Films, rivulets, snapping rivulets, sliding drops, slugs—many flow modes besides filled-tube, Poiseuille-type flow occur in macropores. Some of these fit reasonably into Darcian formulations and the analog of laminar viscous flow in water-filled tubes. But others do not. These exceptions may be the main reason for failures to predict the speed and travel distance of preferential flow. A useful first step for an improved model of macropore flow is the classification of diverse flow modes into categories based on their intrapore boundary conditions. Within a flowing macropore, the gas-liquid and liquid-solid interfaces, with the effects of interfacial constraints such as surface tension and contact angles, determine the geometry of the flowing liquid phase and its controlling frictional influences. A classification scheme with four categories can account for the various flow modes that have been observed in lab and field experiments. This categorization helps to distinguish which flow modes are amenable to Darcian or Poiseuille-type representation and which are not. Some of the exceptions are approachable with wave or film-flow concepts as in several recently-developed models. Yet there are other flow modes that do not fit well in any of these models, and in some cases these may be the most important means of rapid and long-distance transport. Other sorts of physical processes may provide suitable analogs for these, for example free-fall concepts like initial acceleration, speed-dependent frictional forces, and terminal velocity. In any case, the diversity of macropore flow modes needs to be considered in the development of markedly improved models of preferential flow.

Identifying and quantifying preferential flow (PF) through soil—the rapid movement of water through spatially distinct pathways in the subsurface—is vital to understanding how the hydrologic cycle responds to climate, land cover, and anthropogenic changes. In recent decades, methods have been developed that use measured soil moisture time series to identify PF. Because they allow for continuous monitoring and are relatively easy to implement, these methods have become an important tool for recognizing when, where, and under what conditions PF occurs. The methods seek to identify a pattern or quantification that indicates the occurrence of PF. Most commonly, the chosen signature is either (1) a nonsequential response to infiltrated water, in which soil moisture responses do not occur in order of shallowest to deepest, or (2) a velocity criterion, in which newly infiltrated water is detected at depth earlier than is possible by nonpreferential flow processes. Alternative signatures have also been developed that have certain advantages but are less commonly utilized. Choosing among these possible signatures requires attention to their pertinent characteristics, including susceptibility to errors, possible bias toward false negatives or false positives, reliance on subjective judgments, and possible requirements for additional types of data. We review 77 studies that have applied such methods to highlight important information for readers who want to identify PF from soil moisture data and to inform those who aim to develop new methods or improve existing ones.

Preferential flow (PF) in soil causes the rapid transport of water, nutrients, and contaminants into the subsurface, influencing groundwater recharge and streamflow. Data scarcity has hindered the quantification of PF occurrence and the identification of its drivers across diverse ecoregions. We address this gap by analyzing high-frequency, multi-depth soil moisture data across 17 ecoregions in the USA, using similar to 1,500 sensors at 40 sites. We discovered that PF is widespread, with sites experiencing PF in up to 60% of rainfall events >= 2 mm. Multiple approaches consistently show that PF is more likely to occur with increased peak rainfall intensity, finer textured material, low soil moisture variability, humid climate, and higher net primary productivity. This suggests that PF patterns could shift with projected climate changes, increasing uncertainty in predictions of groundwater recharge, water quality, and streamflow generation.

The Richardson-Richards equation (RRE), despite known shortcomings especially in regard to preferential flow, provides the basis for the vast majority of unsaturated flow models in use today. L.F. Richardson published this equation in 1922, nine years before L.A. Richards. Whereas Richards approached this problem directly from the groundbreaking developments of Edgar Buckingham, Richardson, surprisingly, cited as his starting point only the earlier work of L.J. Briggs. Collectively, these four scientists' published and unpublished work reveals that: (1) Briggs' work, though qualitative, captured the essential physical principles needed for quantifying unsaturated flow; (2) Buckingham came very close to deriving the RRE and explained why he stopped short of doing so; (3) derivation of the RRE from the work of either Briggs or Buckingham required only modest developmental work; and (4) besides deriving the RRE, Richards carried through much of the experimental agenda that Buckingham considered a necessary precursor to mathematical treatment.

