencies in hydraulic conductivity of unsaturated soil using newly proposed biochar conductivity factor

In the past few decades,numerous studies have been conducted to promote the use of biochar as a soil amendment and most recently,for compacted geo-engineered soils.In general,the definite trends of biochar effects on water retention and fertility of soils have been confirmed.However,the biochar effects on hydraulic conductivity,particularly unsaturated hydraulic conductivity of soil-biochar mix remain unclear,making it difficult to understand water seepage in both agricultural and geo-engineered infrastructures in semi-arid regions.This study examines the unsaturated hydraulic conductivity function derived based on the measurements of soil water characteristic curves of soil with biochar contents of 0%,5%and 10%.A new parameter“biochar conductivity factor(BCF)”is proposed to evaluate the inconsistency in reported biochar effects on soil hydraulic conductivity and to interpret it from various mechanisms(inter-and intra-pore space filling,cracking,aggregation,bio-film formation and piping/internal erosion).The impact of biochar content on unsaturated hydraulic conductivity appears to reduce as the soil becomes drier with minimal effect in residual zone.Qualitative comparison of near-saturated hydraulic conductivity with test results in the literature showed that the BCF is generally higher for smaller ratio of sand to fine content(clay and silt).Moreover,the particle size of biochar may have significant influence on soil permeability.Future scope of research has been highlighted with respect to biochar production for its applications in agriculture and geo-environmental engineering.Long term effects such as root decay and growth,aggregation and nutrient supply need to be considered.