Variability Of Saturated Hydraulic Conductivity Research Articles

Fully analytical method for probabilistic stability analysis of the rainfall-induced landslide considering variability of saturated hydraulic conductivity and rainfall uncertainty

Fully analytical method for probabilistic stability analysis of the rainfall-induced landslide considering variability of saturated hydraulic conductivity and rainfall uncertainty

Soil freeze/thaw dynamics strongly influences runoff regime in a Tibetan permafrost watershed: Insights from a process-based model

The Tibetan Plateau (TP) is a region rich in extensive frozen ground and the source of many major Asian rivers. However, how soil freeze/thaw (F/T) dynamics influence runoff production at the catchment scale in the TP is poorly understood. This study employs a process-based permafrost hydrology model with a new soil parameterization to investigate soil F/T dynamics and their impact on runoff in a TP permafrost watershed, i.e., the source region of Yangtze River (SRYR). The revised model separates soil evaporation and plant transpiration, and accounts for the influence of soil gravel and organic carbon content, as well as variation in saturated hydraulic conductivity along the soil profile. Validation results demonstrate that the revised model accurately simulates daily soil temperature (mean RMSE of 1.3 °C), soil moisture (mean ubRMSE of 0.05 cm3 cm−3), and runoff discharge (NSE = 0.82). The results reveal different altitudinal patterns of warming trend between permafrost and seasonally frozen ground (SFG). Warming rates in SFG area increase monotonously with elevation, while a turning point is observed in permafrost region around 4800 m. With active layer deepening, deep-soil water content increases but primarily replenishes soil water storage rather than directly contributing to runoff recharge, while rootzone and the middle part of the active layer become drier. Soil F/T cycles in the permafrost region exert stronger influences on runoff process compared to SFG. Delayed soil thaw onset generally results in higher spring runoff coefficient, while delayed soil freeze onset is related to slower runoff recession. The freezing zero-curtain period is likely to impact the continuity of runoff recession processes by affecting the connectivity of groundwater flow channels. These findings uncover the regulatory mechanisms of soil F/T dynamics on runoff production and river discharge characteristics, providing a fundamental basis for predicting permafrost hydrology responses to future climate change in the TP.

Disentangling slope‐scale spatial variability of saturated hydraulic conductivity in the black soil region of northeast China using noise‐assisted multivariate empirical mode decomposition

AbstractReliable estimates of saturated hydraulic conductivity (Ks) are usually difficult to obtain, as Ks is regulated by a variety of soil processes acting at different spatial scales that may obscure each other's impacts. We hypothesized that these scale‐specific relationships could be well characterized with the aid of the noise‐assisted multivariate empirical mode decomposition (NA‐MEMD), thereby serving as a solid foundation for an accurate Ks prediction. The objective was to evaluate whether the incorporation of NA‐MEMD could improve the estimation of Ks based on the multiscale associations it unraveled. On a typical slope transect of 860 m in the black soil region of northeast China, Ks, mollic epipedon thickness, bulk density, soil organic carbon content, total (ϕ) and effective porosities (ϕe), and particle size distribution were investigated at every 20 m. Prior to NA‐MEMD, Ks was most strongly correlated with ϕe, and the linear regression models based on ϕe solely were satisfactory for Ks estimation at the scale of investigation. Adding other predictors significantly improved Ks prediction in calibration, but impaired it in validation. Upon decomposition by NA‐MEMD, Ks was found to be significantly associated with each attribute at two scales of oscillation at least. Summing up the estimates of each Ks component derived from the properties at the equivalent oscillation scales, the results outperformed the traditional multiple linear regressions made at the investigation scale, when the same sets of predictors were used. The application of NA‐MEMD, moreover, could save the tedious measurements of ϕe and ϕ. Excluding these two porosity‐related properties, Ks estimates obtained by incorporating NA‐MEMD were statistically similar or even better than those involving them before NA‐MEMD. These findings demonstrate the great potential of NA‐MEMD in untangling scale‐dependent relationships of Ks with various processes and hold important implications for future estimations of Ks and other hydraulic properties in the black soil region of northeast China and similar regions.

