Effect of macropores on soil freezing and thawing with infiltration

An understanding of heat transport and water flow in unsaturated soils experiencing freezing and thawing is important when considering hydrological and thermal processes in cold regions. Macropores, such as cracks, roots, and animal holes, provide efficient conduits for enhanced infiltration, resulting in a unique distribution of water content. However, the effects of macropores on soil freezing and thawing with infiltration have not been well studied. A one‐directional soil‐column freezing and thawing experiment was conducted using unsaturated sandy and silt loams with different sizes and numbers of macropores. During freezing, macropores were found to retard the formation of the frozen layer, depending on their size and number. During thawing, water flowed through macropores in the frozen layer and reached the underlying unfrozen soil. However, infiltrated water sometimes refroze in a macropore. The ice started to form at near inner wall of the macropore, grew to the centre, and blocked flow through the macropore. The blockage ice in the macropore could not melt until the frozen layer disappeared. Improving a soil freezing model to consider these macropore effects is required. Copyright © 2016 John Wiley & Sons, Ltd.     

Modelling investigation of water partitioning at a semiarid ponderosa pine hillslope

The effects of vegetation root distribution on near‐surface water partitioning can be two‐fold. On the one hand, the roots facilitate deep percolation by root‐induced macropore flow; on the other hand, they reduce the potential for deep percolation by root‐water‐uptake processes. Whether the roots impede or facilitate deep percolation depends on various conditions, including climate, soil, and vegetation characteristics. This paper examines the effects of root distribution on deep percolation into the underlying permeable bedrock for a given soil profile and climate condition using HYDRUS modelling. The simulations were based on previously field experiments on a semiarid ponderosa pine (Pinus ponderosa) hillslope. An equivalent single continuum model for simulating root macropore flow on hillslopes is presented, with root macropore hydraulic parameterization estimated based on observed root distribution. The sensitivity analysis results indicate that the root macropore effect dominates saturated soil water flow in low conductivity soils (K_matrix below 10^?7 m/s), while it is insignificant in soils with a K_matrix larger than 10^?5 m/s, consistent with observations in this and other studies. At the ponderosa pine site, the model with simple root‐macropore parameterization reasonably well reproduces soil moisture distribution and some major runoff events. The results indicate that the clay‐rich soil layer without root‐induced macropores acts as an impeding layer for potential groundwater recharge. This impeding layer results in a bedrock percolation of less than 1% of the annual precipitation. Without this impeding layer, percolation into the underlying permeable bedrock could be as much as 20% of the annual precipitation. This suggests that at a surface with low‐permeability soil overlying permeable bedrock, the root penetration depth in the soil is critical condition for whether or not significant percolation occurs. Copyright © 2010 John Wiley & Sons, Ltd.     

An infiltration model based on flow variability in macropores: development, sensitivity analysis and applications

Simulating infiltration in soils containing macropores still provides unsatisfactory results, as existing models seem not to capture all relevant processes. Recent studies of macropore flow initiation in natural soils containing earthworm channels revealed a distinct flow rate variability in the macropores depending on the initiation process. When macropore flow was initiated at the soil surface, most of the macropores received very little water while a few macropores received a large proportion of the total inflow. In contrast, when macropore flow was initiated from a saturated or nearly saturated soil layer, macropore flow rate variation was much lower. The objective of this study was to develop, evaluate, and test a model, which combines macropore flow variability with several established approaches to model dual permeability soils. We then evaluate the INfiltration–INitiation–INteraction Model (IN³M) to explore the influence of macropore flow variability on infiltration behavior by performing a sensitivity analysis and applying IN³M to sprinkling and dye tracer experiments at three field sites with different macropore and soil matrix properties. The sensitivity analysis showed that the flow variability in macropores reduces interaction between the macropores and the surrounding soil matrix and thus increases bypass flow, especially for surface initiation of macropore flow and at higher rainfall intensities. The model application shows reasonable agreement between IN³M simulations and field data in terms of water balance, water content change, and dye patterns. The influence of macropore flow variability on the hydrological response of the soil was considerable and especially pronounced for soils where initiation occurs at the soil surface. In future, the model could be applied to explore other types of preferential flow and hence to get a generally better understanding of macropore flow.     

