Near-saturated Hydraulic Conductivity Research Articles

Freeze-thaw effects on pore space and hydraulic properties of compacted soil and potential consequences with climate change

Freezing and thawing affect the pore-space structure in agricultural soils with implications for soil hydraulic properties and water flow. Previous studies have focused on the upper few centimeters of the tilled topsoil, where most freeze-thaw (FT) cycles occur, even though deeper soil layers are also subject to freezing and thawing in cold climates. Thus, little is known about how freezing and thawing affect untilled soil layers, which often show high bulk densities that restrict vertical water movement. Furthermore, it remains unclear how shifts in FT patterns with climate change may change the pore-space structure and water flow through these soil layers. Here we investigated the effects of freezing and thawing on X-ray imaged pore-space characteristics, water retention and near-saturated hydraulic conductivity (K) in untilled soil directly below plough depth. Intact cores were sampled at two sites in central Sweden under the same long-term reduced tillage management. The two soils, a silt loam and a silty clay loam, were subjected to three FT scenarios in a laboratory environment intended to represent FT patterns that are considered likely under current and future winter conditions for this region. The latter scenario was characterised by more FT cycles and a lower freezing temperature. Freezing and thawing increased K in the near-saturated range in both soils, which we attribute to observed small (<0.01 mm3 mm−3) increases in the volume of pores of diameters close to the X-ray resolution limit. Concomitant increases in pore network connectivity and critical pore diameter, especially in the denser silty clay loam soil, probably contributed to this increase in K. The water retention data suggested that changes in pore-space characteristics below X-ray resolution also occurred in both soils. Furthermore, our results indicate that both soils may show higher drainage rates due to shifts in FT patterns in the future, although longer-term changes in pore-space structure with an increasing number of FT cycles would mostly be limited to soils with relatively high clay contents. These soils are often more compacted below plough depth and, thus, benefits from improvements in soil structure such as improved root growth and plant water supply are also expected to be larger.

Predicting saturated and near-saturated hydraulic conductivity using artificial neural networks and multiple linear regression in calcareous soils.

Hydraulic conductivity (Kψ) is one of the most important soil properties that influences water and chemical movement within the soil and is a vital factor in various management practices, like drainage, irrigation, erosion control, and flood protection. Therefore, it is an essential component in soil monitoring and managerial practices. The importance of Kψ in soil-water relationship, difficulties for its measurement in the field, and its high variability led us to evaluate the potential of stepwise multiple linear regression (SMLR), and multilayer perceptron (MLPNNs) and radial-basis function (RBFNNs) neural networks approaches to predict Kψ at tensions of 15, 10, 5, and 0 cm (K15, K10, K5, and K0, respectively) using easily measurable attributes in calcareous soils. A total of 102 intact (by stainless steel rings) and composite (using spade from 0-20 cm depth) soil samples were collected from different land uses of Fars Province, Iran. The common physico-chemical attributes were determined by the common standard laboratory approaches. Additionally, the mentioned hydraulic attributes were measured using a tension-disc infiltrometer (with a 10 cm radius) in situ. Results revealed that the most of studied soil structure-related parameters (soil organic matter, soluble sodium, sodium adsorption ratio, mean weight diameter of aggregates, pH, and bulk density) are more correlated with K5 and K0 than particle-size distribution-related parameters (sand, silt, and standard deviation and geometric mean diameter of particles size). For K15 and K10, the opposite results were obtained. The applied approaches predicted K15, K10, K5, and K0 with determination coefficient of validation data (R2val) of 0.52 to 0.63 for SMLR; 0.71 to 0.82 for MLPNNs; and 0.58 to 0.78 for RBFNNs. In general, the capability of the applied methods for predicting Kψ at all the applied tensions was ranked as MLPNNs > RBFNNs > SMLR. Although the SMLR method provided easy to use pedotransfer functions for predicting Kψ in calcareous soils, the present study suggests using the MLPNNs approach due to its high capability for generating accurate predictions.

