Macropore Flow Research Articles

Experimental study on macropore flow effects in unsaturated soil on subsurface drainage and soil desalination*

AbstractDischarging of moisture and salt from unsaturated soil has always been a difficult problem. Therefore, a laboratory experiment in unsaturated soil, under drip irrigation, on subsurface drainage and soil desalination through subsurface drainage pipes (SDP) was conducted for using soil flume, where artificial macropore flow tubes (MFT) were deployed and connected to SDP. We conducted seven different deployment treatments of the artificial MFT, including four deployment tube densities (0, 3, 4, and 5) and two deployment arrangements (curved and straight). The results showed that no flow was observed in the SDP under the base treatment without MFT, increasing flow rates to 9.71 L, 10.7 L, and 12.3 L under the three straight line treatments and 7.81 L, 9.81 L, and 11.2 L under the three curved line treatments. Water content in both the upper and lower soil layers was almost uniform in the treatments with MFT, and an overall desalination trend was observed; the average desalination rate of curved MFT treatments was lower than that of the straight MFT treatments under the same MFT density. The findings serve as the foundation for future studies on drainage and desalination in unsaturated soil and facilitate the design of tile drainage under drip irrigation.

Investigating the impact of exit effects on solute transport in macroporous media

Abstract. The effect of macropore flow on solute transport has spurred much research over the last forty years. In this study, non-reactive solute transport in water-saturated columns filled with porous media crossed by a macropore was experimentally and numerically investigated. The emphasis was put on the study of exit effects, whose very existence is inherent to the finite size of any experimental column. We specifically investigated the impact of a filter at the column outlet on water flow and solute transport in macroporous systems. Experiments involving breakthrough measurements and magnetic resonance imaging (MRI) showed that solute transport displayed some significant non-unidirectional features, with a strong mass exchange at the interface between the macropore and the matrix. Fluid dynamics and transport simulations indicated that this was due to the non-unidirectional nature of the flow field close to the outlet filter. The flow near the exit of the column was shown to be strongly impacted by the presence of the outlet filter, which acts as a barrier and redistributes water from the macropore to the matrix. This impact was apparent on the breakthrough curves and the MRI images. It was also confirmed by computer simulations and could, if not properly taken into account, impede the accurate inference of the transport properties of macroporous media from breakthrough experiments.

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.

Near-saturated soil hydraulic conductivity and pore characteristics as influenced by conventional and conservation tillage practices in North-West Himalayan region, India

Tillage plays an important role in modifying soil hydraulic properties. The objective of the present study was to evaluate the effect of conservation tillage practices in a maize-wheat cropping system on near-saturated soil hydraulic properties and pore characteristics in the North-West Himalayan region, India. Three treatments viz. conventional tillage (CT), minimum tillage (MT), and zero tillage (ZT) were evaluated in terms of field saturated hydraulic conductivity (ks), unsaturated hydraulic conductivity k(h), the inverse of capillary length (α), flow-weighted mean pore radius (r0), numbers of pores per square meter (n0) and water-conducting macroporosity (Ɛ). The above hydraulic conductivity and pore characteristics were derived from steady-state water flux (q) measured using hood infiltrometer at 0, −1, and −3 cm pressure head for each treatment after seven years of establishment of this tillage experiment. Results revealed significantly (p < 0.05) higher values of ks, k(h), α, and Ɛ in ZT as compared with CT. MT had intermediate values. Higher α values suggested a greater gravity-dominated flow under ZT and MT as compared with CT. Analysis of r0 values indicated better connectivity of pores in ZT and MT as compared with CT. Macropore flow suggested that on average pore radii, > 0.50 mm conducted about 63.60, 68.01, and 75.97 % of total flow (at 0 cm pressure head) in the corresponding water-conducting macroporosity of 0.00030, 0.00044, and 0.00069 % of soil volume under CT, MT, and ZT, respectively. Overall, zero-tillage based agriculture system was found to improve near-saturated soil hydraulic properties.