Effects of antecedent moisture and macroporosity on infiltration and water flow in frozen soil

Infiltration into frozen soil plays an important role in soil freeze–thaw and snowmelt-driven hydrological processes. To better understand the complex thermal energy and water transport mechanisms involved, the influence of antecedent moisture content and macroporosity on infiltration into frozen soil was investigated. Ponded infiltration experiments on frozen macroporous and non-macroporous soil columns revealed that dry macroporous soil produced infiltration rates reaching 10³ to 10⁴ mm day⁻¹, two to three orders of magnitude larger than dry non-macroporous soil. Results suggest that rapid infiltration and drainage were a result of preferential flow through initially air-filled macropores. Using recorded flow rates and measured macropore characteristics, calculations indicated that a combination of both saturated flow and unsaturated film flow likely occurred within macropores. Under wet conditions, regardless of the presence of macropores, infiltration was restricted by the slow thawing rate of pore ice, producing infiltration rates of 2.8 to 5.0 mm day⁻¹. Reduced preferential flow under wet conditions was attributed to a combination of soil swelling, due to smectite-rich clay (that reduced macropore volume), and pore ice blockage within macropores. In comparison, dry soil column experiments demonstrated that macropores provided conduits for water and thermal energy to bypass the frozen matrix during infiltration, reducing thaw rates compared with non-macroporous soils. Overall, results showed the dominant control of antecedent moisture content on the initiation, timing, and magnitude of infiltration and flow in frozen macroporous soils, as well as the important role of macropore connectivity. The study provides an important data set that can aid the development of hydrological models that consider the interacting effects of soil freeze–thaw and preferential flow on snowmelt partitioning in cold regions.     

Estimating macropore and matrix flow using the hydrograph separation procedure in an experimental forest plot

This study was conducted to estimate macropore space, macropore flow and matrix flow in an experimental forest plot in the Ouachita Mountains of Arkansas. Lateral soil water fluxes and soil capillary potentials were observed in the isolated plot during applied rainfall experiments. Rainfalls were applied 17 times during the period 17 July to 10 October 1991. The subsurface hydrograph separation technique was used to estimate macropore space, macropore flux and matrix flux. The boundary between macropore and matrix flow was statistically determined by covariance analysis. The maximum estimated lateral macropore space was approximately 0.006 (cm³ cm^?3). The maximum estimated lateral macropore and matrix flow were 0.042 and 0.00066 cm s^?1, respectively. This report also emphasizes the need for further research on the hydrograph separation procedure for estimating macropores and macropore flow.     

Macropores and preferential flow—a love‐hate relationship

Preferential flow is of high relevance for runoff generation, transport of chemicals and nutrients, and the transit time distribution of water in the soil or watershed. However, preferential flow effects are generally ignored in lumped hydrological models. And even most physically‐based models ignore macropores and preferential flow features at the soil and hillslope scale. Keith Beven was never satisfied with this situation and he tried again and again to convince the scientific community to focus their research on the complex topic of macropore and preferential flow. Although he recognized how difficult it is to correctly include preferential flow in hydrological models, he made substantial progress defining and describing macropore flow and showing its relevance, developing models to simulate preferential flow, and in particular, the interaction between macropores and the soil matrix. In this short commentary, I reflect on these achievements and outline a vision for research in preferential flow experiments and modeling.     