Impacts of soil management and climate on saturated and near-saturated hydraulic conductivity: analyses of the Open Tension-disk Infiltrometer Meta-database (OTIM)

Abstract. Saturated and near-saturated soil hydraulic conductivities Kh (mm h−1) determine the partitioning of precipitation into surface runoff and infiltration and are fundamental to soils' susceptibility to preferential flow. Recent studies found indications that climate factors influence Kh, which is highly relevant in the face of climate change. In this study, we investigated relationships between pedoclimatic factors and Kh and also evaluated effects of land use and soil management. To this end, we collated the Open Tension-disk Infiltrometer Meta-database (OTIM), which contains 1297 individual data entries from 172 different publication sources. We analysed a spectrum of saturated and near-saturated hydraulic conductivities at matric potentials between 0 and 100 mm. We found that methodological details like the direction of the wetting sequence or the choice of method for calculating infiltration rates to hydraulic conductivities had a large impact on the results. We therefore restricted ourselves to a subset of 466 of the 1297 data entries with similar methodological approaches. Correlations between Ks and Kh at higher supply tensions decreased especially close to saturation, indicating a different flow mechanism at and very close to saturation than towards the dry end of the investigated tension range. Climate factors were better correlated with topsoil near-saturated hydraulic conductivities at supply tensions ≥ 30 mm than soil texture, bulk density and organic carbon content. We find it most likely that the climate variables are proxies for soil macropore networks created by the respective biological activity, pedogenesis and climate-specific land use and management choices. Due to incomplete documentation in the source publications of OTIM, we were able to investigate only a few land use types and agricultural management practices. Land use, tillage system and soil compaction significantly influenced Kh, with effect sizes appearing comparable to the ones of soil texture and soil organic carbon. The data in OTIM show that experimental bias is present, introduced by the choice of measurement time relative to soil tillage, experimental design or data evaluation procedures. The establishment of best-practice rules for tension-disk infiltrometer measurements would therefore be helpful. Future studies are needed to investigate how climate shapes soil macropore networks and how land use and management can be adapted to improve soil hydraulic properties. Both tasks require large numbers of new measurement data with improved documentation on soil biology and land use and management history.

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.Graphical

Influence of long-term tillage and diversified cropping systems on hydro-physical properties in a sandy loam soil of North-Western India

A study was conducted to determine the combined effects of three tillage practices and four maize (Zea mays L.)-based cropping systems on physical, saturated, and near-saturated hydraulic properties in a sandy loam soil of North-Western India. Split-plot experimental design was adopted with tillage [conventional tillage (CT), zero tillage (ZT), and permanent raised bed (PB)] as the main plot treatments and intensified crop rotations [Maize (Zea mays L.)-Wheat (Triticum aestivum)-Greengram (Vigna radiata L.) (MWGg), Maize-Chickpea (Cicer arietinum L.)-Sesbania (Sesbania aculeata) (MCpSb), Maize-Mustard (Brassica juncea) -Greengram (MMuGg) and Maize-Maize-Sesbania (MMSb)] as subplot treatments. The saturated and near-saturated soil hydraulic conductivity were derived from steady-state infiltration rates measured using a hood infiltrometer on the surface soil at 0, − 1, and − 3 cm pressure heads. The long-term (10 years) study revealed that the bulk density (BD) of the soil under conservation agriculture (CA) practices (PB and ZT) was significantly (P ≤ 0.05) lower than that in CT practices. The soil BD in the MCpSb cropping system was measured to be the lowest (1.24 g cm−3) among all the cropping systems. The soil aggregate mean weight diameter (MWD) under PB and ZT was determined to be 31% and 27% higher than in the CT treatments. In tillage × cropping systems interactions, the highest MWD was observed in the PB×MWGg. The saturated and near-saturated hydraulic conductivity (K(h)) were estimated to have higher values in CA practices (PB and ZT) than in the CT treatments. In the case of cropping systems, the soil's mean field saturated hydraulic conductivity was estimated to be significantly (P < 0.05) higher under the MWGg, MCpSb, and MMuGg than the MMSb. The present study indicates that conservation agriculture-based crop management with diversified maize-based rotation (MCpSb, MWGg, and MMuGg) could be promising alternatives to conventional tillage practices (CT). Among the cropping system choices, MWGg was more effective in improving the soil's hydro-physical properties in the study area.

