Peter F. Germann

Geomorphological control of subsurface hydrology in the creekbank zone of tidal marshes

Abstract A combination of field and numerical modeling methods was used to assess porewater movement in a narrow (20 m) Spartina marsh which was flooded regularly by tidal waters. Soil composition and soil hydraulic properties did not vary across the marsh or with depth. Hydraulic head was monitored on a transect perpendicular to the creekbank. During exposure of the marsh surface, hydraulic gradients were predominantly horizontal; vertical gradients were small or zero. Subsurface flow was directed from the marsh interior toward the creekbank. Approximately 141 of pore water were discharged laterally to the adjacent tidal creek per meter of creekbank over a complete tidal cycle. A numerical hydrological model was modified to simulate subsurface hydraulics in the creekbank vicinity of regularly flooded tidal marshes. The model was parameterized to represent soil conditions, tidal fluctuations and topography at the field site. Observed changes in hydraulic head over complete tidal cycles were accurately predicted by the model. Model simulations identified the vertical infiltration of creek water into the marsh surface at the onset of tidal flooding as the primary source (66%) for the replacement of water drained at the creekbank. Significant replacement (31%) also occurred as discharge from the interior marsh. Horizontal recharge at the creekbank was minimal (3%). A sensitivity analysis was conducted with the model to assess the relative importance of geomorphological factors and soil properties in controlling pore water export at the creekbank of tidal marsh soils. Each parameter was varied systematically over a realistic range for field conditions. Changes in marsh elevation exerted greater control over creekbank discharge than changes in soil hydraulic properties. More rapid turnover of pore water near creekbanks of higher elevation marshes is hypothesized.

Throughflow and solute transport in an isolated sloping soil block in a forested catchment

A 3m wide by 9m long by 1m deep soil block on a forested hillslope near Orono, ME, was isolated by excavation of encircling trenches. A sprinkler system for application of water and potassium bromide tracer was constructed over the plot. Outflow was collected at six locations with troughs. Experiments were conducted at application rates of 2.5, 5, and 10 cm h−1. Pulses of tracer were applied subsequent to attainment of steady flow and breakthrough curves were measured at all outflow points. Recession limbs of outflow hydrographs exhibited distinct breaks when plotted on semilogarithmic axes, indicating drainage from at least two distinguishable pore size classes or flow pathways. Solute breakthrough curves were dominated by a single peak; travel times of solute were inversely related to the application rate. A secondary peak in the outflow curve, which is inconsistent with transport theories for a homogeneous soil, was observed in all cases. This second peak is unexplained, but is conceptually consistent with the notion of transport in at least two pore size classes. An undisturbed soil core (diameter of 30 cm and length of 40 cm) was sprinkled at the same rates as was the soil block, using Methylene Blue as tracer in the last run. Drainage hydrographs and visual examination of dye stains in the block indicated also at this smaller scale that flow and transport are controlled by preferred paths in the soil, paths that cannot be morphologically distinguished from the surrounding soil matrix. Theoretical explanations of processes on such hillslopes need to account for this fact.

Scales and dimensions of momentum dissipation during preferential flow in soils

Momentum dissipation may dominate flow in soils over a considerable distance when input rate and antecedent soil moisture are high enough and when adequate soil structures are present. The concept is derived from momentum balance. It is applied to drainage flow from a column of undisturbed soil and a weighing lysimeter and to water content variations at five depths due to sprinkling. Momentum of input is much lower than momentum during flow in the soil; however, the former is considered important in triggering momentum dissipation within the profile. Drainage flow at a depth of 2.2 m shows flow completely dominated by momentum dissipation, whereas momentum of flow within the soil profile increases with depth, indicating acceleration over a vertical distance from 0.15 to 0.55 m. The Reynolds numbers show laminar flow in all cases.

Inferences about solute transport in macroporous forest soils from time series models

Abstract Solutes can move along macropores and other preferred flow channels in forest soils when input rates at the surface are high enough. In natural systems, the actual flow paths are, to all intents and purposes, indeterminate. The use of transfer function models to represent solute transport has been proposed under these circumstances. We estimated parameters in a discrete-time version of a transfer function model for a sequence of experiments performed by sprinkling an isolated soil block in a forested catchment near Orono, Maine. All experiments were performed at steady flow by adding a quantity of water labeled with potassium bromide. The travel time of solute in the soil block was computed for each of three application rates and the effective mixing volume of the solute within the block was inferred from the time series parameters. Results show that significant macropore flow occurs in the soil block. Despite the importance of macropore flow, the fractional volume of the total pore water that participates in solute transport decreases only slightly with increasing flow rate.

