摘要:

Spatial variability of surface hydraulic properties and the extrinsic (e.g., traffic, cropping, etc.) and intrinsic (e.g., soil type, pore size distribution, etc.) factors associated with these properties are important for infiltration and runoff processes in agricultural fields. Disc infiltrometers measured infiltration at 296 sites arranged on two parallel transects. To examine and differentiate the factors contributing to spatial structure under different field conditions these measurements were made in the corn rows, no‐track interrows, and wheel track interrows of the field using four different soil water tensions Ψ (0, 30, 60, and 150 mm). Unsaturated hydraulic conductivity (K) and saturated hydraulic conductivity (Ks) were maximum in the corn rows and minimum in wheel track interrows, with no‐track interrows intermediate. Exponents (α parameters) ofKsandKrelationships (K=Ksexp−αΨ) for corn rows and no‐track interrows were not significantly different from each other but were significantly different from a for the wheel track interrows atP= 0.01 level. Spatial variability ofKandKsvalues showed some pseudoproportional effect in nugget variance for all three field conditions. No‐track interrows clearly showed an inverse trend for semivariogram ofKwith changing tension (Ψ) values, whereas differences were found for corn rows and wheel traffic interrows. The spatial structure of a for all three field conditions were mostly white noise. Under corn rows, in addition to random variation, a small five‐row periodic variation at theP= 0.20 level, matching the five‐row traffic configuration, was discovered. The spatial structure of α was influenced by soil type for the no‐track interrows. Spatial structure was absent in wheel track interrows, indicating the destruction of pore struct

ISSN:0043-1397    

DOI:10.1029/94WR01052

年代:1994

数据来源: WILEY

摘要:

We present results from experiments on the migration of inorganic colloids through laboratory columns containing clean quartz sand. Particle retention on the quartz collectors was found to be substantially less in experiments using negatively charged silica (SiO2) colloids than in experiments using positively charged anatase (TiO2) or boehmite (AlOOH) colloids. Analysis of these data with respect to two different advection‐dispersion models indicates that deposition of colloidal silica follows a first‐order, reversible kinetics process, while deposition of both anatase and boehmite is more closely depicted by second‐order kinetics. Fitted values of the rate constant used to describe particle attachment vary consistently with the mean grain size of the sand and, for anatase and boehmite, are within a factor of 2 of the values predicted on the basis of colloid filtration t

ISSN:0043-1397    

DOI:10.1029/94WR01046

年代:1994

数据来源: WILEY

摘要:

Water repellency is an important property of many soils. It causes rainwater to penetrate into the soil as preferential flow paths, and solutes can reach the groundwater more rapidly than in the case of a homogeneous wetting. Water repellency depends on several factors which are principally related to the characteristics of the organic matter of the soil. A distinction between “potential” and “actual” water repellency and the assessment of the “critical soil water content” are introduced and highlighted in this paper. Persistence and degree of potential water repellency of dried samples were examined from 10 trenches in a dune sand with grass cover using the water drop penetration time and the alcohol percentage tests. The spatial variability of water repellency and, therefore, soil wetting was extremely high. The actual water repellency was measured on field‐moist samples to obtain critical soil water contents. The soil is wettable above and water repellent below these values. The critical soil water content varies between 4.75 vol % at 5–10 cm and 1.75 vol % at 45–50 cm depth i

ISSN:0043-1397    

DOI:10.1029/94WR00749

年代:1994

数据来源: WILEY

摘要:

The dynamics of fingered flow in a water repellent sandy soil was studied in the field by sampling 10 vertical trenches in a 1‐year cycle. In dry soil, fingers were formed in those places in the top layer which have the lowest degree of potential water repellency. The finger diameters varied roughly between 10 and 50 cm depending on the sequence of weather conditions. The fingers were wet in the topsoil and increasingly drier with depth. In none of the trenches sampled were there any serious indications of finger merger. The actual water repellent soil volumes between fingers were excluded from the transport of water and solutes for at least several hours. The temporal and spatial variability of these actual water repellent soil volumes is illustrated and evaluated with respect to simulation model developmen

ISSN:0043-1397    

DOI:10.1029/94WR00750

年代:1994

数据来源: WILEY

摘要:

Experimental evidence demonstrating gravity‐driven wetting front instability in an initially dry natural fracture is presented. An experimental approach is developed using a transparent analog rough‐walled fracture to explore gravity‐driven instability. Three different boundary conditions were observed to produce unstable fronts in the analog fracture: application of fluid at less than the imbibition capacity, inversion of a density‐stratified system, and redistribution of flow at the cessation of stable infiltration. The redistribution boundary condition (analogous to the cessation of ponded infiltration) is considered in a series of systematic experiments. Gravitational gradient and magnitude of the fluid input were varied during experimentation. Qualitative observations imply that finger development is strongly correlated to the structure of the imbibition front at the onset of flow redistribution. Measurements of fingertip velocity are used to develop a first‐order relationship with fingertip length. Measured finger width is compared to theoretical predictions based on linear stabili

ISSN:0043-1397    

DOI:10.1029/94WR00164

年代:1994

数据来源: WILEY

摘要:

