The presently accepted unsaturated flow theory leaves gaps
in understanding of a wide variety of processes. We are working to extend the
body of theory on several of these, selected according to what is needed for
practical applications, and what is most likely to yield significant advances
based on the tools and knowledge available to us.
Models created by this project help to maximize the value of available measurements, sometimes permitting a small, simple data set to serve in place of a large, complex one.
Frequently there is a tradeoff between the accuracy or quality of data and the spatial range covered. In many of our studies, we make high-quality measurements of hydraulic properties of small core samples, the results being indicative of the properties at one point in the subsurface. The region of interest might be an entire watershed, or the entire area that is vulnerable to contamination from a particular source. At this larger scale, the relevant data are generally much lower in quality, for example remotely sensed seasonal variations in water content with spatial resolution on the scale of tens of meters. Theoretical advances make possible a large-scale treatment of hydrologic processes with some of the accuracy of point measurements applied at the larger scale.
(1) Modify and expand the generally accepted unsaturated-flow theory in ways that strengthen the connection between the underlying physical processes and practical formulations. For example, our soil structural investigations have produced a model that includes aggregate-size distributions as an indicator of soil structure along with the more commonly used particle size distributions (an indicator of soil texture) to more accurately predict soil hydraulic properties. (2) Apply numerical analysis and numerical modeling, e.g. finite-difference, optimization, and nonlinear regression. An example is the use of the VS2DT code (which numerically solves Richards' equation to predict dynamic unsaturated flow) to examine how infiltration and aquifer recharge might be affected by the alteration of natural soil structure during landfill construction. (3) Adapt methods developed for one scale to different (usually larger) scales. An example of this is to use a small number of core-sample hydraulic property measurements, in combination with field-observed characteristics of geologic materials, to estimate the hydraulic properties throughout an unsaturated-zone profile. (4) Create or modify conceptual models. (5) Formulate mathematical representations of hydraulic properties.
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