Comparative Non-Darcian Modeling of Subsurface Preferential Flow Experimental Observations in a Riparian Buffer

HighlightsHigh ecohydrological activity drives macropore prevalence in riparian buffers.An abundance of macropore flow (MF) was confirmed in a field riparian buffer in Kenya.Source-response (SR) and multilayer kinematic diffusive wave (MKDW) MF models are compared.A novel MKDW modeling framework efficiently identifies and predicts preferential flow in riparian buffers.Abstract. The significant ecohydrological activity typical of riparian buffers makes them potential hotspots of macropores, i.e., structured preferential flow pathways, through the soil vadose zone. The prevalence of these preferential pathways can allow transported contaminants to bypass the soil matrix and quickly reach a seasonal shallow water table and the adjacent surface waterbody. This quick transport can ultimately limit the role of riparian buffers for runoff pollution control. Currently, there are no management tools that incorporate macropore flow (MF) when assessing riparian buffer performance. The objective of this study was to experimentally quantify and mathematically simulate macropore flow and arrival time in a riparian buffer under field conditions. Three infiltration experiments were conducted with a grid of 20 time-domain transmission (TDT) dielectric soil moisture sensors along a field riparian buffer transect in Kenya to quantify the presence of macropore flow and to test two non-Darcian soil MF models, including the source-responsive (SR) model and the modified kinematic-dispersive wave (MKDW) model developed in this study, by adding a user-defined multilayer convection scheme and a new hysteresis function between water flux and content. The abundance of MF in the riparian buffer was corroborated experimentally. Modeling results showed that the MKDW model was an efficient (average NSE of 0.937 and 0.721 for calibration and testing, respectively), flexible, and robust method to identify and represent non-linear and non-sequential MF signals at any soil depth and antecedent conditions. The SR model was computationally inexpensive and provided good calibration results (NSE = 0.867) but required piecemeal recalibration of the travel time and maximum water content at each layer and yielded lower performance in testing. The Akaike (AIC) and Bayesian (BIC) information criteria showed that MKDW outperformed SR when accounting for the trade-off between model complexity and efficiency. The results support further research focused on independent characterization of model parameters at the field scale, and the inclusion of MKDW in holistic riparian buffer management and decision-support tools such as VFSmod. Keywords: Kinematic-dispersive wave, Macropore flow, Numerical modeling, Preferential flow, Riparian vadose zone.

The processes of preferential flow in the unsaturated zone

AbstractPreferential flow, a major influence in unsaturated soil and rock almost everywhere, occurs by multiple phenomenologically distinct hydraulic processes. For the mode known as funneled flow, concentrated in particularly conductive portions of the medium, the surface‐tension/viscous‐flow processes of traditional unsaturated flow theory predominate. Fingered flow, through conductive paths of higher water content than surrounding material, requires amendments to traditional theory concerning instabilities and dynamic flow‐regime boundaries. Macropore flow, the most recognized preferential flow mode, poses unanswered questions and major difficulties in practice. Accumulated evidence shows that water flows preferentially mostly through macropores that are (a) only partially filled with water, and (b) surrounded by matrix material that is drier, sometimes much drier, than saturation. With partial filling, geometric characteristics such as aperture have much less influence than was previously thought, and the intra‐macropore configuration of the flowing water phase, about which little is conclusively known, is then a dominant controlling influence. With unsaturated surroundings, macropore/matrix exchange interactions control, for given input and medium, the initiating circumstances, conveyed flux, and duration of macropore flow. The multiple processes in play during such interactions have different sensitivities to the matrix water state and different directions of influence. The net influence of matrix water content on macropore flow is thus highly complex and a major research need. Additional high‐priority topics are: flowpath connectivity, for watersheds as well as small scales; intra‐macropore processes, to discern their importance and possible means of quantification; and the identification of measurable soil and rock properties that can be utilized predictively.

