Effects Of Preferential Flow Research Articles

Water field distribution characteristics and slope stability under the effect of preferential flow on rainfall-driven seepage

Water field distribution characteristics and slope stability under the effect of preferential flow on rainfall-driven seepage

Preferential flow of phosphorus and nitrogen under steady‐state saturated conditions

AbstractRepeated broiler litter application on agricultural lands can cause nutrient enrichment of subsurface effluent, especially with the existence of preferential flow through soil macropores. Previous studies quantifying soil macropores have not attempted to establish a connection of soil macropore characteristics with the subsurface nutrient (nitrogen [N] and phosphorus [P]) losses, across different topographical locations in the field. This study investigated the effect of broiler litter application and preferential flow on subsurface nutrient transport (N and P) at different topographical positions (upslope, midslope, and downslope) in a no‐till pasture field located in Alabama, USA. Twelve intact soil columns (150 mm id and 500 mm length) were used, and the nutrient leaching measurements from laboratory experiments were linked to soil macropore characteristics quantified using X‐ray computed tomography image analysis and solute transport modeling. Treatments included surface broadcast broiler litter (5 Mg ha−1, on dry basis) and unamended control. Leachates were analyzed for dissolved reactive P (DRP), total P (TP), and nitrate + nitrite‐N (NO3− + NO2−–N). The bromide breakthrough curves provided evidence of preferential flow in all columns. Litter application significantly increased leachate P concentrations, and average TP and DRP concentrations were significantly higher in the leachate from upslope columns compared to those at downslope location. The NO3−–N concentrations in leachate exceeded the US EPA drinking water standard of 10 mg L−1 in all the treatment columns. The highest flow‐weighted mean concentrations of TP and DRP, at 2.7 and 2.5 mg L−1, respectively, were recorded in the upslope columns. Soil physicochemical properties and nutrient leaching losses varied substantially across topographical positions, indicating a need for variable litter application rates to reduce P build‐up and subsequent leaching in vulnerable locations within the field. The relevance of the effect of topographic position on nutrient leaching found in this study should be further tested by investigating a wider range of slopes and soil types in pastures.

Effects of soil heterogeneity and preferential flow on the water flow and isotope transport in an experimental hillslope

Soil water movement plays vital roles in hillslope runoff generation and groundwater and surface water interaction. However, there are still knowledge gaps about the impacts of soil heterogeneity and preferential flow on the internal water flow and transport process. In this study, the vertical soil heterogeneity focused on the variations in soil retention capacity, and the consideration of lateral preferential flow emphasized the higher hydraulic conductivity. We combined isotopic tracing and numerical modeling in an artificial hillslope, focusing on monitored processes of the artificial rainfall and isotopic tracing experiment. The results showed that the soil moisture quickly accumulated at the bottom of the hillslope during rainfall events, while the 2H enrichment occurred in the topsoil derived from enriched isotope injection in the second artificial rainfall. The evaporation process slowed down the mixing of new water in the topsoil and old water in the lower layer. We found that the vertical soil heterogeneity had significant influences on the internal water and isotope transport paths within the hillslope. The lateral preferential flow played an important role in the water flux and transport time to the seepage face. The coupling of isotopic tracing, which reflects the water transport and mixing with the hillslope, effectively improved the model simulation and mechanism analysis of hillslope water flow. Our findings provide new insights into the mechanisms governing soil water flow and transport dynamics in hillslopes, taking into account vertical soil heterogeneity and lateral preferential flow.

Experimental study on the failure of loose accumulation landslides under rainfall conditions