Variation in root morphology amongst tree species influences soil hydraulic conductivity and macroporosity

Natural approaches to flood risk management are gaining interest as sustainable flood mitigation options. Targeted tree planting has the potential to reduce local flood risk, however attention is generally focused on the hydrological impacts of catchment afforestation linked to generic tree features, whilst the species-specific impacts of trees on soil hydrology remain poorly understood. This study compared effects of different tree species on soil hydraulic properties. Monocultures of Alnus glutinosa (common alder), Fraxinus excelsior (European ash), Fagus sylvatica (European beech), Betula pendula (silver birch), Castanea sativa (sweet chestnut), Quercus robur (English oak) and Acer pseudoplatanus (sycamore maple) were used to determine effects of tree species identity on soil hydraulic properties (near-saturated K and soil water retention) in a sandy loam soil, North Wales, United Kingdom. The interaction of F. excelsior root properties and soil class on hydraulic conductivity was also examined in four different soils (Rendzic Leptosol, Haplic Luvisol, Dystric Fluvic Cambisol and Dystric Gleysol) across England and Wales. Fine root biomass (FRB) and morphological characteristics were determined at three depths (0–0.1, 0.1–0.2 and 0.2–0.3 m) and complemented by in situ surface measurement of soil hydraulic conductivity. Root morphological traits were closely associated with species identity and pore-size distribution, and FRB was strongly correlated with soil hydraulic conductivity (R2 = 0.64 for 0–0.1 m depth FRB; R2 = 0.69 for 0.1–0.2 m depth FRB). Fine root biomass of F. excelsior was sixfold greater than C. sativa (p < 0.001), and the frequency of 0.01 mm radius soil pores under F. excelsior was twice that of Q. robur. Near-saturated hydraulic conductivity under F. excelsior was 7.91 ± 1.23 cm day−1, double the mean rate of the other species. Soil classification did not significantly influence FRB (p = 0.056) or near-saturated hydraulic conductivity (p = 0.076) in the 0.0–0.1 m depth soil, but soil water retention varied with depth. Species-specific traits of trees should be considered in landscape design to maximise the local hydrological benefits of trees.

Maize cropping degrades soil hydraulic properties relative to grazed pasture in two contrasting soils

Soil hydraulic properties (SHPs), including available water content and near-saturated hydraulic conductivity (Kns), affect hydrological and biochemical processes. The SHPs information is crucial to agricultural water management. The objective of this study using paired sites was to investigate the effects of land use on SHPs in two contrasting soil orders. Soil water retention curves and Kns at three soil depths (0–10, 10–20 and 20–30 cm) were measured under two land uses (pasture, consisting of a rye grass [Lolium perenne L.] and white clover [Trifolium repens L.] mix, and maize [Zea mays L.] cropping > 10 years) in Waikato, New Zealand. For each land use, two soil orders with contrasting soil structural vulnerability were selected: less vulnerable Allophanic soil and more vulnerable Gley soil. Compared with pasture, maize cropping reduced macroporosity, readily available water capacity and Kns of 0–30 cm, and the effect was greater in the deep layer (20–30 cm). This indicated that maize cropping practices result in greater structural degradation to soils compared with pasture, which include the potential for greater subsoil compaction. There was no land use by soil order interaction effect on SHPs, suggesting that the relative SHP degradation under maize cropping compared with pasture grazing was not associated with soil structural vulnerability. Our study emphasised that long-term continuous cropping with maize on the more vulnerable soil (i.e. Gley soil) resulted in the poorest soil physical health.

