Daniel Stephens - Academia.edu (original) (raw)

Papers by Daniel Stephens

Provided by the Department of Hydrology and Water Resources., Mar 1, 1980

Ph. D. dissertation, University of Arizona, …, 1979

Water Resources Research Center for drafting, and Diane Landis for typing the manuscript. I thank... more Water Resources Research Center for drafting, and Diane Landis for typing the manuscript. I thank my wife, Deborah, for her patience, encouragement and love given to me throughout the duration of this endeavor. Lastly, I want to thank my parents who supported and encouraged my pursuit of higher education.

Water Resources Research, Jul 1, 1991

Vadose Zone Journal, 2002

We would like to make your readers aware of the recently completed Department of Energy's (DO... more We would like to make your readers aware of the recently completed Department of Energy's (DOE's) National Roadmap for Vadose Zone Science and Technology, DOE/ID10871. This Roadmap was published in August 2001 at ([U.S. Department of Energy, 2001][1]). Together with

$Research paper thumbnail of Quantifying Return Flow to Groundwater: What\u27s in the Tool Box?$

Return flow to groundwater is the quantity of water applied at or near the land surface which inf... more Return flow to groundwater is the quantity of water applied at or near the land surface which infiltrates back (returns) to the groundwater system. Common uses that lead to return flow are irrigation of agricultural fields, golf courses or lawns, domestic wastewater disposal through septic systems, and artificial recharge. Quantifying the amount of this applied water that percolates to the water table is necessary in many water-short western states to prove beneficial use, evaluate return flow credits, demonstrate the portion of a water right which can be transferred to another party, and for water banking computations in aquifer storage and recovery projects. Return flow analysis is also relevant to municipal water reuse projects, where the impacts to groundwater quality from landscape and golf course irrigation or artificial recharge with treated wastewater are a potential concern. The purpose of this paper is to describe return flow processes and methods available to quantify ret...

Current Practices in Ground Water and Vadose Zone Investigations

Page 97. Jeffrey A. Havlena1 and Daniel B. Stephens2 VADOSE ZONE CHARACTERIZATION USING FIELD PER... more Page 97. Jeffrey A. Havlena1 and Daniel B. Stephens2 VADOSE ZONE CHARACTERIZATION USING FIELD PERMEAMETERS AND INSTRUMENTATION REFERENCE: Havlena, JA and Stephens, DB," Vadose Zone Charac ...

Water Resources Research, 1985

A practical new field method is proposed to evaluate hydraulic conductivity in partially saturate... more A practical new field method is proposed to evaluate hydraulic conductivity in partially saturated media near a water impoundment. The new method is an inverse procedure which utilizes a flow net constructed from the steady state total hydraulic head distribution around the water source. In the vicinity of constant head sources and downstream along any stream tube, wetness and conductivity generally decrease. Knowing the stream tube geometry and hydraulic gradient from the flow net, Darcy's law is used to determine unsaturated hydraulic conductivities within the stream tube relative to some segment of the stream tube where conductivity is known. This approach was used successfully to predict the unsaturated hydraulic conductivity which was input to two variably saturated numerical models, utilizing total hydraulic head fields generated by the models. This procedure is also applied to pressure head and water content data collected in the field surrounding a constant head borehole infiltration test originally designed to determine only saturated hydraulic conductivity above the water table. For practical purposes the new procedure compares very favorably with (1) results of a field experiment to obtain unsaturated hydraulic conductivity in situ using the instantaneous profile method and (2) values of unsaturated conductivity calculated from field and laboratory measurements of water content and pressure head.

Soil Science Society of America Journal, 1994

Extensive studies document that significant recharge can occur in arid and semiarid climates, eve... more Extensive studies document that significant recharge can occur in arid and semiarid climates, even where annual potential evapotranspiration exceeds precipitation. This overview focuses on the importance of considering diffuse recharge mechanisms. The recharge mechanisms discussed include infiltration, redistribution, and evapotranspiration, particularly as they are influenced by climate, spatial heterogeneity, anisotropy, preferential flow, and multidimensional flow. Diffuse recharge, calculated for sites having substantially different geography and soil types, only varies by roughly four orders of magnitude. Temporal variability in recharge is attributed primarily to variability in precipitation patterns, evapotranspiration demands, runoff characteristics, and soil hydraulic properties. Examples of field studies of diffuse recharge emphasize sites in New Mexico. A RID CLIMATE is defined as receiving 25 cm or less average annual precipitation, and semiarid regions as receiving between 25 and 50 cm of precipitation. Together, these two regions comprise about 35% of the earth's surface, excluding the polar deserts (Potter, 1992). Groundwater recharge is a vital component in predicting contaminant transport through the deep vadose zone and is often one of the most im