Rainfall plays a pivotal role in triggering the failure of loose accumulation landslides. To understand their stability, it is essential to explore the deformation, failure patterns, fine particle migration, and infiltration behavior of such landslides under rainy conditions. Employing the Yuqiupo landslide as a case study, this research dissects the fundamental attributes of the Yuqiupo landslide through geological examinations. Moreover, the study replicates the failure of the loose accumulation landslide via artificial rainfall experiments. By employing acquired data on soil volumetric water content and matrix suction from these experiments and observing measurable shifts in the landslide’s failure progression and rainfall circumstances, the water-hydraulic conditions of the accumulation landslide are meticulously dissected. The conducted experiments disclose that the failure of the loose accumulation landslide emanates from the combined effects of preferential flow and matrix flow within unsaturated conditions. Rainfall triggers an elevation in soil moisture content and a concurrent decrease in matrix suction, compromising the slope’s stability and ultimately causing the landslide. Furthermore, our research includes a quantitative analysis of changes in particle size distribution before and after the landslide’s failure. This endeavor underscores the influence of distinct failure modes on the migration of fine particles, with these migration patterns distinctly shaped by prevailing rainfall conditions.

Estimation of Shallow Landslide Susceptibility Incorporating the Impacts of Vegetation on Slope Stability

This study aimed to develop a physical-based approach for predicting the spatial likelihood of shallow landslides at the regional scale in a transition zone with extreme topography. Shallow landslide susceptibility study in an area with diverse vegetation types as well as distinctive geographic factors (such as steep terrain, fractured rocks, and joints) that dominate the occurrence of shallow landslides is challenging. This article presents a novel methodology for comprehensively assessing shallow landslide susceptibility, taking into account both the positive and negative impacts of plants. This includes considering the positive effects of vegetation canopy interception and plant root reinforcement, as well as the negative effects of plant gravity loading and preferential flow of root systems. This approach was applied to simulate the regional-scale shallow landslide susceptibility in the Dadu River Basin, a transition zone with rapidly changing terrain, uplifting from the Sichuan Plain to the Qinghai–Tibet Plateau. The research findings suggest that: (1) The proposed methodology is effective and capable of assessing shallow landslide susceptibility in the study area; (2) the proposed model performs better than the traditional pseudo-static analysis method (TPSA) model, with 9.93% higher accuracy and 5.59% higher area under the curve; and (3) when the ratio of vegetation weight loads to unstable soil mass weight is high, an increase in vegetation biomass tends to be advantageous for slope stability. The study also mapped the spatial distribution of shallow landslide susceptibility in the study area, which can be used in disaster prevention, mitigation, and risk management.

Dynamics and control mechanisms of inorganic nitrogen removal during wetting-drying cycles: A simulated managed aquifer recharge experiment

Managed aquifer recharge (MAR) systems can be operated intermittently through wetting-drying cycles to simultaneously improve the water supply and quality. Although MAR can naturally attenuate considerable amounts of nitrogen, the dynamic processes and control mechanisms of nitrogen removal by intermittent MAR remain unclear. This study was conducted in laboratory sandy columns and lasted for 23 d, including four wetting periods and three drying periods. The hydraulic conductivity, oxidation reduction potential (ORP), and leaching concentrations of ammonia nitrogen and nitrate nitrogen of MAR systems were intensively measured to test the hypothesis that hydrological and biogeochemical controls play an essential role in regulating nitrogen dynamics at different stages of wetting-drying cycles. Intermittent MAR functioned as a sink for nitrogen while providing a carbon source to support nitrogen transformations; however, it occasionally became a source of nitrogen under intense flushes of preferential flow. Nitrogen dynamics were primarily controlled by hydrological processes in the initial wetting phase and were further regulated by biogeochemical processes during the subsequent wetting period, supporting our hypothesis. We also observed that a saturated zone could mediate nitrogen dynamics by creating anaerobic conditions for denitrification and buffering the flush effect of preferential flow. The drying duration can also affect the occurrence of preferential flow and nitrogen transformations, which should be balanced when determining the optimal drying duration for intermittent MAR systems.