Surface soil hydraulic conductivity and macro-pore characteristics as affected by four bamboo species in North-Western Himalaya, India

Soil hydraulic properties are important aspects of the bamboo plantation's eco-hydrological processes as they affect the runoff, soil erosion, and water balance. This study assessed the effect of four commercially important sympodial bamboo species viz., Bambusa vulgaris, Bambusa balcooa, Dendrocalamus hamiltonii, and Dendrocalamus strictus on surface soil saturated and near-saturated hydraulic conductivity (NSHC), macropore characteristics, and the factors influencing the soil hydrological processes. The surface soil hydraulic conductivity at 0, -10, and -30 mm pressure heads and macropore characteristics were estimated from measured steady-state infiltration using a tension (hood) infiltrometer. Soil organic carbon, bulk density, aggregate mean weight diameter (MWD), root biomass, and litterfall was correlated with saturated hydraulic conductivity (SHC) to determine these parameters' relationship with the hydraulic properties. D. hamiltonii showed significantly higher SHC, NSHC, and water-conducting macroporosity than the rest of the species. The correlation analysis revealed that root biomass and soil aggregate MWD were the major determinants of soil hydraulic conductivity characteristics. Bulk density, soil organic carbon, and leaf litter were weakly correlated with SHC. Among the species evaluated, D. hamiltonii was found to be a highly promising species for improvement of soil hydro-physical property and, therefore, can be recommended for increasing infiltration, reducing runoff, controlling soil erosion, and improving groundwater recharge.

Rotation crop root performance and its effect on soil hydraulic properties in a clayey Utisol

Poor soil hydraulic properties (SPH) limit the productivity of clayey Utisol (red soil) under a humid subtropical climate in southern China. The roots performance of rotation crops was compared, and the effects on SPH were investigated in the field. Five treatments (two cultivars of oilseed rape, one-year-old lucerne, and one- and five-year-old vetiver) were employed as pre-crops before summer maize, and their root performances were examined in spring. The soil-water characteristic curve (SWCC) and soil porosity were measured with soil cores, saturated hydraulic conductivity (Ks), and near-saturated hydraulic conductivity with a tension infiltrometer in situ. The two rape cultivars had the thinnest root diameter at an average of 0.45−0.50 mm and the shallowest distribution at a depth of 40 cm, while two vetivers had the thickest mean root diameter at 0.79 mm and the deepest distribution >80 cm. Taproot crops of rape and lucerne did not show stronger rooting ability than fibrous root crops of vetivers. All crops increased soil total and aeration porosity, but no bio-pores larger than 2 mm were formed. Rapes were beneficial for increasing soil aeration porosity (diameter >30 μm), while vetivers increased active capillary porosity (30−0.2 μm). The SHP, namely, SWCC and Ks, was improved by all crops, with the best effects in the five-year old vetiver due to its higher root length density (RLD) and the balanced distribution between diameter classes. Ks was significantly correlated with RLD (p < 0.001) but insignificantly correlated with root diameter (p = 0.9929). The SHP was improved mainly by RLD and influenced by root diameter distribution, suggesting that fine roots played a key role in forming aeration and active capillary pores.