When is porous-media flow preferential? A hydromechanical perspective

Both, preferential flow and dispersive flow (the latter is usually expressed with the Darcy-Richards Equations) are approached with kinematic wave theory. The kinematic wave assumption, l = β γα (where l [LT−1] is volume flux density, β [LT−1] is conductance, ψ [L3L−3] is moisture participating in the flow process, and α is a dimensionless exponent) is validated against data from an infiltration-drainage experiment, performed on a reconstituted soil sample containing artificial macropores. It is proposed that the exponent α may serve as a measure to assess the degree of preferential flow. The model is then applied to four hydrographs obtained from infiltration-drainage experiments at various rates, using a dual-porosity medium made of artificial soil aggregates. The exponent α decreased with increasing application rates of water input to the surface, indicating the dynamic aspect of preferential flow.

A distribution function approach to water flow in soil macropores based on kinematic wave theory

Kinematic wave theory is adapted to gravity-driven flow in narrowly defined macropore groups. The frequency distribution of macropore conductance, b (dimensions L T−1), is obtained by superimposing the predicted drainage hydrographs from the individual groups of macropores and matching predicted outputs to observed drainage hydrographs. The time period over which this concept applies is limited to about 1.5 times the duration of the water input to the soil surface. The drainage hydrographs from four different soils, using three different measuring devices, were analyzed accordingly. The analysis suggests that between 10−2 and 0.2 of the input is required to supply water to the macropore system, while a parameter controlling losses to the matrix varies over only a small range.

Acoustic assessment of flow patterns in unsaturated soil

Abstract Acoustic wavelengths in soils range from meters to millimeters, depending on their frequencies. The spatial and temporal scales of pulse transmission through soils are well suited to investigate transient and presumably heterogeneous water infiltration into and redistribution within soils. Acoustic pulses were transmitted through a column of an undisturbed and partially water-saturated loess soil with height and diameter of 0.8 and 0.3 m, respectively. The maximum frequency of the arriving pulses was 10 kHz, which corresponds to a wavelength of about 50 mm. Both travel velocities and absorption of the acoustic waves reacted in the expected ways on soil moisture variations; however, the two temporal reaction patterns differed considerably. Brutsaerts [J. Geophys. Res. 69 (1964) 243] model was successfully applied to the data. Dye tracers visualized the patterns of water distribution. Their scale compares well with theoretical considerations on the flow paths and the results from the acoustic investigations.

Dual-porosity and kinematic wave approaches to assess the degree of preferential flow in an unsaturated soil

Abstract The purpose of this study was to assess the degree of preferential flow in an unsaturated soil column using two different models: the dual-porosity model, MACRO, and the kinematic wave approach (KWA) based on boundary-layer flow theory. The soil column experiments consisted of six infiltrations with intensities varying from 15 to 101 mm h−1. Bromide solution was also infiltrated at an intensity of 79 mm h−1 and a concentration of 80 mg l−1. Both MACRO and the KWA indicated the absence of pure preferential flow. The KWA indicated intermediate flow with dispersion of the wetting front with depth, whereas MACRO indicated flow dominated by the diffusion of capillary potential. These results shed light on the transition between flows dominated by momentum dissipation and by diffusion of capillary potential. The absence of pure macropore flow in the structured sandy soil is mainly due to efficient lateral mass exchange in this material.

Approaches to rapid and far-reaching hydrologic processes in the vadose zone

Abstract The restrictions imposed by convective-dispersive approaches to water flow and solute transport in porous media are discussed. Examples of transient flow and solute transport are introduced in those the translocations occurred faster than anticipated. Emerging alternate approaches to rapid and far-reaching hydrologic processes are reviewed.

Impact of initial and boundary conditions on preferential flow.

Preferential flow in soil is approached by a water-content wave, WCW, that proceeds downward from the ground surface. WCWs were obtained from sprinkler experiments with infiltration rates varying from 5 to 40 mm h(-1). TDR-probes and tensiometers measured volumetric water contents theta(z,t) at seven depths, and capillary heads, h(z,t) at six depths in a column of an undisturbed soil. The wave is characterized by the velocity of the wetting front, c(W), the amplitude, w(S), and the final water content, theta*. We tested with uni-variate and bi-variate linear regressions the impacts of initial volumetric water contents, theta(ini), and input rates, q(S), on c(W), w(S) and theta*. The test showed that theta(ini) influenced theta* and w(S) and q(S) effected c(W). The expected proportionality of w(S) approximately = qs(1/3) was weak and c(W) approximately = qs(2/3) was strong.