Snowmelt‐driven macropore flow and soil saturation in a semiarid forest

Lateral subsurface flow is generally assumed to occur as a result of the development of a saturated zone above a low‐permeability interface such as at the soil–bedrock contact, and it is often augmented by macropore flow. Our objective was to evaluate the development of lateral subsurface flow and soil saturation at a semiarid ponderosa pine forest in New Mexico with respect to the conceptual model of saturation building above the soil–bedrock contact. At this site, we have long‐term observations of the water budget components, including lateral flow. A 1·5 m deep by 7 m long trench was constructed to observe lateral subsurface flow and development of saturation directly. Our observations are based on flow resulting from a melting snowdrift. The edge of the drift was about 7 m upslope from the trench. Lateral subsurface flow only occurred from root macropores in the Bt soil horizon. Saturation developed and grew outward from flowing root macropores, rather than growing upward from the soil–bedrock interface. This macropore‐centred saturation resulted in a highly heterogeneous distribution of water content until enough macropores began flowing and individual macropore saturated zones grew large enough to coalesce and saturate large volumes of the soil. Our observations are based on one snowmelt event and a relatively short hillslope flow path, and thus do not represent a full range of hydrologic conditions. Nevertheless, the observed behaviour did not conform to the traditional model of soil–bedrock control of saturation and lateral flow. Copyright © 2004 John Wiley & Sons, Ltd.     

Comparing dispersivities and soil chloride concentrations of turfgrass-covered undisturbed and disturbed soil columns

Research relating to soil leaching properties under turfgrass conditions has often been conducted on disturbed soils where macropore structure has been destroyed. The objective of this study was to compare the solute movement characteristics of undisturbed and disturbed soil columns covered with turfgrass. Dispersivities and chloride (Cl) breakthrough curves of undisturbed and disturbed soils were investigated. Soil columns were excavated into three sections after testing, for which the mean bulk density was 1.33 Mg M⁻³ for the undisturbed columns and 1.16 Mg m⁻³ for the disturbed columns. The dispersivity for the undisturbed columns was over three times greater than for the disturbed columns. Chloride concentration found in Layer 1 (0–6.7 cm), Layer 2 (6.7–13.4 cm), and Layer 3 (13.4–20.0 cm) were 2.8, 5.3, and 4.8 times higher, respectively, for the disturbed soils than for the undisturbed. Applying conclusions from solute movement studies using repacked columns covered with turfgrass to actual undisturbed field conditions could lead to errors in interpretation because of the effect of macropores.     

Field study of macropore flow processes using tension infiltration of a dye tracer in partially saturated soils

Macropores are important preferential pathways for the migration of water and contaminants through the vadose zone. The objective of this study was to examine small‐scale preferential flow processes during infiltration in macroporous, low permeability soils. A series of tension infiltration tests were conducted using Brilliant Blue dye tracer at two field sites in southwestern Ontario, Canada. The maximum applied pressure head was varied for each test and the resulting dye stain patterns and macropore networks were characterized by excavation, mapping, photography, and image analysis. Worm burrows were the dominant macropore type, with average macropore densities exceeding 400 m^?2 and peak densities of more than 750 m^?2 at 30 cm depth at both sites. Flow in macropores became significant at infiltration pressures > ? 3 cm, with corresponding increases in infiltration rate, soil water content variability (spatially and temporally), and depth of dye staining. The results demonstrated clear evidence for partially saturated macropore flow under porewater tension conditions and the associated importance of macropore–matrix interaction in controlling this flow. Field observations of transient infiltration showed that film and rivulet flow along macropores yielded vertical flow velocities exceeding 40 m d^?1. Simple calculations showed that film flow along the walls and corners of irregularly shaped macropores could explain the observed results. Copyright © 2009 John Wiley & Sons, Ltd.     

