摘要:

The quasi‐linear parameterization for unsaturated hydraulic conductivityK(Ψ) =Ksexp (αΨ), whereKis hydraulic conductivity, Ψ is soil water matric potential,Ksis saturated hydraulic conductivity, and α is a porous material parameter, has been used in both stochastic and deterministic models of unsaturated water flow in porous materials. In the stochastic approach,Ksis assumed lognormally distributed, but α and the volumetric soil water capacityC=dθ/dΨ, with θ volumetric soil water content, are assumed normally distributed. We point out here that α andKsare related to the same internal pore geometry of the soil. This interrelationship ensures that ifKsis lognormal, then α, and possiblyC, will also be lognormal. Additionally, we present preliminary field results which indicate that α is better described by a lognormal than a normal distribution. The quasi‐linear parameterization can be expected to be correct only in some integral sense. Predictions of increases in the variability of hydraulic conductivity with decreasing Ψ may therefore be prejudiced by the use of the exponential form forK(Ψ). Tests of the sensitivity of stochastic model predictions to both the parameterizations adopted forK(Ψ) and the assumed distribution functions of parameters seem warranted. Reliable experimental evidence on field variability ofK(Ψ) and Ψ(θ) at substantial negative values

ISSN:0043-1397    

DOI:10.1029/91WR02198

年代:1992

数据来源: WILEY

摘要:

A waste load allocation process is developed to determine the maximum effluent concentration of a conservative substance that will not harm fish and wildlife propagation. If this concentration is not exceeded in the effluent, the acute toxicity criterion will not be violated in the receiving stream, and the chronic criterion will not be exceeded in the zone of passage, defined in many state water quality standards to allow the movement of aquatic organisms past a discharge. Considerable simplification of the concentration equation, which is the heart of any waste load allocation, is achieved because it is based on the concentration in the receiving stream when the concentration gradient on the zone of passage boundary is zero. Consequently, the expression obtained for effluent concentration is independent of source location or stream morphology. Only five independent variables, which are routinely available to regulatory agencies, are required to perform this allocation. It aids in developing permit limits which are protective without being unduly restrictive or requiring large expenditures of money and manpower on field investigations.

ISSN:0043-1397    

DOI:10.1029/91WR02480

年代:1992

数据来源: WILEY

摘要:

Two approaches to simulating soil water flow were evaluated: a capacity‐type water flow model combined with a plant water uptake model based on the concept of plant‐available water, and a numerical solution to the Richards equation combined with a potential‐driven water uptake model. Irrigation water was applied at rates ranging from 0.3 to 1.6 potential evapotranspiration to an Oxisol under fallow and planted to corn (Zea maysL.). Results indicate that a two‐parameter soil moisture release curve does not sufficiently describe these soils over the entire plant‐available water range. Field saturated water contents were generally only 70% of total porosity. The potential‐driven model better predicts profile water content distribution under fallow and cropped conditions than the capacity‐type approach (R2= 0.80 and 0.83 versusR2= 0.59 and 0.70). When integrated over the growing season, drainage, evaporation and transpiration rates were very similar for capacity‐type water movement and potential‐driven water movement over the range of irri

ISSN:0043-1397    

DOI:10.1029/91WR02197

年代:1992

数据来源: WILEY

摘要:

Stable S isotopes of SO42‐in precipitation, throughfall, and stream water solutions in a northern hardwood watershed (Bear Brook Watershed, Maine) were examined to determine sources of stream SO42‐and to identify watershed processes that may affect atmospherically deposited SO42‐prior to reaching the streams. Similarity among temporal patterns in δ34S of precipitation, throughfall, stream SO42‐(range: +3.4 to +9.4‰), andof stream SO42‐(5–10%) indicate that the δ34S values of stream SO42‐are controlled principally by atmospheric deposition. Lack of correlation between δ34S of stream SO42‐and stream[SO4 total2‐], elevational position, and stream discharge supports the interpretation that fractionation of stable S isotopes within the watershed appears to have, at most, a minor influence

ISSN:0043-1397    

DOI:10.1029/91WR02481

年代:1992

数据来源: WILEY

摘要:

A new model for erosion of plane soil surfaces by water is developed using physical principles. Raindrop impact and overland flow remove soil from the original cohesive soil. Once eroded soil enters overland flow, either as aggregates or primary particles, a significant proportion of it returns to the soil bed, forming a cohesionless deposited layer from which it can be removed again by the same erosion processes. The action of the eroding agents will be divided between eroding the unshielded original cohesive soil and reintroducing sediment from the deposited layer. The theory recognizes that the nature of the surface is modified by the erosion and deposition processes affecting it. Solutions of the governing differential equations describing sediment concentration are developed for two distinct equilibrium cases. The first case, when the deposited layer completely shields the original soil, appears to correspond with what has been previously called a “transport‐limited” situation. The second case occurs when such shielding is incomplete, and sediment concentration is affected by the cohesive strength of the soil. The resulting equations for sediment concentration at equilibrium are compared with existing equations. Firstly, the equation for the case where the soil is lacking cohesion is shown to be similar to the semiempirical equation of Yang (1973). Secondly, when the soil is cohesive the slope length relationships are shown to be in good agreement with the universal soil loss equation over a wide range of slope stee

ISSN:0043-1397    

DOI:10.1029/91WR02380

年代:1992

数据来源: WILEY

摘要:

The model of soil erosion on a plane slope introduced by Hairsine and Rose (this issue) is further developed by considering rill flow. The rills are considered to be parallel and incised in a homogeneous soil mass and have sediment and water flux contributed to them orthogonally. The erosive action of water flowing down these rills is described using the concept of stream power, which is assumed to be uniformly distributed around the wetted perimeter. The apportioning of the erosion and deposition processes within a rill is then considered to be influenced by the spatial extent of a deposited layer. The theory is developed for trapezoidal rills, with triangular rills and rectangular rills as special cases. Equations describing sediment continuity are presented, and numerical and analytical solutions are described for the entrainment‐limited and transport‐limited situations. The experiments of Meyer and Harmon (1985) are examined using this theory, and consistency of evaluated parameters is demonstrated within this data