DRAINMOD-P: A Model for Simulating Phosphorus Dynamics and Transport in Drained Agricultural Lands: II. Model Testing

HighlightsDRAINMOD-P was tested using a dataset from a drained field with desiccation cracks.Surface and subsurface phosphorus losses were mainly in the particulate form.Surface runoff was a major pathway for phosphorus loss in this field.The model performance in predicting edge-of-field phosphorus loss is promising.Abstract. The recently developed phosphorus (P) model DRAINMOD-P was tested using a four-year dataset from a subsurface-drained field in northwest Ohio with significant potential for desiccation cracking or preferential flow. The model satisfactorily predicted subsurface drainage discharge, with a monthly Nash-Sutcliffe efficiency (NSE) of 0.59 and index of agreement (IOA) of 0.89. Lack of annual water budget closure was reported and was likely caused by uncertainty in measured surface runoff and/or modeling approaches representing macropore flow. More than 80% of predicted surface and subsurface P losses were in the particulate form. Surface runoff was the major pathway for P loss, contributing 78% of predicted total P (TP) load. On average, predicted macropore flow represented about 15% of drainage discharge and contributed 21% of DRP loss via subsurface drains. The performance of DRAINMOD-P in predicting monthly dissolved reactive P and TP losses through subsurface drains can be rated as poor (NSE = 0.33 and IOA = 0.60) and very good (NSE = 0.81 and IOA = 0.95), respectively. DRAINMOD-P demonstrated potential for simulating P fate and transport in drained cropland. More testing is needed to further examine newly incorporated hydrological and biogeochemical components of the model. Keywords: Agricultural drainage, Edge-of-field phosphorus load, Macropore flow, Phosphorus model, Sediment yield, Water quality modeling.

DRAINMOD-P: A Model for Simulating Phosphorus Dynamics and Transport in Drained Agricultural Lands: I. Model Development

HighlightsDRAINMOD-P has been developed to simulate phosphorus (P) dynamics in drained croplands.Key hydrological and biochemical processes affecting P cycling are represented in the model.The model predicts surface and subsurface P losses as affected by weather, soil, and management factors.Abstract. High phosphorus (P) loads to streams and lakes can promote harmful algae blooms and cause water quality deterioration. Recent research has identified subsurface drainage as an important pathway for the transport of dissolved P from drained croplands to receiving surface water bodies, particularly when macropore flow contributes a considerable portion of the subsurface drainage outflow. Currently, a few models are capable of simulating P dynamics in poorly drained soils with artificial drainage systems. The objective of this study was to develop DRAINMOD-P, a field-scale, process-based model that simulates P cycling and transport in drained croplands. Processes represented in the model include atmospheric deposition, organic and inorganic fertilizer applications, plant uptake, sediment-bound and dissolved P losses in both surface runoff and subsurface drainage, tillage practices, and P mineralization and immobilization. The model predicts P losses under different management practices, climatic conditions, drainage systems, and crop rotations. The model is an extension to the nitrogen model DRAINMOD-NII, with full integration of the nitrogen and P model components. DRAINMOD-P uses the recently modified hydrology component that simulates macropore flow. A soil erosion component, based on the RUSLE approach, has been incorporated into the model to estimate sediment loss and associated particulate P loss. Sediment deposition in tile drains is considered to quantify particulate P settling in the drainage system. In this article, we review the approaches used in DRAINMOD-P for simulating P-related processes. Model testing against field-measured data from a subsurface-drained field in northwest Ohio is presented in a companion article. Keywords: Best management practices, Phosphorus model, Phosphorus processes, Soil erosion, Water quality modeling.