Topographic controls upon soil macropore flow

Macropores are important components of soil hydrology. The spatial distribution of macropore flow as a proportion of saturated hydraulic conductivity was tested on six humid–temperate slopes using transects of tension infiltrometer measurements. Automated water table and overland flow monitoring allowed the timing of, and differentiation between, saturation‐excess overland flow and infiltration‐excess overland flow occurrence on the slopes to be determined and related to tension‐infiltrometer measurements. Two slopes were covered with blanket peat, two with stagnohumic gleys and two with brown earth soils. None of the slopes had been disturbed by agricultural activity within the last 20 years. This controlled the potential for tillage impacts on macropores. The proportion of near‐surface macropore flow to saturated hydraulic conductivity was found to vary according to slope position. The spatial patterns were not the same for all hillslopes. On the four non‐peat slopes there was a relationship between locations of overland flow occurrence and reduced macroporosity. This relationship did not exist for the peat slopes investigated because they experienced overland flow across their whole slope surfaces. Nevertheless, they still had a distinctive spatial pattern of macropore flow according to slope position. For the other soils tested, parts of slopes that were susceptible to saturation‐excess overland flow (e.g. hilltoes or flat hilltops) tended to have least macropore flow. To a lesser extent, for the parts of slopes susceptible to infiltration‐excess overland flow, the proportion of macropore flow as a component of infiltration was also smaller compared with the rest of the slope. The roles of macropore creation and macropore infilling by sheet wash are discussed, and it is noted that the combination of these may result in distinctive topographically controlled spatial patterns of macropore flow. Copyright © 2008 John Wiley & Sons, Ltd.     

ABSENCE OF PREFERENTIAL FLOW IN THE PERCOLATING WATERS OF A CONIFEROUS FOREST SOIL

Evidence for the functioning of macropores and the presence of preferential flow in forest soils is equivocal. This is partly because many workers use only one diagnostic technique to indicate whether or not macropore flow occurs. In this paper three lines of evidence are used to suggest that preferential flow does not occur in the percolating waters of a coniferous forest soil under the range of hydrological conditions that prevail in the field. To simulate field conditions, realistic rainfall intensities were used in conservative solute transport experiments on four undisturbed soil columns. A method is described in which breakthrough data can be used to calculate the percentage of antecedent water displaced from a soil column during frontal-type breakthrough experiments. Calculations based on this method using the experimental data show that as little as five percent of the antecedent water was immobile. The simple form of the functional advection–dispersion equation, based on a single value for linear velocity and the dispersion coefficient was fitted to two of the breakthrough curves with reasonable accuracy, further suggesting that preferential flow did not occur in the experiments. Finally, soil moisture characteristic curves were determined for replicate soil samples from the forest soil. The operational water contents of the columns during the breakthrough experiments were compared with the soil moisture characteristics and it was found that pores exerting pressure heads greater than −0·5 kPa did not appear to contribute to flow through the columns, again suggesting an absence of preferential flow. © 1997 by John Wiley & Sons, Ltd.     

Characterizing solute transport in undisturbed soil cores using electrical and X‐ray tomographic methods

Solute transport in undisturbed soil is a complex process and detailed information on the transport characteristics is needed to provide fundamental understanding of the processes involved. X‐ray computer tomography (CT) and electrical resistivity tomography (ERT) have been used to gain information on the transport characteristics. Both methods are non‐intrusive and do not disturb the soil, in contrast to other methods. CT provides high resolution information on bulk density and macropores, while ERT provides a three‐dimensional image of the internal resistivity structure. By adding a suitable solute under steady‐state flow, the internal resistivity changes can be interpreted as a change in resident concentrations. In our experiment two cores from different field sites were investigated. The ERT measurements revealed two transport modes (one fast and one slow) in one of the cores and only one mode in the other. This was consistent with the results of transfer function modelling on the independently measured breakthrough curves (BTCs). The fast transport mode is perhaps a result of many connected macropores, detected by CT, but this could not be verified with the ERT measurements because of the coarser resolution. However, with ERT in both cases we were able to explain the observed BTC qualitatively. Copyright © 1999 John Wiley & Sons, Ltd.     

