New Understanding and Techniques

Unsaturated flow entails complex interactions of widely diverse materials—minerals, air, water in all its phases, living organisms with their products and remnants, and chemicals, both natural and anthropogenic. Many of these processes are poorly understood. Consequently, the generally accepted theory of unsaturated-zone flow routinely fails to predict adequately. Our project aims to develop quantitative generalizations and techniques that are widely applicable, closely tied to the critical issues we need to address, and closely representative of observed phenomena.

Highlights:

• The source-responsive preferential flow model to predict preferential flow-dominated travel times, fluxes, water table fluctuations, and dynamic water contents. This model is unusual in its simplicity and its dependence on input source fluxes in place of highly nonlinear properties and sensitivity to variable moisture content.

• Motivation: numerous observations of unsaturated zone transport that goes faster and farther than accepted theory (Richards’ equation) can predict with any reasonable hydraulic property values.
• Based on findings that (1) remarkably little variation in preferential transport rates occur across diverse scales, media, and moisture conditions; (2) preferential flow depends strongly on the dynamic behavior of the water source (e.g. rainfall, irrigation); and (3) much flow of importance occurs in macropores that are partially-filled.
• Practical means of predicting first-arrival times of subsurface transport.

Nimmo, J.R., 2007, Simple Predictions of Maximum Transport Rate in Unsaturated Soil and Rock: Water Resources Research, v. 43, no. 5. (PDF).

• Prediction of unsaturated fluxes and fluctuating water tables due to preferential flow.

Nimmo, J.R., 2010, Theory for Source-Responsive and Free-Surface Film Modeling of Unsaturated Flow: Vadose Zone Journal, v. 9, no. 2, p. 295–306. (PDF).

• Means of predicting dynamic water contents that evolve irregularly in time and space due to phenomena outside the realm of traditional theory. The model has been added to the USGS software VS2DT, for solving general problems of combined diffuse and preferential flow (Healy, 2016, in review).

Nimmo, J.R., Mitchell, L., 2013, Predicting vertically nonsequential wetting patterns with a source-responsive model: Vadose Zone Journal, v. 12, no. 4. (PDF).

• Prediction of deep unsaturated zone percolation fluxes from sparse data.

Mirus, B.B., and Nimmo, J.R., 2013, Balancing practicality and hydrologic realism--A parsimonious approach for simulating rapid groundwater recharge via unsaturated-zone preferential flow: Water Resources Research, v. 49, no. 3, 2012WR012130, doi: 10.1002/wrcr.20141, p. 1458–1465. (PDF).

Ebel, B.A., and Nimmo, J.R., 2013, An alternative process model of preferential contaminant travel times in the unsaturated zone--Application to Rainier Mesa and Shoshone Mountain, Nevada: Environmental Modeling and Assessment, v. 18, no. 3, doi: 10.1007/s10666-012-9349-8, p. 345-363. (PDF).

• Improved water-balance method for recharge quantification by including preferential bypass processes with the source-responsive model.

Cuthbert, M.O., Mackay, R., and Nimmo, J.R., 2013, Linking soil moisture balance and source-responsive models to estimate diffuse and preferential components of groundwater recharge: Hydrology and Earth System Sciences, v. 17, no. 3, doi: 10.5194/hess-17-1003-2013, p. 1003-1019. (PDF).

• Experimental demonstration that even fractures that are not open to the land surface can be major preferential flow paths, and that seepage of water into fractures occurs under unsaturated conditions.

Su, G.W., Nimmo, J.R., and Dragila, M.I., 2003, Effect of isolated fractures on accelerated flow in unsaturated porous rock: Water Resources Research, v. 39, no. 12. (PDF).

• Use of unsaturated flow characteristics of partially-filled fractures to resolve apparent water-budget imbalances in a fractured-rock unsaturated zone.

Nimmo, J.R., and Malek-Mohammadi, S., 2015, Quantifying Water Flow and Retention in an Unsaturated Fracture-Facial Domain, in Faybishenko, B., Benson, S.M., and Gale, J.E., eds., Dynamics of Fluids and Transport in Fractured Porous Systems, Geophysical Monograph 210, AGU (Wiley), p. 169-179. (PDF).

• Theory and model of water retention to represent unsaturated hydraulic properties at low water contents, and over the complete range from saturation to oven dryness.

Nimmo, J. R., 1991, Comment on the treatment of residual water content in "A consistent set of parametric models for the two-phase flow of immiscible fluids in the subsurface" by L. Luckner et al.: Water Resources Research, v. 27, p. 661-662. (PDF).