Predicting near-saturated hydraulic conductivity in urban soils

Pedotransfer functions (PTFs) provide point predictions of soil hydraulic properties from more readily measured soil characteristics, yet uncertainties and biases in measurement methods, sampling distributions, and boundary conditions can limit accuracy when estimating near-saturated hydraulic conductivity (Kn). These limitations may be particularly problematic in understudied urban landscapes that often contain altered hydraulic properties. To better treat deficiencies in PTF performance, we addressed three objectives, which were to: 1) develop PTFs to predict urban Kn, 2) assess bulk density and coarse fragments as explanatory variables; and 3) evaluate the predictive capability of these PTFs by comparing their output to measured hydraulic conductivity values from three other studies of urban soil hydraulics. We used artificial neural networks (ANN) and random forest (RF) approaches to predict urban Kn, with the training dataset including 307 tension infiltrometer tests and other measurements drawn from urban soil assessments in 11 U.S. cities. The PTFs utilized a hierarchy of inputs, starting with percentage sand, silt, clay, and then adding percentage coarse fragments and bulk density. The ANN models performed similar to the RF models, and all models exhibited similar or better predictive performance as models results collected from published articles. The inclusion of bulk density or coarse fragments did not improve accuracy over soil texture alone. Possible reasons for this result include low correlation between Kn and bulk density and the exclusion of large voids during flow measurements with tension infiltrometers. The models have been made available as an open-source software package to encourage adoption by users working in urban systems.

Variations of soil hydraulic properties along granitic slopes in Benggang erosion areas

As soil hydraulic characteristics, including near-saturated hydraulic conductivity and water retention, influence slope stability and erosion development, this study aimed at the spatial distribution patterns of hydraulic properties along the Benggang slopes for a better understanding of the development mechanisms of Benggang erosion in southern China. Near-saturated hydraulic conductivity and soil water retention at different slope positions in two typical Benggang erosional areas were determined by the in situ infiltration test using tension disk infiltrometer and laboratory test using the high-speed freezing centrifuge method. The soil hydraulic conductivity, effective macropore, and pore size parameter, Gardner α, in lower slopes were greater than those in upper and middle slopes. From upper to lower slope, the total water flow showed an increasing trend in pores > 1 mm in diameter, in contrast to a decrease in pores between > 0.33 mm and < 0.5 mm in diameter, and < 0.33 mm in diameter. The effective macroporosity values in measured soils were statistically greater (p < 0.05) in the lower slope than in the upper or middle slope. Meanwhile, the soil water retention of lower slopes was lower than that of upper and middle slopes, and the lower slope had higher unsaturated hydraulic conductivity than the upper or middle slope. Erosion degree showed significant (p < 0.05, p = 0.016) influence on the soil hydraulic conductivity and effective macropore. Moreover, the soils on the severe erosion slope drain faster than that on the slight erosion slope, and the unsaturated hydraulic conductivity decreased faster on the severe erosion slope than on the slight erosion slope. Vertical differentiation of granitic soil properties is the main factor influencing the soil hydraulic properties of the two slopes with a different erosion degree, while the macropores are the main factor influencing the soil hydraulic properties in different slope positions. Spatial variation may affect the rainwater infiltration during rainfall and soil water redistribution after the rainfall and finally affect the soil mass failure of the collapsing wall.

Long-term effects of conventional tillage and no-tillage on saturated and near-saturated hydraulic conductivity – Can their prediction be improved by pore metrics obtained with X-ray CT?

