摘要:

The permeability of argillaceous formations, although rarely measured and poorly understood, is commonly a critical parameter in analyses of subsurface flow. Data now available suggest a regular relation between permeability and porosity in clays and shales and permeabilities that, even at large scales, are significantly lower than usually assumed. Permeabilities between 10−23and 10−17m2have been obtained at porosities between 0.1 and 0.4 in both laboratory and regional studies. Although it is clear that transmissive fractures or other heterogeneities control the large‐scale hydraulic behavior of certain argillaceous units, the permeability of many others is apparently scale independent. These results have significant implications for understanding fluid transport rates and abnormal pressure generation in basins, and could prove important for waste isolation ef

ISSN:0043-1397    

DOI:10.1029/93WR02930

年代:1994

数据来源: WILEY

摘要:

A general relationship between the contributing area, slope, and mean elevation of catchments with relief declining after a tectonic uplift event is presented. This relationship is based on the continuity equation for runoff and erosion processes in the catchment. The key hypothesis underlying this relationship is that as a catchment declines, the nondimensionalized catchment approaches a constant form. This hypothesis is verified for computer simulated catchments. The area‐slope‐elevation relationship covers several cases: catchments declining toward a peneplain; catchments declining from a high slope dynamic equilibrium (resulting from a high rate of tectonic uplift) to a low slope one (resulting from a lower rate of tectonic uplift); and catchments declining from an elevated initial condition, as, for example, in the decline of a mine spoil heap. A previously published relationship between slope and area for catchments in dynamic equilibrium and based on runoff and erosion physics is shown to be a special case of this general relationship. The new area‐slope‐elevation relationship is compared with data from simulated catchments and a field catchment. It is thus shown that for declining catchments the area‐slope‐elevation relationship is a good predictor of catchment form for catchments with declining relief. It is argued that the slope‐area‐elevation relationship is sufficient, with the planiform drainage pattern, to completely define the elevation properties of the catchment such as, for instance, the hy

ISSN:0043-1397    

DOI:10.1029/93WR01810

年代:1994

数据来源: WILEY

摘要:

A large number of samples of visually classified channel networks are taken from some published works. The samples are representative of the following pattern types: dendritic, parallel, rectangular, trellis, and pinnate. By means of computer techniques, essential parameters of elements of these pattern samples are determined. These include lengths, directions, and degrees of curvedness and meandering of channel segments, as well as confluence angles. The parameters are used to determine pattern attributes such as density, texture, parallelism, rectangularity, and bifurcation ratios. Thresholds are generated for the attributes and used in the construction of classification models for the five pattern types studied. The processes and models are coded in a computer program for use in the automated classification of numerically valued channel networks. After classification the image is displayed with each individual network bearing a color which shows the pattern type to which it has been assigned.

ISSN:0043-1397    

DOI:10.1029/93WR02279

年代:1994

数据来源: WILEY

摘要:

One way to estimate spatial distribution of water content at the soil surface consists of using active microwave remote sensing. It has been theoretically and experimentally demonstrated for unsaturated conditions. Nevertheless, radar data are ambiguous when ponding conditions occur, as in variable source areas, due to the contradictory influence of the dielectric effect and the specular effect on the baekscattering attenuation coefficient, σ0. A procedure is considered, based on a topographic analysis, to take into account the influence of the two effects on radar response. The present results stress the capabilities of the ERS (European Remote Sensing Satellite) radar to survey saturated areas in time and space

ISSN:0043-1397    

DOI:10.1029/93WR02920

年代:1994

数据来源: WILEY

摘要:

Based on Kohsiek's fast air circulation chamber, a method has been developed to measure the surface resistance to vapor diffusion in a drying topsoil. This resistance is important to estimate evaporation from bare soils using an aerodynamic resistance formulation. Measurements were done for a fine sandy loam during a dry down after artificial wetting. Surface resistance started to increase at a moisture content of 15% by volume in the 1‐cm top layer, which is 50% of its moisture content at field capacity. Calculations of the aerodynamic resistance were corrected for stability and were used to isolate the real surface resistance from the bulk resistance. Resistances could be modeled as a function of the top 1 cm soil moisture and varied between approximately 10 s/m for a wet and several thousand seconds per meter for a dry top layer. The measurements demonstrated a very pronounced diurnal course due to drying of the very top layer during the day and recovery of the moisture profile during nighttime hour

ISSN:0043-1397    

DOI:10.1029/93WR02747

年代:1994

数据来源: WILEY

摘要:

TheS11scatter function of a coaxial probe in the frequency domain is shown to be an accurate model for predicting waveforms of a seven‐wire coaxial probe in the time domain. Reproducibility of discrete Fourier analyses carried out on waveforms measured with the Tektronix 1502b cable tester (Tektronix, Incorporated, Beaverton, Oregon) indicated a frequency band width ranging from 20 kHz to approximately 1.5 GHz. The frequency dependent complex dielectric permittivity can be calculated from measured waveforms by inverting theS11scatter function. Results indicate that a seven‐wire coaxial probe, connected to a 1‐m RG214 coaxial cable, emulates a coaxial tube for frequencies up to 150 MHz. Application of the frequency domain model to time domain reflectometry waveforms shows that the apparent dielectric permittivity calculated from the travel time in the probe is very dependent on the relaxation frequency of the material in which the probe is embedded. For materials with low relaxation frequencies the apparent dielectric permittivity is much lower than the static permittivity. Inversion of the frequency domain model allows for solving the frequency dependent complex dielectric permittivity of soils from measured wave

ISSN:0043-1397    

DOI:10.1029/93WR02948

年代:1994

数据来源: WILEY

摘要:

