摘要:

A stochastic production frontier model of irrigation is used to analyze sources of economic inefficiency in irrigation practices of Nebraska com producers and the extent that field information from soil moisture monitoring, commercial scheduling and/or weather reports increases economic efficiency. The results indicate that farm irrigation practices have an average technical efficiency of 81% (defined as the ratio of actualized production to maximum potential production for a level of inputs). Field information from moisture sensors can improve technical efficiency by 3.9%. The value of information provided by moisture sensors depends on the technical efficiency of the farmer and ranges from $58.23 per hectare for an efficient farmer to $40.29 for an inefficient producer. The elasticity of derived demand for water is estimated to be −1.09

ISSN:0043-1397    

DOI:10.1029/91WR02676

年代:1992

数据来源: WILEY

摘要:

The behavior of a bubble plume discharging vertically into a stably stratified water column is described using a simple one‐dimensional plume model. It is confirmed that the behavior can be described with just two dimensionless groupings,CandM, representative of the strength of the stratification and the discharge momentum. Depending on the values of these parameters, the plume can either extend the full depth of the water column or eject water at one or more internal detrainment points. The occurrence of these internal detrainment points can be described in terms of the relative magnitudes of the terms comprising the buoyancy conservation equation. Further, these internal detrainment points can be linked to both the mechanical efficiency and mixing effectiveness of bubble plumes. This is of importance when the bubble plume is being used to act as a destratification device in a lake. Generally, an integral number of internal detrainment points correspond to peak mechanical efficiencies. Maximum mixing effectiveness is favored by lower values of the stratification parameterCand higher values of the source strength parameter

ISSN:0043-1397    

DOI:10.1029/91WR02499

年代:1992

数据来源: WILEY

摘要:

A mathematical model was derived to examine the impact of gas advection, gas diffusion, gas‐water mass transfer, gas‐water partitioning, sorption, and intraaggregate diffusion on subsurface movement of organic vapors. Laboratory experiments were performed to determine the validity of the model and to investigate the impact of the various mechanisms on vapor transport. Columns were packed with a uniform Ottawa sand and an aggregated porous soil material (APSM) to compare transport in different soil structures. Toluene vapor transport was observed in the sand under dry and wet (27% water saturation) conditions. The experiments with the APSM were performed dry and at 67% water saturation. In all the sand and the dry APSM experiments, gas advection and diffusion had the greatest impact. In a wet APSM experiment, intraaggregate (liquid) diffusion was also important to consider for gas velocities greater than approximately 0.05 cm s−1. For both soil materials, sorption of toluene vapors occurred for dry conditions, while vapor sorption was negligible when liquid water was present. These findings imply that vapor extraction performance in moist, aggregated soils will be affected by nonequilibrium transport. Therefore models that are developed for predicting the complete removal of contaminants by vapor extraction must account for nonequili

ISSN:0043-1397    

DOI:10.1029/91WR02661

年代:1992

数据来源: WILEY

摘要:

A model for the entrainment and bed load transport of sediment grains of different sizes, shapes and densities by a unidirectional turbulent flow is developed in terms of (1) sediment types available for transport; (2) the mean and turbulent fluctuating values of fluid forces acting upon the sediment grains; and (3) the nature of the interaction between turbulent fluid forces and available sediment, resulting in entrainment and transport of grains as bed load or in suspension. The behavior of the model is explored extensively, and compared with natural data from flumes and rivers. The predicted threshold of entrainment of individual size fractions within a mixed‐size bed agrees well with observations as long as the pivoting angle is specified appropriately as a function of grain size. The rate and size distribution of bed load transport generally agrees well with natural data as long as effective bed shear stress in the presence of bed forms can be defined, bed load transport measurements are reliable, and the size distribution of available sediment is accurately specifie

ISSN:0043-1397    

DOI:10.1029/91WR02570

年代:1992

数据来源: WILEY

摘要:

A new kind of free boundary is introduced, namely, the projection on the horizontal plane of the interzonal boundary between a seepage face and a phreatic surface. This “seepage boundary” is demonstrated to show certain mathematical similarities to a phreatic surface, viewed in a vertical plane. A variation on Zhukovsk's method is employed to derive a formula for the deformation of the seepage boundary on the side of a hill due to groundwater withdrawal. The modified Zhukovski function, being many valued, is handled by Strack's extension of the Schwartz‐Christoffel transformation. It is finally pointed out that more complicated problems could be solved by a modified hodograph m

ISSN:0043-1397    

DOI:10.1029/91WR02576

年代:1992

数据来源: WILEY

摘要:

An exact mathematical solution to Laplace's equation is presented for appropriate boundary conditions associated with the problem of dual‐pipe subirrigation and drainage. The solution can be used to determine a flow net within the groundwater flow region and the associated water table shape. The solution is general. The effects of several hydraulic and geometrical parameters on the groundwater system, such as thickness of saturated zone, position of subirrigation and drainage pipes, heads in the subirrigation and drainage pipes, crop evapotranspiration, fraction of inflowing subirrigation water that exits through the drains, and the aquifer hydraulic conductivity system are evaluated. Calculations are presented showing how pipe spacing affects the shape of the water table. For example, with hydraulic conductivity of 10 m/d, evapotranspiration of 0.01 m/d, drainpipe radius of 0.05 m, and subirrigation pipe radius of 0.0375 m, calculations show that the maximum water table elevation for a pipe spacing of 40 m is 0.64 m greater than for a pipe spacing of 16 m when 40% of the input subirrigation water volume is being removed from the system by drainage. Finally, the general mathematical solution can be used to predict chemical movement as well as water flow through the syste