Effects of preferential flow on soil nutrient transport in karst slopes after recultivation

In response to the global food shortage, a large amount of abandoned land in karst areas has been reclaimed as cultivated land, causing severe nonpoint source pollution. Preferential flow-driven soil nutrient transport on karst slopes remains poorly studied, though it is a major factor in nonpoint source pollution, as it responds to changes caused by reclamation. We explored the characteristics of soil preferential flow differences in recultivated land, grassland, and shrubland from returning farmland by dye tracer experiments and quantitatively examined the effect of preferential flow on nutrient transport. Under the condition of 40 mm precipitation, the preferential flow paths (PFPs) of the three types of plots were mainly distributed from 0 to 40 cm. The total porosity in the 20–40 cm soil layer was significantly reduced by reclamation, and the number of preferred flow paths in the 20–40 cm soil layer was significantly reduced from 60 to less than 10, which was significantly less than that in grassland and shrubland. But, reclamation results in the transport of more soil nutrients by preferential flow. The contribution rate of preferential flow to other nutrient indexes in the reclaimed land, in contrast to grassland and shrubland, was lower than zero, except for organic matter and total potassium. Moreover, when the PFP was connected to the rock–soil interface, the soil water can leak underground through the rock–soil interface quickly. Therefore, our findings indicated that reclamation reduces the distribution depth of the PFP. Still, the connection of soil preferential flow to the rock–soil interface increases the transport of soil nutrients to deep fissures and even underground rivers, thereby causing recultivated land to become one of the main sources of groundwater pollution in karst areas.

Residual plastic film exerts dual effects of blocking and preferential flow on soil water movement

Plastic-film mulching (PFM) significantly improves farmland productivity in arid and semi-arid regions. However, the residual plastic film (RPF), also named “white pollution”, has become a serious concern. The effect of RPF on soil water flow is unclear, but it has negatively impacted the use of PFM. This study evaluated the effects of RPF on soil physical and hydraulic properties. The impact on infiltration uniformity and preferential flow was examined based on dye tracer technology. The experiments were carried out with 5 RPF levels (0, 150, 300, 450, and 600 kg ha−1), and 4 infiltration amounts (20, 40, 60, and 80 mm). The results showed that increasing RPF amounts decreased soil bulk density and saturated hydraulic conductivity, but increased soil porosity and saturated soil water content in the RPF layer (0–30 cm). RPF accumulation enhanced the blocking effect in the RPF layer promoting preferential flow in the deeper soil layers. In general, with more RPF in soil, dye-stained coverage (DC) and uniform infiltration depth (UniFr) decreased, while the coefficient of variation (CVd), preferential flow fraction (PFF), dye-stained depth (MDD), length index (LI), and the difference of soil water storage between before and after dyeing (∆SWS) increased, especially in the deeper soil layers with high infiltration amount. 450 kg ha−1 RPF produced the strongest preferential flow with a maximum MDD 32.9 cm, PFF 39.2 %, LI 136.8, CVd 0.73 %, and minimum UniFr 8.8 cm. Furthermore, the phenomenon became prominent if higher infiltration amounts were applied; under 450 kg ha−1 RPF, the average CV was 8.0 % higher for 80 mm infiltration compared with 20 mm infiltration. Finally, correlation analysis shows that the RPF was the main driving factor of preferential flow.

Quantifying preferential flows on two farmlands in the North China plain using dual infiltration and dye tracer methods

Preferential flow, as a significant part of soil water infiltration, affects crop water use efficiency and pollutant transport in soil. Although it is important to properly assess the effect of preferential flow on soil water infiltration, there is a lack of widely accepted methods to quantify preferential flow. In this study, a new dual infiltration method (DI method) was used to quantify the contribution of preferential flow at two farmlands in the North China plain. The experiments were performed at two field sites (LY and SZ). To apply the new method, a double-ring infiltrometer was used to measure the steady-state infiltration rate (i.e., when infiltration rate becomes constant or stable with time, SIR) at the field sites, while a constant-head infiltrometer was used to measure the saturated hydraulic conductivity of packed soil columns consisting of disturbed soil obtained from each field site. Water flow in the disturbed soil columns was used to approximate the matrix infiltration rate (MIR), and the difference between SIR and MIR represented the preferential flow infiltration rate (PFIR). A dye tracer was also applied to the infiltrometers to quantify the proportion of preferential flow. For the LY site, the DI method found that the PFIR was 68 times larger than the MIR, while, based on the dye tracer method, the PFIR was only 2 times larger than the MIR. The DI and dye tracer method derived values of PFIR were similar for the SZ site, with MIR accounting for 61% (DI method) and 67% (dye tracer method) of the SIR. Wormholes were an important factor in the preferential flow generated in the LY site. The total areas of wormholes visible at the surface and the PFIR values were significantly positively correlated, r = 0.79 (P < 0.05). The DI method reasonably quantified preferential flow at the field sites, while the dye tracer method might underestimate preferential flow in soil with abundant macropores.