Although time domain reflectometry (TDR) is becoming accepted as an important tool for the measurement of soil water content and bulk soil electrical conductivity, a major part of the method is based on empirical relationships. An improved understanding of dielectric measurements on soils may give more insight into soil properties other than soil water content and bulk soil electrical conductivity. Frequency domain analysis of TDR waveforms enables the measurement of the frequency dependent complex dielectric permittivity of soils. The frequency dependent complex dielectric permittivity of soils can be described with a four‐component complex dielectric mixing model based on the volumetric mixing of the refractive indices of the soil components. The four soil components in the model are air, solids, bound water, and free water. Results indicate that the apparent dielectric permittivity obtained from the travel time of the TDR pulse in the soil is the dielectric permittivity at the highest measurement frequency of the cable tester, probe, and soil system. The model based on the volumetric mixing of real permittivities underestimates the measurements in situations with high values of the imaginary part of the dielectric permittivity. Because the model based on the mixing of the complex dielectric permittivities can describe the data, we conclude that the apparent dielectric permittivity is influenced by the imaginary parts in the dielectric, permittivities of the soil components. Combination of the four‐component complex dielectric mixing model with the complex dielectric permittivity obtained from the frequency domain analysis of TDR waveforms gives a tool for modeling the bulk soil electrical conductivity by separating the conductivity of the soil water into a bound water conductivity and a free water conductiv

ISSN:0043-1397    

DOI:10.1029/93WR02949

年代:1994

数据来源: WILEY

摘要:

The hydraulic conductivity function, which is required to solve the Richards equation, is difficult to measure. Therefore prediction methods are frequently used where the shape of the conductivity function is estimated from the more easily measured water retention characteristic. Errors in conductivity estimations can arise either from an invalidity of the prediction model for a given soil, or from an incorrect description of the retention data. This second error source is particularly important for soils with heterogeneous pore systems that cannot be adequately described by the usually used retention functions. To describe the retention characteristics of such soils, a flexible θ(ψ) function was formed by superimposing unimodal retention curves of the van Genuchten (1980) type. By combining this retention model with the conductivity prediction model of Mualem (1976), conductivity estimations for soils with heterogeneous pore systems are obtained. Estimated conductivities by this model and the classical van Genuchten‐Mualem method can differ by orders of magnitude. Thus reported disagreements between measured and estimated conductivities may in some cases be due to an inadequate description of the retention data rather than due to a failure of the prediction mo

ISSN:0043-1397    

DOI:10.1029/93WR02676

年代:1994

数据来源: WILEY

摘要:

A 10‐mMLiCl solution was hydraulically forced through a 0.3‐cm‐thick smectite membrane at a constant solution flux in order to depict the chemical and isotopic evolution of analogous hyper filtration cells in the subsurface. At steady state (∼19 days) there was, relative to the input solution, a measured 12% buildup of solute within the entire cell and a calculated 248% solute increase at the clay/solution interface. The maximum depletion of7Li (about 13‰ relative to the input solution) was calculated to occur about 1 cm away from the membrane out into the high‐pressure reservoir. When scaled to realistic solution fluxes in natural systems, the position of maximum fractionation would occur meters away from a shale/aquifer contact. A steady state hyperfiltration model indicates that a solute's heavy isotopic species will always be depleted on the high‐pressure side of a clay membrane, a prediction corroborated not only by the Li experiment but also by previous hyperfiltration studies involving stable isotopes of C and Cl. The extent of fractionation increases with higher values of the membrane's reflection coefficient, σ. In the experiment a σ of 0.55 for the 90% porosity clay membrane yielded these results. Because shales have substantially lower porosities with attendant higher values of σ, we conclude that the hyperfiltration phenomenon in the subsurface may result in sampling of waters that are not chemically and isotopically representative of the formation penetr

ISSN:0043-1397    

DOI:10.1029/93WR02682

年代:1994

数据来源: WILEY

摘要:

Soil nitrate‐nitrogen (NO3‐N) data arranged on a three‐dimensional grid were analyzed to compare tillage effect on the spatial distribution of residual NO3‐N in the soil profile of agricultural plots drained by subsurface tiles. A three‐dimensional median‐based resistant (to outlier(s)) approach was developed to polish the spatially located data on soil NO3‐N affected by directional trends (nonstationarity in the mean) in three major directions (row, column, and depth) and along the horizontal diagonal directions of the grid. Effect of preferential or nonpreferential path of transport of NO3‐N in the vertical direction defined as sample hole effect was also removed to make the data trend‐free across holes. Composite three‐dimensional semivariogram models (along horizontal and vertical directions) were used to describe the spatial structure of residual soil nitrate distribution. Two plots in the same field, one under each tillage system (conventional tillage and no tillage), were studied. In each plot, soil samples were collected at five depths (30, 60, 90, 120, and 150 cm) from 35 sites (holes) arranged on a 7×5 regular grid of 7.6×7.6 m. In the conventional tillage plot, residual NO3‐N concentrations decreased gradually to a depth of 90 cm and increased beyond this depth. The coefficient of variation, however, became gradually smaller, showing more uniform distribution for greater depths. In the no‐tillage plot, trends were similar to those in the conventional tillage system, but were spatially more stable across the profile. Structural analyses indicated that under conventional tillage, the semivariogram of residual soil nitrate distribution was linear in the horizontal and vertical directions. In contrast, the semivariograms for no‐tillage showed nugget‐type behavior, indicating a lack of spatial structure

ISSN:0043-1397    

DOI:10.1029/93WR02922

年代:1994

数据来源: WILEY