ISSN:0043-1397    

DOI:10.1029/91WR02484

年代:1992

数据来源: WILEY

摘要:

The concentration, major fractions, and contribution of dissolved organic carbon (DOC) to stream chemistry were examined in two paired streams (East and West Bear Brooks) draining upland catchments in eastern Maine. Concentrations of DOC in East and West Bear Brooks were 183 ± 73 and 169 ± 70 μmol C L−1(mean plus or minus one standard deviation), respectively. Organic acids were the dominant fractions of the stream DOC, with hydrophilic and hydrophobic organic acids representing 44 and 39% of total DOC (average of both streams), respectively. East and West Bear Brooks had mean organic anion concentrations of about 20 μeq L−1during episodic events and 12–13 μeq L−1during base flow. During the episodic events, the maximum organic anion values were 51 and 36 μeq L−1, respectively, for East and West Bear Brooks. Although SO42−was the dominant stream anion, followed by Cl−, organic anions were also determined to be an important component of these stream waters, especially during storm events. This illustrates that even in streams with low DOC, such as those studied here, organic anions can contribute significantl

ISSN:0043-1397    

DOI:10.1029/91WR02180

年代:1992

数据来源: WILEY

摘要:

This paper describes a procedure to upscale unsaturated hydraulic conductivity from the scale at which it is measured to a scale more amenable to actual flow simulations in partially saturated heterogeneous porous formations. In the approach adopted here the block conductivity is defined as the ratio of volume‐averaged flux to the volume‐averaged head gradient. The “point” values of the underlying formation properties (characterized by the saturated conductivityKsand the pore size distribution parameter α) are viewed as realizations of independent random spatial functions. The general approach is applied to a partially saturated, heterogeneous, three‐dimensional statistically anisotropic formation, with statistical isotropy in the horizontal plane and arbitrarily oriented stratification characterized by a finite aspect ratio, for the case where the mean gradient exists only in the vertical direction. The procedure is illustrated by providing a second‐order approximation of the mean (in terms of the components of the block conductivity tensor) and a first‐order approximation of the covariance function of the block conductivity, which depend on the mean capillary pressure headH, the distribution of the “point” values of the underlying log unsaturated conductivity, and the block size. The results of this study allow assessment of the minimal domain size for which the concept of effective properties is appropriate and assessment of the maximal size of conductivity cells which preserve the heterogeneous structure of the porous formation with respect to simulation of flow and transport in the vadose zone. The application of the results to unconditional or conditional simulation of unsaturated flow and transport is straightforward, since knowledge of the mean and the covariance of the block unsaturated conductivity may lead to a relatively simple algorithm for generating its values on a grid at any level of a

ISSN:0043-1397    

DOI:10.1029/91WR02587

年代:1992

数据来源: WILEY

摘要:

We report the first measurements of friction angles for a naturally formed gravel streambed. For a given test grain size placed on a bed surface, friction angles varied from 10° to over 100°; friction angle distributions can be expressed as a function of test grain size, median bed grain size, and bed sorting parameter. Friction angles decrease with increasing grain size relative to the median bed grain size, and are a systematic function of sorting, with lower friction angles associated with poorer sorting. The probability distributions of critical shear stress for different grain sizes on a given bed surface, as calculated from our friction angle data, show a common origin, but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses. The potential mobility of a grain, as defined by its probability distribution of critical shear stress, may be overestimated for larger grains in this analysis, because our calculations do not take into account the effects of grain burial and altered near‐bed flow fie

ISSN:0043-1397    

DOI:10.1029/91WR02529

年代:1992

数据来源: WILEY

摘要:

The relationship of sorptivitySand macroscopic capillary length λcis explored. Sorptivity is the proportionality constant between cumulative intake, expressed as a length, and the square root of time for sorption into an unsaturated soil. The capillary length is defined as the integral of the unsaturated hydraulic conductivity with respect to matric potential normalized by the difference between the conductivity at the two limits of integration. The approximationb= λc(θwct‐ θdry)(Kwct‐Kdry)S−2was found previously to have bounds of 0.5 and π/4, with 0.55 as a good overall approximation. Those results were for two diffusivity functions, an exponentialD=D0exp (Bθ) and forD=D0(θb‐ θ)−2. We now extend the investigation to four additional cases, two for whichSand λcare analytical and two described by the empirical hydraulic functions of Brooks and Corey (1964) and by van Genuchten (1980). The representative valueb= 0.55 was found to be generally correct. For those functions with a finiteDvalue at the wettest value, a refinement from the 0.55 value is offered. The results are believed of value for future parameter estimation problems, including two previous applications related to time of ponding and d

ISSN:0043-1397    

DOI:10.1029/91WR02599

年代:1992

数据来源: WILEY