Preferential flow in the unsaturated zone strongly influences important hydrologic processes, such as infiltration, contaminant transport, and aquifer recharge. Because it entails various combinations of physical processes arising from the interactions of water, air, and solid particles in a porous medium, preferential flow is highly complex. Major research is needed to improve the ability to understand, quantify, model, and predict preferential flow. Toward a solution, a combination of diverse experimental measurements at multiple scales, from laboratory scale to mesoscale, has been implemented to detect and quantify preferential paths in carbonate and karstic unsaturated zones. This involves integration of information from (1) core samples, by means of mercury intrusion porosimeter, evaporation, quasi-steady centrifuge and dewpoint potentiometer laboratory methods, to investigate the effect of pore-size distribution on hydraulic characteristics and the potential activation of preferential flow, (2) field plot experiments with artificial sprinkling, to visualize preferential pathways related to secondary porosity, through use of geophysical measurements, and (3) mesoscale evaluation of field data through episodic master recession modeling of episodic recharge. This study demonstrates that preferential flow processes operate from core scale to two different field scales and impact on the qualitative and quantitative groundwater status, by entailing fast flow with subsequent effects on recharge rate and contaminant mobilizing. The presented results represent a rare example of preferential flow detection and numerical modeling by reducing underestimation of the recharge and contamination risks.

AbstractPreferential flow in the unsaturated zone strongly influences important hydrologic processes, such as infiltration, contaminant transport, and aquifer recharge. Because it entails various combinations of physical processes arising from the interactions of water, air, and solid particles in a porous medium, preferential flow is highly complex. Major research is needed to improve the ability to understand, quantify, model, and predict preferential flow. Toward a solution, a combination of diverse experimental measurements at multiple scales, from laboratory scale to mesoscale, has been implemented to detect and quantify preferential paths in carbonate and karstic unsaturated zones. This involves integration of information from (1) core samples, by means of mercury intrusion porosimeter, evaporation, quasi-steady centrifuge and dewpoint potentiometer laboratory methods, to investigate the effect of pore-size distribution on hydraulic characteristics and the potential activation of preferential flow, (2) field plot experiments with artificial sprinkling, to visualize preferential pathways related to secondary porosity, through use of geophysical measurements, and (3) mesoscale evaluation of field data through episodic master recession modeling of episodic recharge. This study demonstrates that preferential flow processes operate from core scale to two different field scales and impact on the qualitative and quantitative groundwater status, by entailing fast flow with subsequent effects on recharge rate and contaminant mobilizing. The presented results represent a rare example of preferential flow detection and numerical modeling by reducing underestimation of the recharge and contamination risks.

<p>Low-water content soil moisture relations are increasingly important given current trends of climate change, desertification, and growing interest in extreme environments like those of Antarctica and Mars. Whereas most parametric models of soil water retention were developed for the intermediate and wet ranges of moisture, some alternatives published in the last three decades address water retention down to oven dryness. Such models can be strengthened and made more versatile with a deeper understanding of the physical meaning of parameters used in them.</p> <p>For the shape of the dry-range retention curve, a logarithmic relation has repeatedly been shown to work well, and is consistent with accepted theories of adsorption. Fitted values of the log function’s coefficient relate closely to the specific surface area of the medium.</p> <p>The lower limits of water content and matric potential require more explication. Various observers have noted problems that arise with the use of a nonzero residual water content as the lower limit. In practice, this quantity is not measured but obtained as a fitting parameter, whose value depends not on a physical property but on how far the available measurements extend into the dry range. In parametric models it can be useful for applications in which the water content never goes below the intermediate range dominated by capillary processes.</p> <p>The actual lower limit of water content depends on how its zero is defined. The most common definition is based on equilibration in an oven at a particular temperature, commonly 105° C. Ambiguity arises from the dependence of the soil water on the generally uncontrolled relative humidity within the oven. Application of the Kelvin equation with reasonable assumptions about the outside air and its exchange with the inside air can indicate an equivalent matric potential of the oven-dry state, typically about -1 GPa. Logarithmic extrapolations of dry-range retention measurements intersect the water content=0 axis at values comparable to this, with variations likely related to particular conditions in the lab and oven. A way of resolving this ambiguity is to define zero water content not in terms of oven temperature but rather a specified matric potential of equilibration. An attractive possibility, convenient in SI units, is to make it exactly -1 GPa.</p> <p>Neither the traditional nor this proposed definition of zero water content identifies a state where no water molecules remain in the soil. Measurements at temperatures of hundreds of degrees C show that soil water contents can be lower than these defined zero levels by as much as 2% or more. Our standard scale of water content, therefore, is a relative scale, analogous to the Celsius scale for temperature. Consequently negative values of soil water content have a valid physical meaning. To acknowledge this fact and resolve ambiguities, more rigorous definitions as proposed here are thus necessary for applications dealing with the extremely dry conditions that are becoming increasingly important.</p>