Sorption of a reactive solute during transport in a fractured geologic medium is analyzed under the assumption that within each fracture, retardation varies in proportion to the product of a surface distribution coefficient and the specific surface area of the fracture (i.e., its surface area‐to‐volume ratio). This approach is analogous to theKDmodel commonly adopted for granular porous media. Numerical migration experiments in discrete fracture networks show that at the plume scale, retardation is both nonuniform and anisotropic. Different segments of the plume, or equivalently different breakthrough fractions at a downstream boundary, are retarded to a different degree. The degree to which various breakthrough fractions are retarded varies as a function of the orientation of the mean hydraulic gradient relative to the orientation of the fracture sets. This variation can be described in the form of a retardation ellipse. The mean arrival time is the only breakthrough fraction that is consistently described by a uniform, isotropic retardation factor. The degree of nonuniformity and anisotropy in the retardation factor is controlled largely by the difference in the mean apertures of each fracture set, the standard deviation in fracture aperture, and the orientation of the fracture sets relative to the mean hydraulic gradient. A larger variability in fracture aperture, or in the orientation of fractures within each set, promotes nonuniform retardation while reducing the degree of anisotropy in the retardation factor. Our results suggest that a transport model based on the conventional advection‐dispersion equation, using a uniform retardation factor, may be conceptually incorrect, even for a dense fracture network. While the effects of nonuniform retardation modify the dispersive flux in a way that could be described by “effective” dispersion coefficients, it is our opinion that a model formulation is needed that clearly separates chemical effects from those due to hydrodynamic d

ISSN:0043-1397    

DOI:10.1029/94WR01128

年代:1994

数据来源: WILEY

摘要:

Flow leakage through abandoned wells and improperly plugged boreholes drilled between hydrogeologically separated water‐bearing zones represent potential pathways of contamination. The present study provides an analytical evaluation of the transient flow rate through these artificial conduits and the resulting hydraulic head distributions when (1) artificial gradients are created by an injection well operating in one of the aquifers or (2) natural hydraulic head differences are present between two confined aquifers. The analysis is conducted by solving the groundwater flow equations in the confined aquifers that are coupled by the flow through the well or borehole. A case study was investigated involving contamination leakage through abandoned wells and improperly plugged boreholes caused by the operation of deep waste injection well

ISSN:0043-1397    

DOI:10.1029/94WR00952

年代:1994

数据来源: WILEY

摘要:

Numerical procedures for efficient mass conservative solutions of the head‐based form of the Richards equation are presented. Mass conservative solutions are shown to result when the capacity coefficient,C, is formulated by equating the storage term and its chain rule expansion in their discretized forms. Equivalence in the storage term expansion is maintained in finite difference models whenCis evaluated with a standard chord slope approximation. This scheme is shown to produce excellent global mass balance accuracy in simulations of vertical moisture infiltration. An analogous approach to the expansion of the storage term using finite elements results in element dependent expressions ofC. Application of this approach produces mass balance accuracy with errors less than 1%, but also exhibits slow convergence in the consistent form. A nontraditional finite element procedure is presented which maintains equivalence in the storage term expansion whenCis evaluated with the standard chord slope approximation. This scheme exhibits excellent mass balance accuracy, in either the consistent or lumped forms, without significant loss in computational efficienc

ISSN:0043-1397    

DOI:10.1029/94WR01302

年代:1994

数据来源: WILEY

摘要:

Insufficient understanding of winter hydrology conditions still hampers progress in predicting springtime discharge. The least known term in the winter water balance is evaporation, particularly of intercepted snow. Recent studies have shown that the evaporation from intercepted snow can be important. This paper elaborates factors governing evaporation of intercepted snow. Measurements with a cut tree‐weighing device combined with a method to measure throughfall and drip gave a maximum evaporation rate of 0.3 ± 0.06 mm/h or 3.3 ± 0.06 mm/24 hours from a 6‐m‐high spruce. Calculations of evaporation with a combination equation and different ways to calculate the aerodynamic resistance and the evaporation from a snow‐intercepted canopy during melt and sleet events showed that the most important factors for calculating the evaporation were the relative humidity, the aerodynamic resistance, the wind speed, and the intercepted mass. Less important factors were the energy to melt the intercepted snow, the method for calculating reduction in evaporation caused by a partly snow‐covered canopy, accuracy in measurement of wind speed, air temperature, and ne

ISSN:0043-1397    

DOI:10.1029/94WR00873

年代:1994

数据来源: WILEY

摘要:

A model for drainage networks and their associated landscapes has been developed. This model is based on the minimum energy dissipation principle and an empirical relationships∼Qαbetween the slopesof each link in a channel network and the mean annual dischargeQthat flows through it. The model was studied using six different values for the exponent α. The surfaces of these minimum energy dissipation drainage networks appear to be more complex than simple self‐affine fractals. The drainage basins in these optimal drainage networks have power law size (area) distributions with a universal exponent, which is independent of the value of α. For the minimum energy dissipation drainage networks obtained using a particular value for α, the basin shapes are similar to each other. An empirical relationship has also been found to relate the basins' “length‐width” ratio to t

ISSN:0043-1397    

DOI:10.1029/94WR01050

年代:1994

数据来源: WILEY