Rossi, C., and Nimmo, J. R., 1994, Modeling of soil water retention from saturation to oven dryness: Water Resources Research, v.30, p. 701-708. (PDF).

• New theoretical framework for property transfer modeling to estimate soil hydraulic properties (difficult to measure) from geomorphologic data (easy to measure).

Nimmo, J.R., Herkelrath, W.N., Laguna Luna, A.M., 2007, Physically Based Estimation of Soil Water Retention from Textural Data: General Framework, New Models, and Streamlined Existing Models: Vadose Zone Journal 6:766-773. (PDF).

• Discovery of unexpectedly substantial influence of soil microstructure on unsaturated hydraulic conductivity of a sandy soil.

Nimmo, J. R. and Akstin, K. C., 1988, Hydraulic conductivity of a saturated soil at low water content after compaction by various methods: Soil Science Society of America Journal, v. 52, p. 303-310. (PDF).

• Model of soil-water hysteresis that reduces the data requirement for working with the hysteresis inherent in unsaturated hydraulic properties, and provides needed pore size and shape parameters for structural and preferential flow studies.

Nimmo, J. R., 1992, Semiempirical model of soil water hysteresis: Soil Science Society of America Journal, v. 56, p. 1723-173. (PDF).

• Rigorous experimental tests of Darcy’s law and Richards’ equation in unsaturated soil.

Nimmo, J. R., Rubin, J., and Hammermeister, D. P., 1987, Unsaturated flow in a centrifugal field: measurement of hydraulic conductivity and testing of Darcy's law: Water Resources Research, v. 23, p. 124-134. (PDF).

Nimmo, J. R., 1990, Experimental testing of transient unsaturated flow theory at low water content in a centrifugal field: Water Resources Research, v. 26, p. 1951-1960. (PDF).

• Review articles and science-historical investigations

• Book chapters on fundamental unsaturated zone topics.

Nimmo, J.R., 2005, Unsaturated Zone Flow Processes, in Anderson, M.G., and Bear, J., eds., Encyclopedia of Hydrological Sciences: Chichester, UK, Wiley, p. 2299-2322. (PDF).

Nimmo, J.R., 2009, Vadose Water, in Likens, G.E., ed., Encyclopedia of Inland Waters: Oxford, UK, Elsevier, v. 1, p. 766-777. (PDF).

Nimmo, J.R., 2013a, Porosity and Pore Size Distribution, Reference Module in Earth Systems and Environmental Sciences, Elsevier, doi: 10.1016/B978-0-12-409548-9.05265-9. (PDF).

Nimmo, J.R., 2013b, Aggregation--Physical Aspects, Reference Module in Earth Systems and Environmental Sciences, Elsevier, doi: 10.1016/B978-0-12-409548-9.05087-9. (PDF).

• Review of book on preferential flow theory.

Nimmo, J.R., 2015, Preferential Flow—Stokes Approach to Infiltration and Drainage [Book review]: Vadose Zone Journal, v. 14, no. 7, doi:10.2136/vzj2013.03.0054. (PDF).

• Journal articles to expand knowledge and interest in the history of unsaturated flow science.

Landa, E.R., and Nimmo, J.R., 2003, The life and scientific contributions of Lyman J. Briggs: Soil Science Society of America Journal, v. 67, no. 3, p. 681-693, cover, i. (PDF).

Nimmo, J.R., and Landa, E.R., 2005, The soil-physics contributions of Edgar Buckingham: Soil Science Society of America Journal, v. 69, no. 2, p. 328-342. (PDF).

Nimmo, J.R., 2008, The Public Fountains of the City of Dijon by H. Darcy--translated by P. Bobeck [Book Review]: Vadose Zone Journal, v. 7, no. 4, p. 1311-1312. (PDF).

In 2016 and later:

Develop a self-consistent model of hydraulic properties that control the transfer of water and dissolved substances between preferential flow channels and the surrounding matrix material, as needed especially in contaminant transport problems.

Evaluate the various possible physical modes of preferential flow, including filled and unfilled conduits, sliding drops, rivulets, and films. Apply hydrodynamic theory to develop appropriate mathematical representations for them in a subsurface flow model.

Conduct lab experiments to evaluate the possibility of thresholds in water content or applied flux at which preferential flow begins to be generated, a major issue with respect to predicting when preferential flow does and does not occur.

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Last modified: Feb. 2016