Soil heterogeneity influence on the distribution of heavy metals in soil during acid rain infiltration: Experimental and numerical modeling

Acid rain is a global environmental problem that mobilizes heavy metals in soils, while the distribution and geochemical fraction of heavy metals during acid rain infiltration in heterogeneous soils are still unclear. In this study, we performed column experiments to investigate the distribution and geochemical fraction of Cu, Pb, Ni and Cd in heterogeneously layered soils during continuous acid rain infiltration. Chloride ion used as a conservative tracer was found to be uniformly distributed during acid rain infiltration, showing insignificant preferential flow effects in the column. In contrast, however, the distribution of heavy metals was highly non-uniform, especially in the silty soil at the lower part of the column, indicating a heterogeneous distribution of adsorption capacity. In addition, in the control experiments with neutral rain infiltration, uniform distribution of heavy metals was observed, indicating that the heterogeneous distribution of adsorption coefficient during acid rain infiltration was mainly caused by different pH buffering capacities. A numerical model considering water flow and solute transport was developed, where the average water-solid distribution coefficient (Kd) in Layer 2 was only 1.5–12.5% of that in Layer 1 during acid rain infiltration. The model could predict the variation of heavy metal concentrations in soil with the majority of error less than 35%, confirming that different Kd induced the heterogeneous distribution of heavy metals. In addition, the geochemical fraction of heavy metals in the upper coarse sand layer remained stable, while the acid-extractable fractions in the lower loam and silt loam layer gradually increased. Our findings suggest that soil heterogeneity, especially chemical heterogeneity affected by rainfall acidity, has an important influence on the infiltration, migration and geochemical fraction of heavy metals in soils. This study could help guide the risk assessment of heavy metal-contaminated sites that were polluted by acid rain or landfill leachate.

Preferential flow improves root-soil system on a small scale: A case study of two ecotypes of Phragmites communis

The effects of preferential flow on plant root and soil have not been deeply studied. Research on the combination of wetland vegetation, hydrology, and soil can provide a theoretical basis for wetland vegetation restoration and soil salinization control. We integrated preferential flow, root, and soil to study the distribution characteristics of the root architecture and soil nutrient of two different ecotypes of Phragmites communis (P. communis) in the Yellow River Delta on a small scale. We found that both root architectures included root length, width, surface area, volume, and biomass and soil nutrients included soil organic carbon, organic matter and total nitrogen decreased with the increase of soil depth. Large P. communis (LP) not only had significantly larger root length, width, surface area, but also had significantly higher soil organic carbon, organic matter, total nitrogen, and available phosphorus than small P. communis (SP). The root architectures of SP in the preferential flow area (PFA) were significantly greater than those in the matrix flow area (MFA). The contents of soil organic carbon, organic matter, and total nitrogen in LP grown in the PFA were obviously higher than those in the MFA. Based on correlation analysis, root architectures had remarkably positive correlations with soil organic carbon, organic matter, and total nitrogen. These findings suggest that preferential flow improves the root-soil system. By artificially adding peat and biochar and changing irrigation methods, the possibility of preferential flow can be increased, plant root growth and soil nutrient can be improved, so as to resist the damage of soil salinization. In addition, different ecotypes of P. communis should be planted to improve soil salinization. LP and SP are suitable for slight and heavy soil salinization areas respectively.