Infiltration pattern in a regolith–fractured bedrock profile: field observation of a dye stain pattern

We examined the infiltration pattern of water in a regolith–bedrock profile consisting of two overburdens (OB1 and OB2), a buried rice paddy soil (PS), two texturally distinctive weathered materials (WM1 and WM2) and a fractured sedimentary rock (BR), using a Brilliant Blue FCF dye tracer. A black‐coloured coating in conducting fractures in WM1, WM2 and BR was analysed by X‐ray diffraction and scanning electron microscopy. The dye tracer penetrated to greater than 2 m depth in the profile. The macropore flow and saturated interflow were the major infiltration patterns in the profile. Macropore flow and saturated interflow were observed along fractures in WM1, WM2 and BR and at the dipping interfaces of PS–WM1, PS–WM2 and PS–BR respectively. Heterogeneous matrix flow occurred in upper overburden (OB1) and PS. Compared with OB1, the coarser textured OB2 acted as a physical barrier for vertical flow of water. The PS with low bulk density and many fine roots was another major conducting route of water in the profile. Manganese oxide and iron oxide were positively identified in the black coating material and had low crystallinity and high surface area, indicating their high reactivity with conducting contaminants. Copyright © 2006 John Wiley & Sons, Ltd.     

Synergistic effects of water repellency and macropore flow on the hydraulic conductivity of a burned forest soil,south‐east Australia

Research shows that water repellency is a key hydraulic property that results in reduced infiltration rates in burned soils. However, more work is required in order to link the hydrological behaviour of water repellent soils to observed runoff responses at the plot and hillslope scale. This study used 5 M ethanol and water in disc infiltrometers to quantify the role of macropore flow and water repellency on spatial and temporal infiltration patterns in a burned soil at plot (<10 m²) scale in a wet eucalypt forest in south‐east Australia. In the first summer and winter after wildfire, an average of 70% and 60%, respectively, of the plot area was water repellent and did not contribute to infiltration. Macropores (r > 0·5 mm), comprising just 5·5% of the soil volume, contributed to 70% and 95%, respectively, of the field‐saturated and ponded hydraulic conductivity (K_p). Because flow occurred almost entirely via macropores in non‐repellent areas, this meant that less than 2·5% of the soil surface effectively contributed to infiltration. The hydraulic conductivity increased by a factor of up to 2·5 as the hydraulic head increased from 0 to 5 mm. Due to the synergistic effect of macropore flow and water repellency, the coefficient of variation (CV) in K_p was three times higher in the water‐repellent soil (CV = 175%) than under the simulated non‐repellent conditions (CV = 66%). The high spatial variability in K_p would act to reduce the effective infiltration rate during runoff generation at plot scale. Ponding, which tend to increase with increasing scale, activates flow through macropores and would raise the effective infiltration rates at larger scales. Field experiments designed to provide representative measurements of infiltration after fire in these systems must therefore consider both the inherent variability in hydraulic conductivity and the variability in infiltration caused by interactions between surface runoff and hydraulic conductivity. Copyright © 2010 John Wiley & Sons, Ltd.     

The influence of preferential flow on hillslope hydrology in a semi‐arid watershed (in the Spanish Dehesas)

Preferential flow is known to influence hillslope hydrology in many areas around the world. Most research on preferential flow has been performed in temperate regions. Preferential infiltration has also been found in semi‐arid regions, but its impact on the hydrology of these regions is poorly known. The aim of this study is to describe and quantify the influence of preferential flow on the hillslope hydrology from small scale (infiltration) to large scale (subsurface stormflow) in a semi‐arid Dehesa landscape. Precipitation, soil moisture content, piezometric water level and discharge data were used to analyse the hydrological functioning of a catchment in Spain. Variability of soil moisture content during the transition from dry to wet season (September to November) within horizontal soil layers leads to the conclusion that there is preferential infiltration into the soils. When the rainfall intensity is high, a water level rapidly builds up in the piezometer pipes in the area, sometimes even reaching soil surface. This water level also drops back to bedrock within a few hours (under dry catchment conditions) to days (under wet catchment conditions). As the soil matrix is not necessarily wet while this water layer is built up, it is thought to be a transient water table in large connected pores which drain partly to the matrix, partly fill up bedrock irregularities and partly drain through subsurface flow to the channels. When the soil matrix becomes wetter the loss of water from macropores to the matrix and bedrock decreases and subsurface stormflow increases. It may be concluded that the hillslope hydrological system consists of a fine matrix domain and a macropore domain, which have their own flow characteristics but which also interact, depending on the soil matrix and macropore moisture contents. The macropore flow can result in subsurface flow, ranging from 13% contribution to total discharge for a large event of high intensity rainfall or high discharge to 80% of total discharge for a small event with low intensity rainfall or low discharge. During large events the fraction of subsurface stormflow in the discharge is suppressed by the large amount of surface runoff. Copyright © 2008 John Wiley & Sons, Ltd.     