Soil Science Society of America Journal, 1985

The soil-water characteristic curve, was determined for an undisturbed core of fine sand over a r... more The soil-water characteristic curve, was determined for an undisturbed core of fine sand over a range of negative pressure heads observed in the field, 0 to 150 cm, below about the 30 cm depth. These data were used to predict relative hydraulic conductivity in a computer code of closed-form analytical solutions developed by van Genuchten for the theoretical models of Burdine and Mualem. The equations to predict conductivity are based upon two parameters fit to observed data by a non-linear least squares procedure. The two parameters are influenced by the choice of residual water content, and unsaturated water content, If is unknown, it is estimated in the program from observed data. A sensitivity analysis was performed to evaluate the reliability of the models. For this soil, calculated conductivities appear to be sensitive to If data are available over only a limited range, or if is too large, modelpredicted values may be near zero, whereas at large may actually be about 0.06 to 0.08 cm/cm. Predicted conductivities may differ by more than one order of magnitude at about =150 cm, depending on the choice of Additional Index Words: residual water content, saturated water content, instantaneous profile method. View complete article To view this complete article, insert Disc 4 then click button8

Groundwater Monitoring & Remediation, 1994

Page 1. Landfill Performance Assessment at a Semiarid Site: Modeling and Validation by Daniel B. ... more Page 1. Landfill Performance Assessment at a Semiarid Site: Modeling and Validation by Daniel B. Stephens and Larry M. Coons Abstract ho IIHI.P model (Version >.i)5) was applied to simulalo the long-term per-colation i'rcim ;i proposed landfill in southern New Mexico. ...

Ground Water, 2009

Anecdotal evidence, mostly from the United States, suggests that it has become increasingly diffi... more Anecdotal evidence, mostly from the United States, suggests that it has become increasingly difficult to find well-trained, entry-level ground water hydrologists to fill open positions in consulting firms and regulatory agencies. The future prospects for filling positions that require training in ground water hydrology are assessed by considering three factors: the market, the numbers of qualified students entering colleges and universities, and the aging of the existing workforce. The environmental and water resources consulting industry has seen continuous albeit variable growth, and demand for environmental scientists and hydrologists is expected to increase significantly. Conversely, students' interest and their enrollment in hydrology and water resources programs have waned in recent years, and the interests of students within these departments have shifted away from ground water hydrology in some schools. This decrease in the numbers of U.S. students graduating in hydrology or emphasizing ground water hydrology is coinciding with the aging of and pending retirement of ground water scientists and engineers in the baby boomer generation. We need to both trigger the imagination of students at the elementary school level so that they later want to apply science and math and communicate the career opportunities in ground water hydrology to those high school and college graduates who have acquired the appropriate technical background. Because the success of a consulting firm, research organization, or regulatory agency is derived from the skills and judgment of the employees, human resources will be an increasingly more critical strategic issue for many years.

A laboratory experiment was conducted to demonstrate the behavior of dense nonaqueous phase liqui... more A laboratory experiment was conducted to demonstrate the behavior of dense nonaqueous phase liquid (DNAPL) 1,1,1-trichloroethane (TCA) in a complex aquifer system. The system simulated consists of two unconfined aquifers separated by a low permeable bedrock perching layer that contains a fracture. In contrast to previous work by other researchers of DNAPL migration in hydrostatic conditions, in this experiment, prior to and during the DNAPL release, ground water flow occurs from the upper to lower aquifer via the fracture. Although the experiment illustrates that DNAPL may migrate rapidly in narrow pathways, the observations were predicted rather well by a simplified analytical solution and a numerical simulation with the multi-phase flow and transport code TOUGH-2. With recharge to the lower aquifer occurring through the fracture, a DNAPL pool did not develop prior to its entry into the fracture.

Vadose Zone Journal, 2010

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