Effects of preferential flow induced by desiccation cracks on slope stability

Abstract Desiccation cracks on a soil slope can significantly increase permeability, reduce shear strength, and potentially result in shallow landslides. To reveal the slope failure mechanism induced by desiccation cracks, a full-scale model test was conducted on a cracked soil slope under rainfall–evaporation cycles. Image processing techniques were used to quantify the crack characteristics at the slope crest (SC), around the slope shoulder (SS), and at the slope foot (SF), and hydrologic sensors were used to monitor the moisture content, matric suction, and pore water pressure at different depths in the crack areas. The results showed dynamic variations in the desiccation crack patterns in accordance with their position on the slope and the rainfall–evaporation cycle. Preferential flow induced by the desiccation cracks in response to rainfall was detected earlier by the lower hydrologic sensors than the upper ones, and the desiccation cracks significantly increased the infiltration depth by up to four or five times the crack depth. Experimental evidence confirmed that preferential flow through desiccation cracks can trigger slope failure or landslides by forming local perched water zones near the crack tips. Based on this investigation, the failure process of the cracked soil slope was separated into three stages according to the crack patterns and failure modes: (I) generation of desiccation cracks, with surface erosion as the failure mode; (II) development and transformation of cracks, with flow-slip and local failure as the failure modes; and (III) renewal and further development of cracks, with overall failure as the failure mode. These conclusions suggest that when simulating the seepage and stability of a cracked soil slope, further modifications should consider the dynamic changes that occur within desiccation cracks. In addition, the use of specific treatment measures to avoid slope failure during the different stages is suggested.

Effects of Infiltration Amounts on Preferential Flow Characteristics and Solute Transport in the Protection Forest Soil of Southwestern China

Preferential flow has an important role as it strongly influences solute transport in forest soil. The quick passage of water and solutes through preferential flow paths without soil absorption results in considerable water loss and groundwater pollution. However, preferential flow and solute transport under different infiltration volumes in southwestern China remain unclear. Three plots, named P20, P40 and P60, were subjected to precipitation amounts of 20, 40 and 60 mm, respectively, to investigate preferential flow and solute transport characteristics via field multiple-tracer experiments. Stained soils were collected to measure Br− and NO3− concentrations. This study demonstrated that precipitation could promote dye tracer infiltration into deep soils. The dye tracer reached the maximum depth of 40 cm in P60. Dye coverage generally reduced with greater depth, and sharp reductions were observed at the boundary of matrix flow and preferential flow. Dye coverage peaked at the soil depth of 15 cm in P40. This result demonstrated that lateral infiltration was enhanced. The long and narrow dye coverage pattern observed in P60 indicated the occurrence of macropore flow. Br− and NO3− were found at each soil depth where preferential flow had moved. Increasing precipitation amounts increased Br− and NO3− concentration and promoted solute movement into deep soil layers. Solute concentration peaked at near the end of the preferential flow path and when preferential flow underwent lateral movement. These results indicated that the infiltration volume and transport capacity of preferential flow had important effects on the distribution of Br− and NO3− concentrations. The results of this study could help expand our understanding of the effects of preferential flow on solute transport and provide some suggestions for protection forest management in southwestern China.

Wavelet analysis of soil water state variables for identification of lateral subsurface flow: Lysimeter vs. field data

AbstractPreferential and lateral subsurface flow (LSF) may be responsible for the accelerated transport of water and solutes in sloping agricultural landscapes; however, the process is difficult to observe. One idea is to compare time series of soil moisture observations in the field with those in lysimeters, where flow is vertically oriented. This study aims at identifying periods of deviations in soil water contents and pressure heads measured in the field and in a weighing lysimeter with the same soil profile. Wavelet coherency analysis (WCA) was applied to time series of hourly soil water content and pressure head data (15‐, 32‐, 60‐, 80‐, and 140‐cm depths) from Colluvic Regosol soil profiles. The phase shifts and periodicities indicated by the WCA plots reflected the response times to rain events in the same depth of lysimeter and field soil. For many rain events and depths, pressure and moisture sensors installed in the field soil responded earlier than those in the lysimeter. This could be explained by either vertical preferential flow or LSF from upper hillslope positions. Vice versa, a faster response in the lysimeter soil could be indicative for vertical preferential flow effects. Dry weather conditions and data gaps limited the number of periods with elevated soil moisture in 2016–2018, in which LSF was likely to occur. The WCA plots comprise all temporal patterns of time shifts and correlations between larger data time series in a condensed form to identify potentially relevant periods for more detailed analyses of subsurface flow dynamics.