A Review of Gas Flow and Its Mathematical Models in Shale Gas Reservoirs

The application of horizontal wells with multistage hydraulic fracturing technologies has made the development of shale gas reservoirs become a worldwide economical hotspot in recent years. The gas transport mechanisms in shale gas reservoirs are complicated, due to the multiple types of pores with complex pore structure and special process of gas accumulation and transport. Although there have been many attempts to come up with a suitable and practical mathematical model to characterize the shale gas flow process, no unified model has yet been accepted by academia. In this paper, a comprehensive literature review on the mathematical models developed in recent years for describing gas flow in shale gas reservoirs is summarized. Five models incorporating different transport mechanisms are reviewed, including gas viscous flow in natural fractures or macropores, gas ad-desorption on shale organic, gas slippage, diffusion (Knudsen diffusion, Fick diffusion, and surface diffusion), stress dependence, real gas effect, and adsorption layer effect in the nanoshale matrix system, which is quite different from conventional gas reservoir. This review is very helpful to understand the complex gas flow behaviors in shale gas reservoirs and guide the efficient development of shale gas. In addition to the model description, we depicted the type curves of fractured horizontal well with different seepage models. From the review, it can be found that there is some misunderstanding about the essence of Knudsen/Fick diffusion and slippage, which makes different scholars adopt different weighting methods to consider them. Besides, the contribution of each mechanism on the transport mechanisms is still controversial, which needs further in-depth study in the future.

Concentration–Discharge Relationships in Runoff Components during Rainfall Events at the Hydrohill Experimental Catchment in Chuzhou, China

Concentration–discharge (C-Q) relationships are a convenient and increasingly popular tool for interpreting the episodic hydrochemical response to the varying discharge in small basins, providing insights into solute transport and streamflow generation. While most studies are focused on total runoff, this study quantified C-Q relationships in four runoff components during precipitation events at the Hydrohill experimental catchment in Chuzhou, China. This unique artificial catchment is carefully engineered, allowing observations of the interacting runoff components that collectively determine total flow issuing from the catchment. The four runoff components, or flow paths, include surface runoff (SR), shallow interflow at 0–30 cm depth (SSR30), deeper interflow at 30–60 cm depth (SSR60), and groundwater flow at 60–100 cm depth (SSR100). Water samples were collected during three consecutive precipitation events to study how the concentrations of primary solutes vary with flow. Analysis of C-Q relationships reveals that concentrations of Na+, Ca2+, Mg2+, SO42−, and HCO3− in the four runoff components had a negative relationship with discharge, while the concentration of K+ and Cl− were negatively correlated with discharge in SR and SSR30 but positively correlated in SSR60 and SSR100. Further insights were gained from principal component analysis. Three eigenvectors explained 92% of the variability in hydrochemistry in surface runoff, while two eigenvectors explained most of the variability in the hydrochemistry of subsurface flows observed at various depths in the soil profile (73% for SSR30, 79% for SSR60, and 76% for SSR100). PC1 (the first Principal Component) can be interpreted as a salinity factor, deriving from carbonate minerals such as dolomites and limestone minerals. Results indicated that leaching and dilution processes, water–soil interaction, and macropore flows in soils are the primary factors controlling the C-Q relationships. Our work sheds light on the coupled processes and streamflow generation mechanisms that control water quality at the catchment scale.

Application methods of tracers for N2O source determination lead to inhomogeneous distribution in field plots.

Source determination of N2O has often been performed using stable isotope incubation experiments. In situ experiments with isotopic tracers are an important next step. However, the challenge is to distribute the tracers in the field as homogeneously as possible. To examine this, a bromide solution was applied as a stand-in tracer using either a watering can, a sprayer, or syringes to a relatively dry (25% gravimetric moisture content) or wet (30%) silt loam. After 1h, samples were taken from three soil depths (0-10cm), and analyzed for their water content and bromide concentration. The application with syringes was unsuccessful due to blocked cannulas. Therefore, further laboratory experiments were conducted with side-port cannulas. Despite a larger calculated gravimetric soil moisture difference with watering can application, more Br- tracer was recovered in the sprayer treatment, probably due to faster transport of Br- through macropore flow in the wetter conditions caused by the watering can treatment. The losses of Br- (33% for the watering can, 28% for the sprayer treatment) are equivalent to potential losses of isotopic tracer solutions. For application of 60 at% 15NH4 +, this resulted in theoretical enrichments of 44-53 at% in the upper 2.5cm and 7-48 at% in 5-10cm. As there was hardly any NO3 - in the soil, extrapolations for 15NO3 - calculated enrichments were 57-59 at% in the upper 2.5cm and 26-57 at% in 5-10cm. Overall, no method, including the side-port cannulas, was able to achieve a homogeneous distribution of the tracer. Future search for optimal tracer application should therefore investigate methods that utilize capillary forces and avoid overhead pressure. We recommend working on rather dry soil when applying tracers, as tracer recovery was larger here. Furthermore, larger amounts of tracer lead to more uniform distributions. Further studies should also investigate the importance of plant surfaces.