Abstract Because tension infiltrometers apply water through a disk of finite size, the infiltrated water moves laterally as well as downward. Only the vertical component of this flow is indicative of the hydraulic conductivity K, so the algorithm for computing K must include a way of isolating that component from the total flow. Some commonly used formulas correct for the multidimensional effects by subtracting an estimate of the laterally spreading flow. For disks smaller than about 200 mm in diameter, however, lateral spreading constitutes so much of the total flow that these subtractive formulas lose considerable accuracy, and sometimes overcorrect so severely as to produce a negative number for K. Other methods rely on empiricisms that are not completely consistent with unsaturated‐flow theory and that require prior knowledge of certain soil properties. We developed a new formula that uses a multiplicative factor instead of a subtracted term to achieve the needed correction. For testing we conducted numerical experiments with synthetic data produced by solving the Richardson‐Richards equation using the code VS2DRTI, for diverse media and a range of disk sizes, including the widely used 45‐mm diameter. We compared K values calculated from our formula to the actual K used to generate the simulated data, as well as to results from other published formulas. This comparison shows that our method provides an algorithm based in unsaturated‐flow theory that produces more reliable values for small disks without requiring prior knowledge of soil properties.

Abstract The infiltration of water into soil has profound importance as a central component of the hydrologic cycle and as the means of replenishing soil water that sustains terrestrial life. Systematic quantitative study of infiltration began in the 19th century and has continued through to the present as a central topic of soils, soil physics, and hydrology. Two forces drive infiltration: gravity, and capillarity, which results from the interaction of air-water surface tension with the solid components of soil. There are also two primary ways water moves into and within the soil. One is diffuse flow, through the pores between individual soil grains, moving from one to the next and so on. The other is preferential flow, through elongated channels such as those left by worms and roots. Diffuse flow is slow and continues as long as there is a net driving force. Preferential flow is fast and occurs only when water is supplied at high intensity, as during irrigation, major rainstorms, or floods. Both types are important in infiltration. Especially considering that preferential flow does not yet have a fully accepted theory, this means that infiltration entails multiple processes, some of them poorly understood. The soil at a given location has a limit to how much water it can absorb—the infiltration capacity. The interplay between the mode and rate of water supply, infiltration capacity, and characteristics of the soil and surrounding terrain determines infiltration into the soil. Much effort has gone into developing means of measuring and predicting both infiltration capacity and the actual infiltration rate. Various methods are available, and research is needed to improve their accuracy and ease of use.

AbstractMany studies have shown that fracture networks can provide rapid pathways for water infiltration and time‐dependent recharge in the vadose zone of consolidated fractured rock formations. To investigate the control of fracture networks and fracture–matrix interaction on preferential flow and infiltration dynamics, we constructed a simple network system consisting of m × 2 sandstone blocks placed in between two glass plates. Water was injected from a point source directly into the fracture system with a constant flow rate of 1.5 g min−1. We used a dual‐porosity non‐equilibrium model to model the discharge dynamics and the internal fracture–matrix mass exchange and observed strong deviations from the laboratory results when the original parameterization was used. This specifically concerns the matrix–fracture volume ratio κ and the fracture flow velocity v that require modifications in terms of their definition and underlying process description. Although the original model assumes a perfectly coupled fracture and matrix domain, in the experiments the discrete nature of the fracture network led to a much stronger dominance of the rapid flow domain and hence to a reduction of κ. The newly introduced parameter κ* includes additional effects and processes related to the time‐dependent evolution and smaller size of the fracture–matrix interface. Furthermore, experiments of varying total vertical system size reveal convergence toward a unique parameter set and the existence of a representative elementary volume (REV) for the chosen setup. Though it performs less well for very small systems below REV scale, the unique parameter set describes discharge dynamics in sufficiently large systems with high accuracy.