Effect of soil characteristics on preferential flow of Phragmites australis community in Yellow River delta

Wetland ecosystem, with their precious natural resources and important ecological functions, is an integral part of the global ecosystem. Preferential flow is a form of water and solute transport that is widely present in all types of soils and has a great impact on nutrient transport and environmental pollution in soils. In this paper, we observe the phenomenon of preferential flow within a wetland ecosystem and discuss the possible effects of preferential flow within a wetland on the wetland ecosystem as a background. We conducted dye tracing experiments outdoors. The preferential flow was qualitative analysis using soil composition. It was found that two types of preferential flow existed in the experimental area, namely finger flow and macropore flow. Quantitative analysis of the preferential flow was performed using PS and Image-Pro Plus software to obtain the intensity data of the preferential flow. The results show that the preferential flow phenomenon in the experimental area is obvious and the preferential-flow intensity data is high. We also found a clear correlation between initial soil–water content, which is an environmental factor characteristic of wetlands, and the preferential-flow data (p = 0.04 < 0.05). The correlation coefficient between the two sets of data is 0.45. This paper systematically investigates the preferential flow phenomenon in wetlands, aiming to provide directions and basic data for the subsequent research on preferential flow

Dual‐permeability modeling of preferential flow and snowmelt partitioning in frozen soils

AbstractThe infiltrability of frozen soils modulates the partitioning of snowmelt between infiltration and runoff in cold regions. Preferential flow in macropores may enhance infiltration, but flow dynamics in frozen soil are complicated by soil heat transfer processes. We developed a dual‐permeability model that considers the interacting effects of freeze–thaw and preferential flow on infiltration and runoff generation in structured soils. This formulation was incorporated into the fully integrated groundwater–surface water model HydroGeoSphere, to represent water–ice phase change in macropores such that porewater freezing is governed by macropore–matrix heat exchange. Model performance was evaluated against laboratory experiments and synthetic test cases designed to examine the effects of preferential flow on snowmelt partitioning between infiltration, runoff, and drainage. Simulations were able to reproduce experimental observations of rapid infiltration and drainage behavior due to macropores very well, and approximated soil thaw to an acceptable degree. Simulation of measured data highlighted the importance of macropore hydraulic conductivity, as well as macropore–matrix heat and water transfer, on controlling preferential flow dynamics. Test cases replicated a range of snowmelt partitioning behavior commonly observed in frozen soils, including subsurface conditions that produce rapid infiltration and deeper drainage, the contrast between limited vs. unlimited infiltration responses to snowmelt, and the temporal evolution of runoff generation. This study demonstrates the important influence that water freezing along preferential flowpaths can have on infiltrability and runoff characteristics in frozen soils and provides a physically based description of this mechanism that links infiltration behavior to hydraulic and thermal properties of structured soils.

Characterizing Preferential Flow Paths in Texturally Similar Soils under Different Land Uses by Combining Drainage and Dye-Staining Methods

Preferential flow paths have been widely characterized by many visualization methods. However, the differences in preferential flow paths under various land uses and their relationships to hydraulic properties remain uncertain. The objectives of this study are to (1) characterize preferential flow paths under various land uses (forest and orchard) by combining drainage and dye-staining methods and to (2) build a connection between preferential flow paths and hydraulic-related parameters and extract the proportion of preferential flow paths from the compounding effects of matrix flow and preferential flow. The dye-staining experiments were conducted in five sandy soils and one sandy clay loam in situ, including four soils from forest and two soils from orchards. A total of 47 soil cores, 4 cm in height and 9 cm in diameter, were collected in each layer of the dye-stained soils for drainage experiments in the laboratory. Dye coverage and hydraulically equivalent macropore parameters (macroporosity, pore size distribution, and number of macropores) and their relationships were analyzed. The results show that the volume of preferential flow is partly affected by the total macropore volume. The effect of macropores on preferential flow varies by macropore size distribution. Dye coverage exhibited a significant (P &lt; 0.01) correlation with macroporosity (correlation coefficient 0.83). Based on the value of macroporosity or steady effluent rates, the part of the dye coverage that was due to preferential flow on the surface dye-stained soil (resulting from both matrix and preferential flow) could be identified in this study. Compared with orchards, forestland has more preferential flow paths in both surface soil and subsoil. Further studies are needed to quantify the 3-D preferential flow paths and build a connection between preferential flow paths and hydraulic properties.