Tillage practices have a profound impact on soil structure and soil hydrology, which may affect ecosystem functions like plant productivity. There is an ongoing debate whether a conversion from conventional tillage (CT) to no-till (NT) leads to an increase in (near-)saturated hydraulic conductivity. This is because true effects are often disguised by large spatial and temporal variability, but also by the deficiencies in the measurement technique.In this paper, we measured (near-) saturated hydraulic conductivity (Ks and K-2) in a long-term tillage trial (26 years) in Germany with three different methods: hood infiltrometer (HI) in the field, tension disc infiltrometer (TI) on undisturbed soil cores and direct simulation (DS) of water flow on X-ray CT images of macropore structure in these soil cores with a Stokes-Brinkmann solver. On average the absolute values varied by two orders of magnitude in the order TI < HI < DS with very low correlation (R2 < 0.05) between log10(Ks) measurements. The conversion from CT to NT caused an increase in bulk density, a decrease in air capacity and a small but consistent decrease in grain yields. Ks was increased with NT when measured with HI but decreased in TI and indifferent when measured with DS. This inconsistency is caused by the proportion at which large biopores that are more frequent in NT soil due to higher earthworm abundance contribute to total flow in each measurement technique. Regression analyses between pore space attributes measured with X-ray CT and Ks (and K-2) showed very strong agreement with DS values, but poorer agreement with HI and TI, suggesting that those values are afflicted with measurement artifacts like poor contact, entrapped air, different average volumes and so on.The pore metric with highest predictive power (>90%) on simulated Ks in NT soil cores was the critical pore diameter because it represents the bottleneck that restricts a large contribution to flow by elongated biopores. However, in plowed soil (CT) pore metrics that best describe flow through the loose soil matrix, like macroporosity and pore connectivity, have a higher predictive power and the critical pore diameter is rendered meaningless. The relative importance of various pore metrics as good predictors of hydraulic conductivity does not only change in a very small pressure range (Ks vs. K-2) but also between measurement techniques (HI vs. TI vs. DS). These inconsistencies raise the question if and how existing pedo-transfer functions for estimating (near-) saturated hydraulic conductivities can be extended by image-derived pore metrics in a meaningful way.

Effects of land-use/cover change on soil hydraulic properties and pore characteristics in a semi-arid region of central Iran

The semi-arid regions of central Iran have vastly undergone the manipulation and conversion of rangelands to dry farmlands. Soil hydraulic properties and pore characteristics may differ in various land-use/cover types. This study evaluated the effects of good and poor rangeland, dry farmland and abandoned farmland on the soil hydraulic properties and pore characteristics in a semi-arid region of central Iran. A completely randomized design was used to analyze the effects of land-use/cover on soil hydraulic properties and pore characteristics. Water infiltration into the soil at inlet matric suction (h) values of 2, 5, 10 and 15 cm was measured using a tension infiltrometer in different land-use/cover types with 18 replications. Wooding's analytical method was used to model the infiltration data and the best-fit values for Gardner’s parameters of macroscopic capillary length (λc) and saturated hydraulic conductivity (Ks) were estimated. Pore characteristics were also estimated using the Watson and Luxmoore method. The results indicated that saturated and near-saturated hydraulic conductivity values [Kh] and λc were significantly influenced by the land-use/cover type. For h < 5 cm, good rangeland and dry farmland had the highest and lowest means of hydraulic conductivity, steady-state flux and sorptivity, respectively. The K values at h lower than 5 cm were found to be as follows: good rangeland > poor rangeland > abandoned farmland > dry farmland. Good rangeland had a greater number of large pore-size class (i.e., > 0.06 cm) and total porosity. Dry farmland and good rangeland had the lowest and highest proportions of large pore-size class (> 0.06 cm), respectively. Inappropriate management practices such as over-grazing of poor rangeland, cultivation and harvesting machinery stress and soil organic carbon decomposition in the dry farmland decreased the frequency of very large pore-size classes (i.e., > 0.15 cm). Although very large and large pores contributed to less than 1% of the soil volume, more than 50% of the total water flow would happen through these pore-size classes. Preserving rangelands in good condition can maintain soil structure and stability and would enhance water infiltration into the soil. These findings can be used by decision makers and land managers for holistic management in ecosystems of semi-arid areas.

Effect of Initial Compaction State on Near-Saturated Hydraulic Conductivity

AbstractQuantification of water infiltrating into the subsurface is mandatory for hydrological modeling of irrigation and drainage projects. Infiltration is influenced primarily by the soil type an...