ISSN:0043-1397    

DOI:10.1029/91WR02381

年代:1992

数据来源: WILEY

摘要:

Models of unsteady unsaturated flow in large‐scale heterogeneous soils are developed considering finite flow domains and nonstationarity of the soil properties and flow characteristics. The problem is cast into a more general stochastic framework than the originally proposed stationary spectral framework (Mantoglou and Gelhar, 1987a,b,c). The methodology considers the three dimensionality of the local governing flow equation, the nonlinear dependence of the local output on the local soil properties, as well as the effect of finite flow domains and nonstationarity of the soil properties and flow characteristics. The large‐scale model representation is in the form of a partial differential equation, with large‐scale “effective” parameters, subject to a set of initial and boundary conditions. The effective model parameters are related to a set of fluctuation covariance equations obtained by using a linearized fluctuation equation. This set of covariance equations and the corresponding large‐scale model of the system are coupled and must be solved simultaneously. Particular cases of interest where stationarity in two or three spatial dimensions occurs are investigated, and, using spectral representations, the dimensionality of the covariance equations is reduced. Simple closed‐form and practical expressions for the effective parameters valid in specific situations are presented, and illustrative examples are discussed. The theory and the models presented provide a more complete view of the large‐scale unsaturated flow problem and can prove useful for evaluation of unsaturated flow phenomena of paramount importance in practical applications, for example, for predicting the movement of liquid wastes in the u

ISSN:0043-1397    

DOI:10.1029/91WR02232

年代:1992

数据来源: WILEY

摘要:

A stochastic unsaturated flow theory and a numerical simulation model have been coupled in order to estimate the large‐scale mean behavior of an unsaturated flow system in a spatially variable soil. On the basis of the theoretical developments of Mantoglou and Gelhar (1987a,b,c), the theory assumes that the local hydraulic soil properties are realizations of three‐dimensional random fields, and it derives the large‐scale flow structure by averaging the local governing flow equation over the ensemble. The resulting mean model representation is in the form of a partial differential equation, similar to the local flow equation, in which effective model parameters occur (e.g., effective hydraulic conductivity). Further, the theory predicts the variance (prediction error) of the capillary tension head due to the spatial variability of the local hydraulic soil properties. The governing unsaturated flow equation representing the mean system behavior is solved using a finite difference numerical solution technique. The effective parameters are evaluated from the stochastic theory formulas before entering them into the numerical solution for each iteration. The stochastic model is applied to a field site in Denmark, where information is available on the spatial variability of soil parameters and variables. Numerical simulations have been carried out, and predictions of the mean behavior and the variance of the capillary tension head and the soil moisture content have been compared to field observations. The stochastic model predicts the average system behavior reasonably well. Reasonable results are also obtained for the range of variation of the capillary tension head, despite the limited number of measurements and corresponding statistical inference issues. The stochastic model presented here seems to offer a rational framework for modeling large‐scale unsaturated flow and estimating areal averages of soil‐hydrological processes in spatially varia

ISSN:0043-1397    

DOI:10.1029/91WR02233

年代:1992

数据来源: WILEY

摘要:

The distribution of anthropogenic36Cl and3H was used along with numerical flow simulations to evaluate the relative importance of liquid and vapor flow in the shallow unsaturated zone of an area within the Chihuahuan Desert of Texas. Chlorine 36 is nonvolatile and is restricted to liquid phase flow, whereas tritiated water is volatile and can move in both liquid and vapor phases. Tritium penetrated 1 m deeper than36Cl, although3H fallout occurred later than that of36Cl. Deeper penetration of3H relative to that of36Cl was attributed to enhanced downward movement of3H in the vapor phase. The moisture flux calculated from the36Cl/Cl peak at 0.5‐m depth was 1.4 mm yr−1, whereas that based on the3H peak at 1.4‐m depth was 7 mm yr−1. The difference in moisture fluxes between the two tracers suggests a vapor flux of approximately 6 mm yr−1. The vapor flux hypothesis was tested using nonisothermal liquid and vapor flow simulations with the computer code SPLaSHWaTr. Simulations of 5‐day periods in the winter and summer were conducted to represent the extremes in temperature gradients. The calculated vapor flux was two to eight orders of magnitude greater than the liquid flux for the periods simulated. Predicted vapor fluxes were upward in the top 0.04 m of the unsaturated zone in the summer and winter in response to steep water potential gradients induced by surface evaporation. Below the evaporation front, from depths of 0.15 to 1 m, downward vapor fluxes in the summer were much greater than generally upward vapor fluxes in the winter. These results suggest an annual net downward vapor flux that is consistent with the chemical

ISSN:0043-1397    

DOI:10.1029/91WR02200

年代:1992

数据来源: WILEY

摘要:

We have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers. Distance of movement was scaled using the mean distance of movement of the median size group of the surface material; size was scaled using the medianbaxis diameter of the subsurface material. The scaled data are fitted with a nonlinear function. Clasts larger than the median of surface material exhibit a steep decline of mobility, with an upper limit for mobility nearDi/D50sub= 10. Smaller materials exhibit a less sensitive relation. We suppose that trapping of small clasts affects their travel distance. These results indicate that the movement of free surface stones is size preferred. A subordinate analysis of free, constrained and buried particles reveals that bed surface structure acts to reduce differential movement.

ISSN:0043-1397    

DOI:10.1029/91WR02523

年代:1992

数据来源: WILEY