DRAINMOD Simulation of macropore flow at subsurface drained agricultural fields: Model modification and field testing

Macropores are critical pathways through which water and pollutants can bypass the soil matrix and be rapidly transported to subsurface drains and freshwater bodies. We modified the DRAINMOD model to simulate macropore flow using a simple approach as part of developing the DRAINMOD-P model to simulate phosphorus dynamics in artificially drained agricultural lands. The Hagen-Poiseuille’s law was used to estimate the flow capacity of macropores. When ponding depths on the soil surface are greater than Kirkham’s depth, water is assumed to flow through macropores directly to tile drains without interaction with the soil matrix. In the modified model, macropore size is adjusted based on wet or dry conditions while connectivity is altered by tillage. The model was tested using a 4-year data set from a subsurface drained field in northwest Ohio. The soils at the field are classified as very poorly drained and are prone to desiccation cracking. The modified model predicted the daily and monthly subsurface drainage with average Nash-Sutcliffe efficiency (NSE) values of 0.48 and 0.59, respectively. The cumulative drainage over the 4-year simulation period was under-predicted by 8%. The new macropore component was able to capture about 75% of 60 peak drainage flow events. However, surface runoff was over-predicted for the entire study period. Annual water budgets using measured data (precipitation, subsurface drainage, and surface runoff) and model predictions (evapotranspiration, vertical seepage, and change in storage) were not balanced with an average annual imbalance of 6.4 cm. The lack of closure in the water balance suggests that errors may have occurred in field measurements, particularly, surface runoff. Overall, incorporating macropore flow into DRAINMOD improved predictions of daily drainage peaks and enabled the model to predict subsurface drainage flux contributed by macropore flow, which is critical for expanding DRAINMOD to simulate phosphorus transport in subsurface drained agricultural land.

Impacts of preferential flow and agroecosystem management on subsurface particulate phosphorus loadings in tile-drained landscapes.

Recent research on tile-drainage has placed emphasis on dissolved reactive phosphorus (DRP) delivery and transport pathways but less emphasis on particulate P (PP), resulting in its exclusion from agricultural water management models. In this study, we quantified the fluxes, mechanisms, and factors driving PP delivery into tiles through statistical analysis of a long-term hydrologic and water quality dataset. The dataset includes 5 yr of surface and tile discharge, total P (TP), DRP, total nitrogen (TN), and dissolved inorganic N concentrations from two edge-of-field study sites with contrasting soil and management practices. Hydrograph recession techniques were coupled with multiple linear regression for understanding hydrologic flow pathways, and empirical mode decomposition (EMD) time-series analysis was used to determine the significance of PP seasonality processes and the effect of management practices. The analysis of hydrologic flow pathways demonstrated that quickflow contributed 66 and 36% of subsurface discharge in the clay and loam sites, respectively. Phosphorus loading analysis showed that macropore flow plays a significant role in PP delivery to subsurface P loading and that PP significantly contributed to TP and DRP delivery; however, greater PP loadings were observed at the clay site despite greater subsurface discharge and soil test P levels at the loam site. Furthermore, PP delivery was significantly affected by environmental conditions and management practices. We highlight the efficacy of hydrograph recession analysis for identifying macropore and diffuse drainage, of P/N ratios to characterize sediment delivery mechanisms in tiles, and of EMD to detect management impacts on TP and DRP at the field scale.