The role of overland flow and subsurface flow on the spatial distribution of soil moisture in the topsoil

Many investigations show relationships between topographical factors and the spatial distribution of soil moisture in catchments. However, few quantitative analyses have been carried out to elucidate the role of different hydrological processes in the spatial distribution of topsoil moisture in catchments. A spatially distributed rainfall—runoff model was used to investigate contributions of subsurface matric flow, macropore flow and surface runoff to the spatial distribution of soil moisture in a cultivated catchment. The model results show that lateral subsurface flow in the soil matrix or in macropores has a minor effect on the spatial distribution of soil moisture. Only when a perched groundwater table is maintained long enough, which is only possible if the subsurface is completely impermeable, may a spatial distribution in moisture content occur along the slope. Surface runoff, producing accumulations of soil moisture in flat flow paths of agricultural origin (field boundaries), was demonstrated to cause significant spatial variations in soil moisture within a short period after rainfall (<2 days). When significant amounts of surface runoff are produced, wetter moisture conditions will be generated at locations with larger upstream contributing areas. Copyright © 2001 John Wiley & Sons, Ltd.     

Tracing solute infiltration using a combined method of dye tracer test and electrical resistivity tomography in an undisturbed forest soil profile

An accurate prediction of solute infiltration in a soil profile is important in the area of environmental science, groundwater and civil engineering. We examined the infiltration pattern and monitored the infiltration process using a combined method of dye tracer test and electrical resistivity tomography (ERT) in an undisturbed field soil (1 m × 1 m). A homogeneous matrix flow was observed in the surface soil (A horizon), but a preferential flow along macropores and residual rock structure was the dominant infiltration pattern in the subsurface soil. Saturated interflow along the slopping boundaries of A and C1 horizons and of an upper sandy layer and a lower thin clay layer in the C horizon was also observed. The result of ERT showed that matrix flow started first in A horizon and then the infiltration was followed by the preferential flows along the sloping interfaces and macropores. The ERT did not show as much detail as the dye‐stained image for the preferential flow. However, the area with the higher staining density where preferential flow was dominant showed a relatively lower electrical resistivity. The result of this study indicates that ERT can be applied for the monitoring of solute transportation in the vadose zone. Copyright © 2010 John Wiley & Sons, Ltd.     

Sensitivity analysis on processes affecting bypass flow

Bypass flow in structured soils is dominated by soil hydrological processes, such as rain intensity, initial pressure head of the soil, surface storage of rain, horizontal contact area and absorption rate, and hydraulic conductivity of the soil matrix. This study was conducted to determine the relative impact of these processes in different soil types. A quasi 3-dimensional simulation model was used to calculate the effects of these soil hydrological input parameters on surface infiltration, macropore flow (with related horizontal absorption) and drainage. For light textured soils, surface infiltration was the most important term in the water balance. Heavy textured soils, in contrast, had drainage as the main term. In the latter soils bypass flow, when occurring, was almost equal to the amount of rain applied, indicating that absorption processes were strongly reduced. Lateral absorption on macropore walls was a minor fraction in the total mass balances, due to limited contact area and relatively weak diffusivity forces. Surface infiltration is a crucial parameter in bypass flow and is mainly dependent on rain intensity, initial pressure head and conductivity of the soil matrix. This requires measurement methods for hydraulic conductivity that specifically consider the effect of macropores.