Temporal variability of saturated hydraulic conductivity on a typical black soil slope of northeast China

Saturated hydraulic conductivity (Ks) is highly variable in both space and time. Compared to its spatial variability, however, the temporal Ks variability has rarely been studied or taken into account in the hydrological modelling. The objectives were to characterize the slope-scale temporal variability of Ks and to diagnose the primary factors regulating Ks variation during the growing season. In the black soil region of northeast China, a typical slope farmland dominated by silt loam was selected and divided into five sections with respect to soil erosion. Soil samples were collected in each section at approximately every 3 weeks during the maize growing season, and Ks, bulk density (BD), land surface roughness, wet-aggregate stability (WAS), soil organic carbon content (SOC), clay and sand contents (SAND) were investigated. Mainly owing to root growth, precipitation and agricultural management, strong temporal variability of Ks was observed in each of the five sections, as the corresponding coefficients of variation all exceeded 35 %. Using the redundancy analysis (RDA) and classical correlation analyses, the primary variables controlling Ks distribution varied among sections, i.e., precipitation (P) and SAND in the erosive section and BD in the others. The predominance of BD was also manifested when performing RDA and structural equation modelling (SEM) over the entire slope covering all five sections.The other significant factors included SAND, SOC, P and WAS, in the decreasing order of direct path coefficient. In addition, these factors also indirectly affected Ks through their influence on BD and/or other factors. These findings demonstrate the strong temporal variability of Ks throughout the maize growing season and intricate interactions of Ks with related factors, and have important implications for hydrological modelling and agricultural production in the black soil region of northeast China and other similar regions.

Stability analysis of heterogeneous infinite slopes under rainfall-infiltration by means of an improved Green–Ampt model

Rainfall infiltration analysis has a great significance to the mitigation and risk assessment of rainfall-induced landslides. The original Green–Ampt (GA) model ignored the fact that a transitional layer exists in infiltration regions of soils under the rainfall permeation; moreover, it cannot effectively analyze the rainfall-infiltrated heterogeneous slope considering the spatial variability of saturated hydraulic conductivity ( ks ). In this study, an improved GA model is proposed for the rainfall-infiltration analysis of heterogeneous slopes. Four common slope cases are investigated to validate the effectiveness of the proposed model. An infinite slope model is taken as an illustrative example to investigate the distributions of volumetric water content and slope stability under the rainfall infiltration. The results show that the distributions of volumetric water content and factor of safety ( Fs) obtained from the proposed model are in very good agreement with the numerical results of the Richards' equation. In contrast, the modified GA model obtains biased distributions of volumetric water content and smaller Fs for the same cases. The results show that the proposed GA model can accurately identify the location of critical slip surface of the slope, and as such it provides an efficient method for risk analysis and control of slopes susceptible to landslide.

Probabilistic Slope Seepage Analysis under Rainfall Considering Spatial Variability of Hydraulic Conductivity and Method Comparison

Due to the spatial variability of hydraulic properties, probabilistic slope seepage analysis becomes necessary. This study conducts a probabilistic analysis of slope seepage under rainfall, considering the spatial variability of saturated hydraulic conductivity. Through this, both the commonly used Monte Carlo simulation method and the proposed first-order stochastic moment approach are tested and compared. The results indicate that the first-order analysis approach is effective and applicable to the study of flow processes in a slope scenario. It is also capable of obtaining statistics such as mean and variance with a high enough accuracy. Using this approach, higher variabilities in the pressure head and the fluctuation of the phreatic surface in the slope are found with a higher value of the correlation length of the saturated hydraulic conductivity. The Monte Carlo simulation is found to be time-consuming: at least 10,000 realizations are required to reach convergence, and the number of realizations needed is sensitive to the grid density. A coarser grid case requires more realizations for convergence. If the number of realizations is not enough, the results are unreliable. Compared with Monte Carlo simulation, the accuracy of the first-order stochastic moment analysis is generally satisfied when the variance and the correlation length of the saturated hydraulic conductivity are not too large. This study highlights the applicability of the proposed first-order stochastic moment analysis approach in the slope scenario.