The effect of density‐driven flow on the transport of high‐concentration solutes in the hyporheic zone

AbstractIn this study, both laboratory experiments and numerical simulations were conducted to investigate the effect of density‐driven flow on the transport of high‐concentration pollutants in the hyporheic zone. The results show that the density gradient can change the flow of pore water and the strong density‐driven flow can lead to an unstable flow, which increases the effect of preferential flow and thus causes the appearance of solute fingers in the hyporheic zone. Notably, these solute fingers become more obvious with the increase of depth. The appearance of solute fingers depends on the relative strength of the pumping exchange and density gradient, which are represented by the dimensionless number M* and N* respectively. Finger flows appear near the interface when M* is less than 0.5 N*. This study may contribute to better understanding the transport and destination of solutes and thus may provide some insights into the assessment on pollution incidents.

Acidification and weathering associated with deep tongues in sandy Spodosols, Michigan, USA

We studied the effects of preferential flow on weathering of primary minerals in well-drained, sandy Spodosols (Podzols) in northern Michigan, USA. Preferential flow in these soils is manifested as deep eluvial/illuvial tongues. We hypothesized that sands in the best developed (strong) tongues would be more weathered than in tongues that are short and/or thin (weak tongues), and especially so when compared to pedons that lack tongues (off-tongues). Across these three types of morphologies, we sampled at a variety of depths and horizons to establish correspondence between tongue strength and primary mineral weathering, using (Ba+Rb)/Sr data from a portable xrf instrument as a weathering surrogate. Soil acidity and primary mineral weathering vary predictably: strong tongues > weak tongues > off-tongues. This trend continues well below the profile, as BC and C horizons below strong tongues are also weathered, even though morphological indicators of pedogenesis and weathering here are not present. E horizons, which experience the most cumulative percolation of soil water, are strongly weathered, regardless whether they are located above a tongue, or not. Thus, we conclude that the cumulative percolation of acidic water, rich in organo-metallic compounds, drives weathering in these soils, even at depths to which the organics and illuvial materials have not yet reached. That is, the effects of podzolization and weathering extend well below what is morphologically considered the profile. Our data indicate that (a) coatings of organo-metallic substances in B horizons do not “protect” the underlying grains from dissolution and weathering-related reactions, and (b) pedogenesis and weathering can extend far below the profile, beneath areas of preferential flow.

Field monitoring of preferential infiltration in loess using time-lapse electrical resistivity tomography

The effect of preferential flow (PF) in loess is of direct interest as flood irrigation is commonly found in loess regions where precipitation is scarce. This study was to assess the applicability of time-lapse electrical resistivity tomography (ERT) data on monitoring infiltration in loess and to analyze the effects of preexisting preferential path on infiltration process at a localized scale. An in situ single-ring infiltrometer (SRI) test was conducted above a preexisting near-surface crack under a constant influx (10 cm water head for 8h), and the test was monitored by the means of 2-D time-lapse ERT and validated by an exploratory trench. The ERT results observed some increasing uncertainties in the electrical resistivity in part due to daily temperature variation; however, they showed that the infiltration process under the presence of preferential path can be effectively monitored by time-lapse ERT with a combination of unit electrode spacing (UES). The test results showed a high relative reduction of electrical resistivity (>80%) due to matrix flow (MF) and PF within 0–0.5h of the test, and followed by a transformation of preferential flow to a more matrix-dominated unsaturated flow, i.e. preferential-to-matrix flow (PMF), with continuous downward progression into the deeper subsurface (greater than 1.2 m) while MF stagnated at the near-surface (about 0.4 m). The results suggested that the presence of even a near-surface preferential path significantly accelerate the infiltration into the deeper loess by the means of PMF, as compared to observed slow infiltration of pure matrix flow in previous studies.