Variations in near-saturated hydraulic conductivity of arable mineral topsoils in south-western and central-eastern Finland

Soil hydraulic properties play an important role in determining the level of crop productivity and the extent of environmental loading. Consequently, measurement of near-saturated soil hydraulic conductivity (K(h)) by tension infiltrometer (TI) is an interesting technique. To provide reference values for near-saturated K(h) in arable mineral topsoils of Finland and to investigate variability in these values caused by tillage, antecedent soil moisture content and earthworm burrows, field measurements were conducted in south-western and central-eastern Finland using the TI technique, at supply pressure head –6, –3, and –1 cm. The range of near-saturated K(h) values obtained was 0.02–12.6 mm h-1 at –6 cm, 0.30–85.9 mm h-1 at –3 cm, and 2.55–250 mm h-1 at –1 cm. At –6 cm pressure head, clay soils tended to show lower values than coarser-structured soils, but the order was reversed at –1 cm. The spatial variation in near-saturated K(h) was moderate and was exceeded by the temporal variation.

Estimating Soil Hydraulic Conductivity at the Field Scale With a State-Space Approach

A precise description of saturated (Ks) and near-saturated hydraulic conductivity (K−10) and their spatial variability is important for understanding water/solute transport in the vadose zone. However, it is laborious to measure K directly. Alternatively, K could be predicted from easily measurable soil properties using pedotransfer functions (PTFs). Because PTFs ignore the spatial relationships and covariance between soil variables, they often perform unsatisfactorily when field-scale estimations of K are needed. Therefore, the objective of this study was to improve the estimation of K at field scale through consideration of spatial dependences between soil variables. K was measured at 48 locations in a 71 × 71-m grid within a farmland under no-till. An autoregressive state-space approach was used to quantify the spatial relations between K and soil properties and to analyze the spatial variability of K in the field. In comparison, multiple linear regression (MLR) was used to derive PTFs for K estimation. Using various combinations of variables, state-space analysis outperformed PTFs in estimating spatial K distribution across the field. While state-space approach explained 69%, MLR method explained only 6% of the total variation in Ks. For K−10, the best state-space model included silt, clay, and macroporosity and performed almost perfectly (R >95%) in characterizing the spatial variability of K−10. In that case, the best MLR-type PTF explained only 60% of the variation. The results indicate that, by considering the spatial relations between soil variables, state-space approach is an effective tool for analyzing the spatial variability of K at field scale.

Tillage effect on hydrophobicity and hydrological properties of oil-contaminated sediments in a hyper-arid region

A crude oil spill in 2014 led to the contamination of an extensive area in the Evrona Nature Reserve at the hyper-arid Arava Valley of Israel. In 2015, as a part of the restoration efforts, the oil-polluted braided ephemeral stream channels underwent a single session of tillage, to a depth of 10 cm. The rationale of this treatment was to break the hard, impermeable crust of the residual oil, aiming to reduce soil hardness and water repellency, and to improve the hydrological properties of the soil. In 2017, a study was conducted to assess the effect of chiseling tillage on the soil's hydrophobicity, mechanical strength, and infiltration capacity. Water drop penetration time, critical surface tension, penetration resistance, and near-saturated hydraulic conductivity were measured on-site, at depths of 0, 5, and 10 cm in contaminated-tilled, contaminated-non-tilled, and clean plots. We found that at the ground surface, tillage had a slight and non-significantly positive effect on the nearly-saturated hydraulic conductivity and a negative effect on the penetration resistance, while no improvement was recorded for the other properties. Further, some of these properties were inferior in the contaminated-tilled sediments compared to those in the contaminated-non-tilled sediments, an effect that could be attributed to the: (1) tillage-induced inversion of the most polluted surface layer into the sub-soil, (2) absence of a clayey sublayer, and (3) limited oil degradation due to the extremely dry conditions. Therefore, the results indicate that tillage is not an effective practice for restoring oil-polluted soils and sediments in hyper-arid regions.