Assessment of the Effects of the 2021 Caldor Megafire on Soil Physical Properties, Eastern Sierra Nevada, USA

The Caldor fire burned ~222,000 acres of the Eastern Sierra Nevada during summer–fall 2021. We evaluated the effects of this “megafire” on the physical properties of a sandy soil developed from glacial tills to document fire-induced soil modifications in this region. We measured soil water retention and hydraulic conductivity functions as well as the thermal properties of five core samples from control (unburned) areas and eight core samples from burned soil of the same soil unit. Soil water repellency was measured in terms of water drop penetration time (WDPT) in the field and apparent contact angle in the laboratory on control and burned soil as well as ash samples. Soil organic matter (SOM) and particle and aggregate size distributions were determined on control and burned soil samples. Additionally, scanning electron microscopy (SEM) was used to image microaggregates of control and burned soil samples. We found a significant difference in SOM content and sand and silt aggregate size distribution between control and burned samples, which we associated with the disintegration of microaggregates due to the fire. We found no significant difference between soil water retention and hydraulic conductivity functions of control and burned soil but observed greater variation in saturated hydraulic conductivity and systematic shifts in thermal conductivity functions of burned compared to control samples. WDPT and apparent contact angle values were significantly higher for burned soils, indicating the occurrence of fire-induced soil hydrophobicity (FISH). Interestingly, the average apparent contact angle of the control soil was >90°, indicating that even the unburned soil was hydrophobic. However, the ash on top of the burned soil was found to be hydrophilic, having apparent contact angles <10°. Our results indicate that SOM and microaggregates were readily affected by the Caldor fire, even for sandy soil with a weakly developed structure. The fire seemed to have moderated thermal properties, significantly and soil wettability but had only minimal effects on water retention and hydraulic conductivity functions. Our findings demonstrate the complex nature of fire-soil interactions in a natural environment and highlight the need for additional investigation into the causes and processes associated with FISH and structure alterations due to fire to improve our ability to rapidly determine potential problem areas in terms of hazards commonly associated with fire-affected soils.

A statistical approach for the assessment of the saturated hydraulic conductivity applied to an Austrian region

Study regionThis study refers to the Hydrological Open Air Laboratory (HOAL) watershed, located in South-West Austria. Study focusThe spatial variability of saturated hydraulic conductivity, Ks, is sometimes synthetized by geometric mean, K̃s, and coefficient of variation, while areal-average infiltration models rely upon the arithmetic mean, K¯sl, associated to log-transformed Ks, and relative coefficient of variation, CVal. Robust estimation of K¯sl and CVal, as well as of K̃s and associated coefficient of variation, CVg,would require a large number of Ks observations. The determination of the minimum number of Ks measurements, n*, for obtaining sufficiently accurate values of each aforementioned quantity over an area is an open issue addressed here. A statistical approach has been applied to Ks datasets on three grassy plots for an uncertainty analysis based on the non-parametric bootstrap method with replacement. The uncertainty of each quantity has been derived for different observation numbers and areas. New hydrological insights for the regionConsidering different sub-regions in the largest plot the uncertainty is almost invariant with increasing the sub-region area beyond a threshold. Furthermore, for a given n*, the uncertainty of K¯sl and CVal is much smaller than that of K̃s and CVg. Our approach defines a methodology for determining over an area the n* associated to a fixed uncertainty level in the joint estimation of the selected quantities. Guidelines for investigations over different plots are also proposed.

SPATIAL VARIABILITY OF HYDRO-RELATED PHYSICAL PROPERTIES OF AL-RASHEED LOAM

A field study was conducted on 2 hectare area at Al- Rashid district, south of Baghdad to analyze the spatial variability of saturated hydraulic conductivity (Ks), initial infiltration rate (IR), Porosity (F) and bulk density (BD). Based on measured BD values Rosetta software was used in this study to estimate water retention parameters, water content at θ33 and θ1500 kPa and unsaturated hydraulic conductivity at 33 kPa(k33), 100 kPa(k100) and 1500 kPa(k1500) according to Van Genuchten-Mualem model. Measured and predicted data were analyzed both statistically and geostatistically and, the results showed a strong to moderate spatial dependence for the studied characteristics. The spatial correlation values (r2) were obtained with a spherical model for Ks, θ33, θ1500, k33, k100 and k1500, an exponential model for IR, and a Gaussian model for F and BD. Ks increased significantly with increasing of IR (r2 = 0.49**) and decreased with increasing of F and BD, IR increased with decreasing of BD (r2 = -0.326*) and BD increased with increasing of F (r2 = 0.989 **). In general the spatial distribution was moderately skewed (-0.5 to 0.5) for the studied characteristics with pronounced kurtosis (>2.5). The limit distance for the search radius to estimating spatial dependency varied from 29.9 m for BD to 105 m for IR.

Mechanical sowing alters slope-scale spatial variability of saturated hydraulic conductivity in the black soil region of Northeast China

Saturated hydraulic conductivity (Ks) and its spatial variability are essential for the simulation of water and solute transport in the vadose zone. Mechanical operations have been increasingly employed in agricultural systems during recent decades to improve crop establishment, but their impact on Ks spatial variability has rarely been evaluated. The objectives were to characterize the slope-scale variability of Ks and to identify the primary controls dominating Ks distributions before and after mechanical sowing. Along a 900 m slope transect in the black soil region of Northeast China, soil samples were collected in an interval of 20 m, and Ks, bulk density (BD), particle-size distribution, wet-aggregate stability (WAS), soil organic carbon content (SOC) and land surface roughness (SR) were investigated before and after mechanical sowing in the spring. The results show that sowing significantly decreased Ks along the transect (p < 0.01). Partly due to the strong comparisons among three different sections with respect to soil erosion, i.e., original, erosive and deposited sections, large Ks spatial variability was detected both before and after sowing. According to the semivariogram analysis, nevertheless, sowing decreased the semivariances of Ks overall and altered its spatial structure, i.e., from an exponential semivariogram possessing a sill of 0.72 mm2 min−2 and exhibiting a moderate spatial dependency before sowing to a pure nugget effect value of 0.27 mm2 min−2 corresponding to random distribution after sowing. Using the state-space approach, BD, CLAY and SR were identified as the main factors regulating Ks spatial distributions, whether before or after sowing. These findings demonstrate the remarkable impact of mechanical sowing on Ks and its spatial variability, and hold important implications for hydrological modeling and agricultural management in the black soil region of Northeast China.

Rainfall Infiltration Process of a Rock Slope with Considering the Heterogeneity of Saturated Hydraulic Conductivity

In order to analyze the effects of rainfall events on the stability of an open-pit rock slope, with considering the spatial variability of saturated hydraulic conductivity, based on the unsaturated seepage theory and the random filed theory, modified functions of the unit saturation, the hydraulic conductivity (k), and the shear strength parameters are established for unsaturated slope, by using FISH and the non-intrusive stochastic method. A saturated-unsaturated seepage random field model is proposed. And then the impacts of the rainfall intensity, the rainfall duration, and the spatial variability of saturated hydraulic conductivity (ks) on the infiltration process and stability of the unsaturated rock slope are analyzed. The results show that the proposed model can estimate rainfall infiltration of rock slope accurately. Rainfall mainly affects the seepage field in the shallow layer of the slope, where a transient saturated zone can be formed. With the development of the rainfall duration, the weight of the rock mass increased, the matric suction reduced, the negative pore pressure, the degree of saturation, and the infiltration depth of the rock slope increased, and the water in the slope root connects with the initial water table gradually, the unsaturated zone shrinks, which causes the safety factor of the model decreases, but the trend of change slows down gradually. As the rainfall intensity strengthened, the infiltration depth increased and the safety factor of the slope reduced, while the changing rate increases first and then decreases. Increasing the correlation length of k can reduces the infiltration depth and safety factor of the slope. Increasing the variation coefficient of k will increase the infiltration depth, while the safety factor of the slope decreases. The infiltration depth and safety factor of the slope are most affected by rainfall duration, but its sensitivity to the variability coefficient of k will be strengthened when the rainfall intensity exceeds the infiltration capacity. This conclusion can provide reference significance for the risk estimation of slope geological hazards, which are induced by the rainfall infiltration.

Spatial variation of saturated hydraulic conductivity in a riparian zone: A case study of the Baoan Lake in Hubei Province, China

Saturated hydraulic conductivity (Ks, [L T−1]) is an important physical property of soil in riparian zones that influences water and solutes exchange between surface and subsurface water systems. Spatial variation of Ks should be considered in the assessment of its effects on the interaction of surface and subsurface waters in riparian zones. In this study, 133 disturbed and 399 undisturbed soil samples were collected from the riparian zones of the Baoan Lake in Hubei Province, China. Spatial distribution patterns of Ks in rural riparian zones were determined, and their variabilities were analyzed to understand the influences of soil, land use, vegetation, hydrological, and geological factors. The spatial variations of Ks indicated the following: (1) the mean Ks value decreased with soil depth in the riparian zone; (2) Ks values showed increasing and fluctuating patterns in the lateral direction of the lakeshore; (3) the study region was divided into high-, medium-, and low-permeability areas; and (4) Ks values of influent tributaries decreased from upstream to downstream reaches in the periodic inundation zone (Zone 1), whereas the effluent tributary exhibited the opposite trend. Permeabilities were low at the confluences of the lake and its tributaries compared to that of the other reaches of the same tributaries. Riparian soil displayed fast to slow infiltration patterns between 0–20 cm and 20–40 cm soil layers. The degree of vertical variation caused by the periodic inundation of water was smaller. Permeability based on land use decreased in the order of farmland, forests, and fishponds. Studying the permeation of riparian zones and associated ecosystems provides a scientific basis for their comprehensive management and efficient utilization, and this work provides a framework for the same through a case study.

Spatial variability of saturated hydraulic conductivity and its links with other soil properties at the regional scale

Saturated hydraulic conductivity (K) is a key property for evaluating soil water movement and quality. Most studies on spatial variability of K have been performed soil at a field or smaller scale. Therefore, the aim of this work was to assess (quantify) the spatial distribution of K at the larger regional scale in south-eastern Poland and its relationship with other soil properties, including intrinsic sand, silt, and clay contents, relatively stable organic carbon, cation exchange capacity (CEC) and temporally variable water content (WC), total porosity (FI), and dry bulk density (BD) in the surface layer (0–20 cm). The spatial relationships were assessed using a semivariogram and a cross-semivariogram. The studied region (140 km2) with predominantly permeable sandy soils with low fertility and productivity is located in the south-eastern part of Poland (Podlasie region). The mean sand and organic carbon contents are 74 and 0.86 and their ranges (in %) are 45–95 and 0.002–3.75, respectively. The number of individual samples varied from 216 to 228 (for K, WC, BD, FI) to 691 for the other soil properties. The best fitting models were adjusted to the empirical semivariogram (exponential) and the cross-semivariogram (exponential, Gaussian, or linear) used to draw maps with kriging. The results showed that, among the soil properties studied, K was most variable (coefficient of variation 77.3%) and significantly (p < 0.05) positively correlated with total porosity (r = 0.300) and negatively correlated with soil bulk density (r = – 0.283). The normal or close to the normal distribution was obtained by natural logarithmic and root square transformations. The mean K was 2.597 m day−1 and ranged from 0.01 up to 11.54 m day−1. The spatial autocorrelation (range) of K in the single (direct) semivariograms was 0.081° (8.1 km), while it favourably increased up to 0.149°–0.81° (14.9–81 km) in the cross-semivariograms using the OC contents, textural fractions, and CEC as auxiliary variables. The generated spatial maps allowed outlining two sub-areas with predominantly high K above 3.0 m day−1 in the northern sandier (sand content > 74%) and less silty (silt content < 22%) part and, with lower K in the southern part of the study region. Generally, the spatial distribution of the K values in the study region depended on the share of individual intrinsic textural fractions. On the other hand, the ranges of the spatial relationship between K and the intrinsic and relatively stable soil properties were much larger (from ~ 15 to 81 km) than between K and the temporally variable soil properties (0.3–0.9 km). This knowledge is supportive for making decisions related to land management aimed at alteration of hydraulic conductivity to improve soil water resources and crop productivity and reduce chemical leaching.

Variation in Saturated Hydraulic Conductivity at the Outcrop Scale, the Whanganui Basin, New Zealand.

Groundwater flow and contaminant transport are strongly influenced by hydrogeological spatial variation. Understanding the textural heterogeneity of aquifer and aquitard units is critical for predicting preferential flow pathways, but is often hindered by sparse hydrogeological data, widely spaced data points, and complex stratigraphy. Here, we demonstrate the application of a relatively new air permeameter technology, providing a cost-effective, rapid alternative for characterizing hydrostratigraphic units in the field. The aim of this research is to (1) characterize the variation of saturated hydraulic conductivity across shallow-marine hydrostratigraphic units of the Whanganui Basin, New Zealand, and (2) assess the variation of saturated hydraulic conductivity within individual hydrostratigraphic units and relate these changes to facies and depositional environments. Results suggest heterogeneity within fine-grained aquitard units is controlled by bioturbation, whereby burrowing, ingestion and defecation results in grain size segregation and differential micrite cementation. Coarse-grained heterolithic aquifer facies display sharp changes in permeability across planar to cross-bedded sets, related to current and wave energy fluctuations within shallow-marine depositional settings. Bedding plane orientation creates high permeability zones that promotes down dip subsurface flow. Down dip gradation of coarse-grained nearshore facies into fine-grained shelf facies along the paleo shoreline-shelf transect is suggested to promote lateral and vertical groundwater flow within the basin fill. Air permeameter techniques have potential for application within groundwater basins around the world, providing datasets that facilitate greater understanding of groundwater systems, informing practices and policies for targeted water quality management.