Groundwater recharge moves downward from the land surface and reaches the groundwater to replenish aquifers. Despite its importance, methods to directly measure recharge remain cost and time‐intensive. Recharge is usually estimated using indirect methods, such as the widely used water‐table fluctuation (WTF) method, which is based on the premise that rises in groundwater levels are due to recharge. In the WTF method, recharge is calculated as the difference between the observed groundwater hydrograph and the hydrograph obtained in the absence of water input. The hydrograph in the absence of rise‐producing input is estimated based on a characteristic master recession curve (MRC), which describes an average behavior for a declining water‐table. Previous studies derive MRC using recession data from all seasons. We hypothesize that for sites where groundwater table is shallow, using recession data from periods with high groundwater‐influenced evapotranspiration (ET) rates versus all periods will yield significantly different MRC, and consequently different estimates of recharge. We test this hypothesis and show that groundwater recession rates are significantly greater in warm months when the groundwater‐influenced ET rates are higher. Since obtaining seasonal recession rates is challenging for locations with a limited amount of data and is prohibitive if it is to be obtained for any given season of a particular year, we propose two novel parsimonious methods to obtain recession time constants for distinct seasons. The proposed methods show the potential to significantly improve the estimates of seasonal recession time constants and provide a better understanding of seasonal variations in recharge estimates.

The generally accepted theory of unsaturated flow, encapsulated in the hundred-year-old Richardson-Richards equation (RRE), has been successful in many situations, especially for diffuse flow through homogeneous granular media with grains and pores of sand-size or smaller. Since the late twentieth century, some version of it has also been the most commonly applied predictor of preferential flow, typically in combination with the RRE in a dual-domain framework in which the parameters take different values in the two domains. Current knowledge of preferential flow processes, however, shows that this extension of its original use is inappropriate. Various alternative formulations have been developed for preferential flow, many of them based on film and wave concepts, but these also have limits on their applicability. They also can be prohibitively awkward to combine with RRE to account for the totality of flow in an unsaturated medium. Given the different dominant processes of diffuse and preferential flow, the widely used dual-domain framework is appropriate. The RRE is available for flow in the diffuse domain, but improved methods are needed for the other two fundamental components: flow in the preferential domain, and the exchange of water between domains. For the preferential domain, I suggest these concepts and guiding principles: (1) A flow-velocity parameterization that is generalized, not specifically tied to a particular geometrical form such as films. (2) Variability of volumetric flux that is independent of flow velocity, not inextricably linked to velocity as in the gravity term of the RRE. (3) Gravity is the only significant driving force. (4) The essential constancy and uniformity of gravitational force is a tremendous advantage, and with the absence of pure diffusive flow, it reduces the required variables to just two, flux and water content, as opposed to the triply-coupled water-content/matric-potential/conductivity variables in the RRE. Further consequences are that (a) the basic continuity equation is the central component of a partial differential equation operative within the preferential domain, and (b) flux boundary conditions are the only type possible for this domain. Some guidelines for domain exchange are: (1) Flow can go in either direction, seepage as well as abstraction, depending on the diffuse-domain water content. (2) The exchange can be represented as a first-order diffusion process, from the domain interface to an internal position within the diffuse domain; this requires an additional parameter representing the effective lateral distance that this introduced water travels within the diffuse domain. A formulation based on these principles would require much development and testing, but if implemented with a minimal number of parameters, each of them having a physically meaningful interpretation, it could lead to a more versatile and acceptable way to predict preferential flow than is presently available.

Fracture networks often provide rapid pathways for water infiltration and play an important role for the time-dependent recharge in the vadose zone of consolidated fractured rock and karst formations. Such systems are often conceptualized using a dual-domain approach, since they can be divided into a fracture and a matrix domain. The fracture domain, especially when well connected, provides fast preferential flow paths, whereas the matrix domain usually acts as a storage due to the high contrast in hydraulic conductivities. Under partially saturated conditions, fracture-matrix interactions, i.e., imbibition of water from the fracture system into the matrix, strongly control the fracture flow progression. We conducted infiltration experiments in simple fracture-matrix systems of varying vertical length consisting of sandstone blocks, and use a dual-porosity non-equilibrium model to model the discharge dynamics and the internal fracture-matrix mass exchange. The results show strong deviations from the experimental observations when the original parameterization and model assumptions are not modified. The domain coupling, i.e., the (activated) interface area for fracture-matrix interaction, described by the matrix-fracture volume ratio (κ) was found to be the critical parameter in order to reproduce the data. While the original model assumes a perfectly coupled fracture and matrix domain, in the experiments the discrete nature of the fracture network leads to a much stronger dominance of the rapid flow domain and hence to a reduction of κ. The newly introduced (calibrated) parameter κ* includes additional effects and processes related to the time dependent evolution and smaller dynamic size of the fracture-matrix interface. Furthermore, experiments of varying total vertical system size reveal convergence toward a unique parameter set and the existence of a representative elementary volume (REV) for the chosen setup. Though it performs less well for very small systems below REV scale, the unique parameter set describes discharge dynamics in sufficiently large systems with high accuracy.

The generally accepted theory of unsaturated flow, encapsulated in the hundred-year-old Richardson-Richards equation (RRE), has been successful in many situations, especially for diffuse flow through homogeneous granular media with grains and pores of sand-size or smaller. Since the late twentieth century, some version of it has also been the most commonly applied predictor of preferential flow, typically in combination with the RRE in a dual-domain framework in which the parameters take different values in the two domains. Current knowledge of preferential flow processes, however, shows that this extension of its original use is inappropriate. Various alternative formulations have been developed for preferential flow, many of them based on film and wave concepts, but these also have limits on their applicability. They also can be prohibitively awkward to combine with RRE to account for the totality of flow in an unsaturated medium. Given the different dominant processes of diffuse and preferential flow, the widely used dual-domain framework is appropriate. The RRE is available for flow in the diffuse domain, but improved methods are needed for the other two fundamental components: flow in the preferential domain, and the exchange of water between domains. For the preferential domain, I suggest these concepts and guiding principles: (1) A flow-velocity parameterization that is generalized, not specifically tied to a particular geometrical form such as films. (2) Variability of volumetric flux that is independent of flow velocity, not inextricably linked to velocity as in the gravity term of the RRE. (3) Gravity is the only significant driving force. (4) The essential constancy and uniformity of gravitational force is a tremendous advantage, and with the absence of pure diffusive flow, it reduces the required variables to just two, flux and water content, as opposed to the triply-coupled water-content/matric-potential/conductivity variables in the RRE. Further consequences are that (a) the basic continuity equation is the central component of a partial differential equation operative within the preferential domain, and (b) flux boundary conditions are the only type possible for this domain. Some guidelines for domain exchange are: (1) Flow can go in either direction, seepage as well as abstraction, depending on the diffuse-domain water content. (2) The exchange can be represented as a first-order diffusion process, from the domain interface to an internal position within the diffuse domain; this requires an additional parameter representing the effective lateral distance that this introduced water travels within the diffuse domain. A formulation based on these principles would require much development and testing, but if implemented with a minimal number of parameters, each of them having a physically meaningful interpretation, it could lead to a more versatile and acceptable way to predict preferential flow than is presently available.

Recharge dynamics within the vadose zone (variable saturation conditions) of consolidated fractured rock formations are an ongoing challenge when it comes to process understanding and predictive modeling. The proper delineation of fast (macropores, fractures, conduits) and slow (matrix) flow components in these systems and their interaction with each other remains a complex puzzle and holds a key to enhance process-based infiltration models.We conducted laboratory and field experiments to study infiltration dynamics through porous-fractured systems. Laboratory experiments were carried out with analogue fracture networks on meter scale. Orthogonal networks were created by placing equally sized blocks with a constant gap between to glass plates, which were mount by metal clamps. Vertical flow through different network configurations (apertures, intersection types, topology, flow rates) was studied for (1) porous media (sandstone) and (2) non-porous media (glass) to delineate the control of network features on flow dynamics, as well as the effect of fracture-matrix interaction. Matrix imbibition was found to strongly control the preferential flow velocity during flow path evolution. Higher infiltration rates lead to more by-pass at fracture intersections, whereas low infiltration rates favor flow partitioning into horizontal fractures. Vertical flow progression within the non-porous network is significantly faster due to the lack of imbibition. Semi-analytical tools, such as transfer functions, and source-responsive dual-domain models are tested to reproduce the experimental data and to incorporate key features of fracture networks in future modeling approaches. We additionally obtained experimental data from infiltration dynamics at porous-fractured field sites on meter scale to compare them to the well-controlled laboratory experiments and to evaluate the applicability of the results to actual field processes.

Most models of soil water retention represent the wettest range simplistically, reflecting a high priority on facilitating computation without recognition of the active physical processes. Commonly the wet range is misleadingly represented by a straight line of zero slope, or by default using the same formulation as for the middle range, even though the mechanisms of water retention are different for the wet and middle portions of the range. Though adequate for some purposes, such treatment causes problems for applications that are sensitive to wet-range processes. It prevents accurate prediction of critical but challenging wet-range phenomena such as domain exchange between preferential flow paths and soil matrix. It limits the choices available for quantifying flow problems, for example a blowing-up of derivatives on approach to saturation prohibits the use of diffusivity-based formulations. A new model addresses these issues for the important case where the medium is soil matrix material exclusive of macropores, thus having a well-defined air-entry value, and the moisture dynamics are the typical wet and dry cycling that achieves maximum wetness at field saturation, with a presence of trapped air at zero matric potential. The range between the air-entry value and field saturation is dominated by trapped air expansion in response to pressure change, as well as a process that increases the sensitivity to changing matric pressure. This enhanced sensitivity may be related in part to a collapse of liquid bridges between air pockets as they expand. For this wet range, the new model incorporates the Boyles’ law inverse-proportionality of trapped air volume and pressure, amplified by an empirical factor to account for the additional processes. To cover the full range of possible moisture, this wet-range formula is supplemented by two others. The middle range of capillary advance/retreat and Haines jumps is represented by a new adaptation of the lognormal distribution function. The adsorption-dominated dry range is represented by a logarithmic relation used in earlier models. Joined together with a continuous first-derivative constraint, the overall formulation recognizes the dominant processes within three segments of the full range. Optimization of five parameters can fit the model to a full data set. Tests have demonstrated excellent fits, using measured data that have many closely spaced points in the wet and middle ranges. With their basis in process, the model’s parameters have a strong physical interpretation, and potentially can be assigned values without fitting, from knowledge of fundamental relationships or individual measurements. This basis in process also may permit accommodation of hysteresis by a systematic adjustment of the relation between the wet and middle ranges, and with minimal additional data may serve to facilitate estimation of other properties such as hydraulic conductivity, diffusivity, and sorptivity.

Preferential 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.

The unsaturated zone (UZ) extends across the Earth’s terrestrial surface and is central to many problems related to land and water resource management. Flow of water through the UZ is typically thought to be slow and diffusive, such that it could attenuate fluxes and dampen variability between atmospheric inputs and underlying aquifer systems. This would reduce water resource vulnerability to contaminants and water-related hazards. Reducing or negating that effect, however, spatially concentrated and rapid flow and transport through the unsaturated zone is surprisingly common and becoming more so with the increasing frequency and magnitude of extreme hydroclimatic events. Arising from the wide range in the rates and complex modes of nonlinear flow processes, these effects are among the most poorly characterized hydrologic phenomena. Issues of scale present additional difficulties. Equations representing unsaturated processes have been developed and tested on the basis of field and laboratory measurements typically made at scales from pore size to plot size. In contrast, related problems of significant interest to society, including floods, aquifer recharge, landslides, and groundwater contamination, range from watershed to regional scales. The disparity between the scale of our understanding and the scale of interest for societal problems has spurred application of these model equations at increasingly coarse resolutions over larger areas than can be justified by existing measurements or theory. This mismatch in scales requires an assumption that spatially averaging slow diffusive flow and rapid preferential flow can effectively represent the influence of both processes across vast areas. Given the currently inadequate recognition and quantitative characterization of focused and rapid processes in unsaturated flow, these phenomena are critically in need of expanded attention and effort.