The drought or base flow characteristics of six basins in the Finger Lakes region are obtained by considering for each available record the lower envelope of |dQ/dt| as a function of Q, where Q is the flow rate. This procedure avoids the uncertainty regarding a proper time reference after each rainfall event, and it eliminates the effects of evapotranspiration. The results suggest that among several expressions, Boussinesq's nonlinear solution of free surface groundwater flow is best suited to parameterize the observed hydrographs. The obtained parameters can be related to the basin characteristics, viz., drainage area and the total stream length, in accordance with relationships derived on the basis of the Dupuit‐Boussinesq aquifer model. This result allows the determination of drought flow parameters for ungaged sites within the region.

Abstract:Runoff from hillslopes is generated by processes such as infiltration-excess and saturation overland flow, subsurface stormflow and subsurface return flow. Preferential flow through macropores can affect any one of these runoff generation processes. Field studies at the Hitachi Ohta Experimental Watershed in Japan have noted that self-organization processes may manifest the connectivity of such subsurface flow paths, particularly via macropores, from hillslopes to stream channels. It is well established from soil physics principles that the connectivity of macropore networks depends on soil wetness and this has been shown experimentally at Hitachi Ohta where subsurface flow and streamflow respond to thresholds of wetness. Numerical solutions to the three-dimensional Richards equation are derived for a sloping soil block containing a population of disconnected macropores of various sizes, shapes and orientations. Solutions for the case of steady water flux applied to the surface of the soil block are evaluated to determine the conditions where the disconnected macropores become active in the flow process. Results show that subsurface flow is directed through the preferential flow network in the saturated portion of the soil but bypasses the macropores in the drier regions. The preferential flow network expands as the degree of saturation increases. The expanding network of active macropores leads to less resistance to overall flow in the domain and access to increased volumes of the flow domain. Although the individual macropores are disconnected, it is argued that large localized hydraulic gradients can potentially lead to preferred zones of subsurface erosion. In addition to the importance of these findings related to stormflow generation in catchments, they add support to the concept of self-organization of subsurface flow systems in soils.

Although it is generally well known that water repellent soils have distinct preferential flow patterns, the physics of this phenomenon is not well understood. In this paper, we show that water repellency affects the soil water contact angle and this, in turn, has a distinct effect on the constitutive relationships during imbibing. Using these constitutive relationships, unstable flow theory developed for coarse grained soils can be used to predict the shape and water content distribution for water repellent soils. A practical result of this paper is that with a basic experimental setup, we can characterize the imbibing front behavior by measuring the water entry pressure and the imbibing soil characteristic curve from the same heat treated soil. ᭧

Abstract. With prolonged rainfall, infiltrating wetting fronts in water repellent soils may become unstable, leading to the formation of high-velocity flow paths, the so-called fingers. Finger formation is generally regarded as a potential cause for the rapid transport of water and contaminants through the unsaturated zone of soils. For the first time, field evidence of the process of finger formation and finger recurrence is given for a water repellent sandy soil. Theoretical analysis and model simulations indicate that finger formation results from hysteresis in the water retention function, and the character of the formation depends on the shape of the main wetting and main drainage branches of that function.Once fingers are established, hysteresis causes fingers to recur along the same pathways during following rain events. Leaching of hydrophobic substances from these fingered pathways makes the soil within the pathways more wettable than the surrounding soil. Thus, in the long-term, instability-driven fingers might become heterogeneity-driven fingers.

[1] Stability analysis of gravity-driven unsaturated flow is examined for the general case of Darcian flow with a generalized nonequilibrium capillary pressure-saturation relation. With this nonequilibrium relation the governing equation is referred to as the nonequilibrium Richards equation (NERE). For the special case where the nonequilibrium vanishes, the NERE reduces to the Richards equation (RE), the conventional governing equation for describing unsaturated flow. A generalized linear stability analysis of the RE shows that this equation is unconditionally stable and therefore not able to produce gravity-driven unstable flows for infinitesimal perturbations to the flow field. A much stronger result of unconditional stability for the RE is derived using a nonlinear stability analysis applicable to the general case of heterogeneous porous media. For the general case of the NERE model, results of a linear stability analysis show that the NERE model is conditionally stable, with lower-frequency perturbations being unstable. A result of this analysis is that the nonmonotonicity of the pressure and saturation profile is a requisite condition for flow instability.

[1] Understanding colloid transport behavior in unsaturated porous media is important because mobile colloid-contaminant complexes and colloidal pathogens can degrade groundwater quality. Visual evidence suggests that colloids are retained at the air-water meniscus-solid (AW m S) interface in unsaturated porous media, but there is no quantitative theoretical explanation of this retention mechanism to date. A theoretical study is presented here to quantify energy potentials represented by capillary forces on colloids at the AW m S interface in unsaturated porous media. Our calculations indicate that the capillary energy potentials for colloids retained in films or at the AW m S interface range from 10 7 to 10 8 kt, which are several orders greater than the Derjaguin-Landau-VerweyOverbeek energy potentials. Capillary forces for colloids at the AW m S interface can be decomposed into a force that pushes the colloid back in the bulk solution and one that pins the colloid against the surface of the grain. A friction force generated between the colloid and grain can prevent the colloids from moving back into solution. When drag forces can be ignored, colloids will remain at the AW m S interface as long as the static friction coefficient is greater than the tangent of the water-grain contact angle. It is independent of the surface tension.Citation: Gao, B., T.

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Gas diffusion is important in determining the aeration status of soils for crop production and in providing estimates of transport of volatile chemicals at waste disposal or chemical spill sites. This study investigated the effects of compaction on the gas diffusion coefficient in four soils. The experiment involved equilibrating bulk loose soil to a known matric potential in a pressure chamber, compressing the equilibrated moist soil in metal cores at a given applied stress, and measuring the concentration of N 2 diffusing through these cores into a diffusion chamber. The diffusion coefficient was calculated by fitting an analytical solution of the transient diffusion equation to the measured N, concentration in the diffusion chamber as a function of time. The diffusion coefficient of N 2 in four soils at four water contents and four applied loads varied exponentially as a function of air-filled porosity and was nearly the same function for all four soils. At an airfilled porosity of-10%, the diffusion coefficient was close to zero, reflecting a discontinuity in the pathways at an air-filled pore space of 10% or lower. A model for predicting the diffusion coefficient of gases in soils based on the diffusion coefficient of individual soil constituents was tested with the data set. For known shape factors for soil solid and soil water, the model overpredicted the diffusion coefficient for all four soils. A reduction factor suggested in the literature to account for blocked air slightly improved predictions. The ratio of the measured to predicted diffusion coefficient vs. degree of air-filled saturation suggests a second-degree correction factor to account for blocked air in soils.