SPATIAL VARIABILITY OF THE SATURATED HYDRAULIC CONDUCTIVITY OF SOIL IN COCOA FARMING IN RECÔNCAVO BAIANO

ABSTRACT Irrigated cocoa cultivation opened the way for production in Coastal Tablelands soils. However, in this region, the cohesive layer formed near the surface can be a limiting factor for production. The knowledge of physical soil water attributes enables the efficient irrigation management of cohesive soils. This study characterized and modeled the spatial variability of saturated hydraulic conductivity (K0) in a Distrocoeso Oxisoil of the Recôncavo Baiano Coastal Tablelands. The soil sampling was performed as undeformed structures from 50 spaced points in an 8.0 to 8.0 m area, at three different depths in the experimental area of the Federal University of Bahia Recôncavo in the Cruz das Almas-BA cultivated with cocoa (‘CCN 51’). In the laboratory, K0 was determined by permeameter method constant load, and the pore size distribution was determined using the voltage table and the soil density (Ds). Data were analyzed using descriptive statistics and geostatistics. On average, the K0 values were 40.41, 26.49, and 37.82 mm-1 h-1 at the depths from 0.0-0.15 m, 0.15-0.30, and 0.30-0.45 m. The Gaussian model was the best fit to the K0 data set. For soil class, the K0 showed a strong spatial dependence due to their relationship with the physical properties of the soil, its use, and handling. Since an important attribute for the delimitation of homogeneous areas for specific site management purposes as well be considered.

Performance of a Hydraulically Linked and Physically Decoupled Stormwater Control Measure (SCM) System with Potentially Heterogeneous Native Soil

This study shows that a physically decoupled but hydraulically linked design focusing on surface infiltration components (i.e., excluding underdrain and infiltration bed systems) can be the preferred way to have a low-cost and robust stormwater control measure (SCM) system. The SCM under investigation in Philadelphia, PA, is a green infrastructure (GI) and has a mirrored design of two sets of hydraulically linked planters. Each planter has an overflow pipe connected to an underground infiltration bed. The system showed excellent overall performance as no overflow/bypass entering the combined sewer. A large variation of saturated hydraulic conductivity was found for the planter soil, and the planter with lower saturated hydraulic conductivity created surface runoff that overflows to the next planter in line. Due to the linked design, the unexpected deviation of performance of a single planter did not affect overall system performance. The infiltration bed showed great variation in water drawdown rate at different water depth, which could be caused by the possible high heterogeneity of the native soil. The study argued that overflow systems, which handled only about 18% of runoff in this study, can be replaced by slightly larger surface area for lower building cost, lower maintenance cost, and more reliable performance.

Effect of soybean roots and a plough pan on the movement of soil water along a profile during rain

The movement of water into soil is the primary driving force in nutrient transport for crops, and the laws governing this movement can be affected by crop roots and plough pans. The main objectives of this study were to monitor the movement of water during rain along a soil profile influenced by soybean roots and a plough pan and to assess whether these factors influenced the efficacy of using the Richards equation to simulate the transport of soil water along a profile associated with a single rain. Experiments and model simulations were carried out. Our results suggested that the soybean field soil may have had preferential flow paths. Soybean stems and leaves transferred rainwater that otherwise would have fallen farther from the central stalk. The transmission of rainwater by the branches and leaves had a larger effect than the interception of rainwater by the branches and leaves. The effect of the root system on the soil particles may increase soil capillary action and ultimately increase saturated hydraulic conductivity. Saturated hydraulic conductivity was higher near the surface than at depth, because the transmission of water by the root system and the effect of large gaps were greater in the soil near the surface. Saturated hydraulic conductivity for both bare land and land covered with soybean plants decreased linearly from depths of 0–15 cm along the soil profile, particularly in the presence of a plough pan. We developed a linear equation to represent the variation in saturated hydraulic conductivity in the 0–15-cm layer in the black soil area of northeastern China. Using our linear model for saturated hydraulic conductivity as a parameter, the Richards equation more accurately simulated soil water transport along a profile influenced by soybean roots and a plough pan in rainy conditions.

