HR: 15:25h
AN: H53H-08    [Abstracts]
TI: Augmentation of the Darcy-Buckingham-Richards Formulation to Account for Fast Response to Infiltration of Soil Moisture at Depth
AU: * Nimmo, J R
EM: jrnimmo@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: The generally accepted quantitative model of unsaturated flow is Richards' equation, based on concepts put forward by Darcy and Buckingham. This continuum approach is normally implemented using a fine discretization and a representative elementary volume conceived on the scale of some fraction of a meter. Traditionally, it has typically been used where time scales of interest range from several hours for diurnal soil-water-plant response, to years for long-range contaminant transport. Its predictions depend strongly on quantitative knowledge of unsaturated hydraulic properties of soil and rock, and on initial and boundary conditions in a subsurface domain. A growing accumulation of observational evidence indicates that moisture in the unsaturated zone, even several meters below the land surface, may respond rapidly (minutes to hours) to larger-scale hydrologic conditions such as fluctuating rainfall intensity. Rapid responses have been documented in terms of water content, arrival of tracer, or water-table fluctuation. The Darcy-Buckingham-Richards (DBR) approach alone is often ill-suited for these phenomena because their short time scale and relatively large spatial scale contrast markedly with those of diffuse unsaturated flow occurring at the same time. The size of representative elementary volume needed to justify the continuum approach for diffuse flow may be much too small to also represent preferential flow. The model of Nimmo (2007) predicts travel times of the preferential component of flow using alternative concepts such as constant-rate progress through preferential paths, without regard to traditional unsaturated flow properties of the subsurface media. It formulates the problem with strong reliance on the dynamic character of the infiltration rather than particular properties of the soil and rock. Where preferential and diffuse flow are both significant, an interactive combination of this approach with the DBR approach improves prediction of net vertical unsaturated-zone fluxes in response to hydraulic inputs and the evolving distribution of soil moisture.
DE: 1816 Estimation and forecasting
DE: 1829 Groundwater hydrology
DE: 1865 Soils (0486)
DE: 1866 Soil moisture
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting