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Project Hypothesis or Objectives:
Much unsaturated-zone water moves not as typically slow diffuse flow, but rapidly through preferential flow channels such as root holes and fractures. Impacts are profound and far-reaching for issues such as contaminant transport, ecohydrology, and aquifer recharge. Understanding is limited and accepted theory is lacking, making preferential flow a crucial and exciting area of earth science.
Although preferential flow has been a major research focus for more than three decades, most application-directed quantification is still based on formulations that treat all unsaturated-zone flow as diffuse flow or that use some adaptation of diffuse-flow techniques for the preferential component. In many ways, however, the processes of preferential flow are critically different from those of diffuse flow. For this reason our approach is to develop formulas that directly describe these processes, and to combine them with traditional formulations as needed. Because the mathematical models of diffuse flow, though conceptually simple, are difficult for practical implementation and solution, and because evidence suggests preferential flow processes may be approached with simpler math, this separate-process approach may lead to new models that are both more tractable and more accurate for predicting transport in the unsaturated zone.
Current research in pursuit of these goals emphasizes theory and model development, utilizing published and recently-obtained data from field and lab observations under both preferential-dominated and diffuse-dominated flow conditions. Principal research activities include development of models, formulas, and algorithms through iterative hypothesizing and testing against data. Various possibilities are available for pursuit of a chosen aspect of the overall problem. Examples of our methods and tools include the Source-Responsive family of preferential flow models (e.g. Nimmo and Mitchell, 2013, VZJ, v. 12, no. 4), and analysis of time-series water-content and well-level data to evaluate signature behaviors of preferential flow processes (e.g. Nimmo and Perkins, 2018, VZJ, v. 17, no. 1).
Duration: Up to 12 months
Internship Location: Menlo Park, California
Field(s) of Study: Geoscience
Applicable NSF Division: EAR Earth Sciences, CBET Chemical, Bioengineering, Environmental, and Transport Systems, PHY Physics
Intern Type Preference: Either Type of Intern
Keywords: Unsaturated zone, vadose zone, preferential flow, recharge, contaminant transport, ecohydrology, soil-water, fractured rock
Expected Outcome:
The project will produce journal publications, or substantial material to go into them, that advance the understanding and theory of preferential flow, and build a foundation for practical models and techniques that solve critical problems such as rapid contaminant transport and episodic aquifer recharge. The USGS will benefit from this contribution to its mission of evaluating the problems and effective utilization of water resources. The student will benefit by first-hand acquaintance with the ideas and research practices of USGS scientists, practical experience with cutting-edge research techniques, and achieving research contributions to thesis work and to published scientific knowledge.
Special skills/training Required:
The student should have a Bachelor’s or Master’s degree in hydrology, geology, soil science, physics, environmental science, or a related discipline. The student will need good understanding and ability in physics and math.
Duties/Responsibilities:
The student will collaborate as a member of our team, and take responsibility for one or more particular research components. Such a component might be a set of lab or field experiments, development of a formula or algorithm for predicting the behavior or consequences of preferential flow, or developing a new technique for measurements or calculations. The student will participate in planning, interpreting, and preparing for publication of the research.