Spatial variability of saturated hydraulic conductivity and measurement-based intensity-duration thresholds for slope stability, Santa Ynez Valley, CA

Shallow landslides are commonly triggered by positive pore-fluid pressures generated at the soil-bedrock interface and are linked to heavy rainfall. In this study, in situ measurements of saturated hydraulic conductivity (Ksat) were performed at three historic landslide sites to determine its variability and implications on predicting rainfall-rate thresholds for shallow landslides in the Santa Ynez Valley, CA. Falling head tests were performed in the soil column and at the soil-bedrock interface to estimate Ksat. It was found that overlying soils from the A and B horizons exhibited higher infiltration rates ranging from 37 to 138 mm/h and 24 to 77 mm/h, respectively, whereas the failure plane (coincident with the soil-bedrock interface) exhibited Ksat values ranging from 4 to 33 mm/h. Below the soil-bedrock interface, Ksat ranged from 5 to 123 mm/h, signifying a pattern of fast-slow-fast infiltration. Measurements of grain size distributions and bulk density do not provide insight into the cause of low-conductivity at the soil-bedrock interface. Rather, field observations suggest that the infilling of fractures and pore spaces with fine-grained particles and caliche are a possible cause of the low-conductivity layer. Measurements of soil mechanical and hydrologic properties were used in a finite slope-stability model as well as a simple 1-D flow model to predict intensity-duration (I-D) thresholds for shallow landsliding in this area. Results of this analysis indicate rainfall intensities >20–30 mm/h that are sustained for >2–4 h are capable of triggering widespread shallow landsliding. This study also suggests that the variability in Ksat at the failure plane can produce significant changes in I-D thresholds for failure and is an important factor in predicting slope stability over any spatial scale.

Quantification of the structure evolution in a garden soil over the course of two years

In this proof-of-concept, we demonstrate the potential of quantifying the structural evolution in an individual soil sample with the help of X-ray imaging. The soil sample was acquired in summer 2013 after a manual seedbed preparation and scanned with X-ray CT on six occasions during the following two years. After each imaging session, the soil sample was re-installed into the field. We focused on analyzing the evolution of soil morphologic measures that are thought to be fundamental to air and water flow in soil. We also quantified deformation of the soil matrix during the experiment. Our results illustrate the effects of several biotic and abiotic processes on the evolution of soil structure. A well-connected inter-aggregate pore network after seedbed preparation was replaced by a sparser network of larger biopores. Macro-faunal burrowing activity generally increased morphological measures associated with larger air and hydraulic conductivity as well as a better aeration. Soil settling and the growth of a dandelion tap-root acted in the opposite direction. Soil settling and compaction continued during the entire experimental period, but was restricted to soil depths below 20 mm. Other noteworthy observations that appear worth investigating in follow-up experiments were i.) the strong variation in the critical pore diameter, which could explain the commonly noted large temporal variability of saturated hydraulic conductivity, ii) the much greater extent of lateral compaction due to tap root growth than macro-faunal burrowing, and iii.) the short life-span of large biopores. We conclude that the approach presented here shows great potential for quantifying soil structural dynamics pertaining to individual structure-forming and degrading processes under field conditions. This kind of data could also prove very useful for constructing and testing ‘next-generation’ models that link a dynamic description of soil structure to various processes and functions in the soil-plant system.

Two‐Dimensional Probabilistic Infiltration Analysis in a Hillslope Using First‐Order Moment Approach

A first-order moment analysis method is introduced to evaluate the pore-water pressure variability within a hillslope due to spatial variability in saturated hydraulic conductivity (Ks ) during rainfall. The influences of the variance of the natural logarithm of Ks (ln Ks ), spatial structure anisotropy of ln Ks , and normalized vertical infiltration flux (q) on the evaluations of the pore-water pressure uncertainty are investigated. Results indicate different responses of pressure head variability in the unsaturated region and the saturated region. In the unsaturated region, a larger variance of ln Ks , a higher spatial structure anisotropy, and a smaller q lead to a larger variability in pressure head, while in the saturated region, the variability in pressure head increases with the increase of variance of ln Ks , the decrease of spatial structure anisotropy, or the increase of q. These variables have great impacts on the range of fluctuation of the phreatic surface within the hillslope. The influences of these three variables on the variance of pressure head within the saturated region are greater than those within the unsaturated region, and the variance of ln Ks has the greatest impact. These results yield useful insight into the effects of heterogeneity on pressure head and uncertainty associated with predicted flow field.