Field observations of soil hydrological flow path evolution over 10 millennia

Abstract. Preferential flow strongly controls water flow and transport in soils. It is ubiquitous but difficult to characterize and predict. This study addresses the occurrence and the evolution of preferential flow during the evolution of landscapes and here specifically during the evolution of hillslopes. We targeted a chronosequence of glacial moraines in the Swiss Alps to investigate how water flow paths evolve along with the soil-forming processes. Dye tracer irrigation experiments with a Brilliant Blue FCF solution (4 g L−1) were conducted on four moraines of different ages (30, 160, 3000, and 10 000 years). At each moraine, three dye tracer experiments were conducted on plots of 1.5 m ×1.0 m. The three plots at each moraine were characterized by different vegetation complexities (low, medium, and high). Each plot was further divided into three equal subplots for the application of three different irrigation amounts (20, 40, and 60 mm) with an average irrigation intensity of 20 mm h−1. The day after the experiment five vertical soil sections were excavated, and the stained flow paths were photographed. Digital image analysis was used to derive average infiltration depths and flow path characteristics such as the volume and surface density of the dye patterns. Based on the volume density, the observed dye patterns were assigned to specific flow type categories. The results show a significant change in the type of preferential flow paths along the chronosequence. The flow types change from a rather homogeneous matrix flow in coarse material with high conductivities and a sparse vegetation cover at the youngest moraine to a heterogeneous infiltration pattern at the medium-age moraines. Heterogeneous matrix and finger flow are dominant at these intermediate age classes. At the oldest moraine only macropore flow via root channels was observed in deeper parts of the soil, in combination with a very high water storage capacity of the organic top layer and low hydraulic conductivity of the deeper soil. In general, we found an increase in water storage with increasing age of the moraines, based on our observations of the reduction in infiltration depth as well as laboratory measurements of porosity. Preferential flow is, however, not only caused by macropores, but especially for the medium-age moraine, it seems to be mainly initiated by soil surface characteristics (vegetation patches and microtopography).

Quantification of macropore flow in Danish soils using near-saturated hydraulic properties

Larger pores allowing macropore flow can rapidly transport contaminants as well as nutrients from the soil surface to the subsoil and groundwater. Modeling of the involved macropore flow is complex due to large number of required parameters. One way to deal with this difficulty is the precise evaluation of the soil hydraulic conductivity function using robust pedotransfer functions for the near-saturated part. Aiming to develop pedotransfer functions to estimate saturated and unsaturated hydraulic conductivities [Ks and K(h)] as well as water contents at specific pressure heads, we used Gaussian Process Regression (GPR), a non-parametric machine learning model, to obtain van Genuchten (1980) retention and conductivity parameters for Danish soils based on soil texture and organic matter. We defined K10 as K(h) at − 10 cm pressure head. The difference between the logarithm of Ks and K10 [log(Ks) − log(K10)] was denominated log(Kjump) and was assumed to convey the potential degree of macropore flow considering only soil hydraulic properties. Macropore flow in Denmark was evaluated through the developed PTFs for log(Ks) and log(K10) with an average RMSE of 0.635 and 0.594 (for K in cm d−1) under a confidence interval obtained by bootstrapping. Dynamics of pressure head in an initially saturated soil column with a zero-flux top boundary condition and a 2 m deep groundwater level were simulated using the Hydrus-1D model. The result of this modeling showed that soils in the eastern part of Denmark had a higher likelihood to experience macropore flow. Finally, using the spatial distribution of meteorological data, a macropore flow map of Denmark was produced. Precipitation was a dominating factor on macropore flow in sandy clay loam and sandy loam soils when compared to sandy and loamy sand soils. Finally, maps of relative risk classes of macropore flow qualitatively differentiated land areas in Denmark for vulnerability of nutrient loss and groundwater contamination.