Estimates of Tillage and Rainfall Effects on Unsaturated Hydraulic Conductivity in a Small Central European Agricultural Catchment

In arable land, topsoil is exposed to structural changes during each growing season due to agricultural management, climate, the kinetic energy of rainfall, crop and root growth. The shape, size, and spatial distributions of soil aggregates are considerably altered during the season and thus affect water infiltration and the soil moisture regime. Agricultural topsoils are prone to soil compaction and surface sealing which result in soil structure degradation and disconnection of preferential pathways. To study topsoil infiltration properties over time, near-saturated hydraulic conductivity of topsoil was repeatedly assessed in a catchment in central Bohemia (Czech Republic) during three consecutive growing seasons, using a recently developed automated tension minidisk infiltrometer (MultiDisk). Seasonal variability of soil bulk density and saturated water content was observed as topsoil consolidated between seedbed preparations. Topsoil unsaturated hydraulic conductivity was lower in spring and increased in the summer months during two seasons, and the opposite trend was observed during one season. Temporal unsaturated hydraulic conductivity variability was higher than spatial variability. Cumulative kinetic energy of rainfall, causing a seasonal decrease in soil macroporosity and unsaturated hydraulic conductivity, was not a statistically significant predictor.

Soil hydraulic conductivity differences between upslope and downslope of two coniferous trees on a hillslope

ABSTRACTStanding trees influence the redistribution of soil water around the trunk and alter water movement under the canopy on hillslopes. We investigated soil properties around trees to clarify the effect of trees on soil hydraulic properties of soil surface layers on a slope. The intact tree and cut tree plots of two coniferous tree species, Cryptomeria japonica and Chamaecyparis obtusa, were considered in the northern part of Kyushu, Japan. The soil properties on the upslope and downslope of individual trees or stumps were compared. The soil near-saturated hydraulic conductivity (kns) on the upslope of individual trees and stumps was higher than that on the downslope for both species. Although the distribution patterns of kns were similar in all plots, the determinant factors of kns seemed to be different. In the Cr. japonica intact tree plot, more litter tended to accumulate on the upslope of individual trees, and it seemed to indirectly enhance kns. However, the results of the cut tree plot suggested that kns was affected by factors other than the litter layer biomass. In the Ch. obtusa intact plot, high water repellency of soil on the downslope of individual trees reduced kns. Organic matter tended to accumulate on the upslope of Ch. obtusa individual stumps and in combination with water repellency appeared to affect kns. Our findings showed that soil hydraulic conductivity was affected by the distance and direction from a tree on a slope and that the effect was maintained for a few years after cutting.

In situ measurements of near-surface hydraulic conductivity in engineered clay slopes

In situ measurements of near-saturated hydraulic conductivity in fine grained soils have been made at six exemplar UK transport earthwork sites: three embankment and three cutting slopes. This paper reports 143 individual measurements and considers the factors that influence the spatial and temporal variability obtained. The test methods employed produce near-saturated conditions and flow under constant head. Full saturation is probably not achieved due to preferential and by-pass flow occurring in these desiccated soils. For an embankment, hydraulic conductivity was found to vary by five orders of magnitude in the slope near-surface (0 to 0.3 metres depth), decreasing by four orders of magnitude between 0.3 and 1.2 metres depth. This extremely high variability is in part due to seasonal temporal changes controlled by soil moisture content, which can account for up to 1.5 orders of magnitude of this variability. Measurements of hydraulic conductivity at a cutting also indicated a four orders of magnitude range of hydraulic conductivity for the near-surface, with strong depth dependency of a two orders of magnitude decrease from 0.2 to 0.6 metres depth. The main factor controlling the large range is found to be spatial variability in the soil macro structure generated by wetting/drying cycle driven desiccation and roots. The measurements of hydraulic conductivity reported in this paper were undertaken to inform and provide a benchmark for the hydraulic parameters used in numerical models of groundwater flow. This is an influential parameter in simulations incorporating the combined weather/vegetation/infiltration/soil interaction mechanisms that are required to assess the performance and deterioration of earthwork slopes in a changing climate.