Landmark Papers: No. 9 Jarvis, N.J. 2007. A review of non‐equilibrium water flow and solute transport in soil macropores: Principles, controlling factors and consequences for water quality. European Journal of Soil Science, 58, 523–546

Landmark Papers: No. 9 Jarvis, N.J. 2007. A review of non‐equilibrium water flow and solute transport in soil macropores: Principles, controlling factors and consequences for water quality. <i>European Journal of Soil Science</i>, 58, 523–546

A review of non‐equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

SummaryThis review discusses the causes and consequences of ‘non‐equilibrium’ water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger thanc. 0.3 mm in equivalent cylindrical diameter allow rapid non‐equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non‐equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non‐equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non‐equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

Reflection on Jarvis (2007), “Review of non‐equilibrium water flow and solute transport in soil macropores: Principles, controlling factors and consequences for water quality”. European Journal of Soil Science, 58, 523–546

Reflection on Jarvis (2007), “Review of non‐equilibrium water flow and solute transport in soil macropores: Principles, controlling factors and consequences for water quality”. European Journal of Soil Science, 58, 523–546

Spatial Variability of Preferential Flow and Infiltration Redistribution along a Rocky-Mountain Hillslope, Northern China

Rock fragments in soil strongly increase the complexity of hydrological processes. Spatial variability of preferential flow and infiltration characteristics, especially along a rocky-mountain hillslope are poorly understood. In this study, five rainfall–dye tracer experiments were performed in the rocky Taihang Mountains, northern China, to investigate the spatial variability of preferential flow and infiltration redistribution on different hillslope positions. Tracers were used to distinguish macropore flow and actual water flow patterns, and preferential flow indices and spatial non–uniformity of the infiltration redistribution were calculated using image analysis. Results showed increasing trends in the dye coverage, maximum infiltration depth, and steady infiltration rate with increased hillslope position, with a preferential flow fraction of 0.10, 0.11, 0.15, 0.29, and 0.26 for the bottom–, down–, mid–, upper–, and top–slope positions, respectively. With increased hillslope position, the spatial non–uniformity of the infiltration redistribution gradually increased in orthogonal and parallel directions to the stained section, and was supported by the fractal dimensions. Positive (gravel mass ratio, saturated water content, altitude, hydraulic conductivity and roots) and negative (bulk density and clay content) impacts on preferential flow and infiltration redistribution were quantitatively emphasized. The characteristic and mechanism of infiltration process were further identified along a rocky-mountain hillslope.

Linking soil macropores, subsurface flow and its hydrodynamic characteristics to the development of Benggang erosion

The increasing population in south China is exposed to Benggang erosion that deteriorates slope stability and agricultural production. However, the influence of subsurface flow, especially macropore flow, on the development of Benggang erosion and the collapse of Benggang remains inadequately studied. To address this issue, we measured the spatial distribution of soil macropores on an active Benggang-eroded slope in a subtropical region in China. During the experimental period of 20151221 to 20180116, two collapse events were observed on the study slope. In addition, the soil water content at the depths of 20, 40, 60, and 80 cm was monitored at a time interval of five minutes at six sites along the slope. The velocities of vertical subsurface flow traveling between measured soil depths were determined based on the difference in the response time of soil water content to rainfall along the depth. Further, the Reynolds number (Re) of subsurface flow in different water patterns, e.g., Darcy flow (DF) and preferential flow (PF) was calculated based on soil pore size and flow velocity. Results indicate that the number of macropores near the Benggang wall (i.e., the most active area of erosion on a Benggang slope) was almost twice that on the upper slope. The presence of abundant macropores near the Benggang wall facilitated faster subsurface flow and a greater variation in soil water content in time and space, which significantly influenced Benggang erosion and collapse. For both the DF and PF, the maximum Re was detected near the Benggang wall. The Re of PF was ten times higher than that of DF. The Re being greater than 3 was the threshold to initiate PF. Moreover, the larger Re of PF likely enhanced the leaching of fine soil particles and promoted the occurrence of Benggang erosion. This study confirms the key role of macropore flow in Benggang erosion, which helps better understanding, modeling and controlling of Benggang erosion.