HR: 08:45h
AN: H11H-04    [Abstracts]
TI: Three Decades of Field Experiments Show Minimal Variability in Maximum Preferential Flow Speed
AU: * Nimmo, J R
EM: jrnimmo@usgs.gov
AF: USGS, 345 Middlefield Rd., MS-421, Menlo Park, CA 94025 United States
AB: Evaluation of about 50 published field tests shows that the maximum transport speed in macropores and other preferential channels varies surprisingly little. The tests differ substantially: in type of medium, including fractured rock and various soil textures; in travel distance, ranging from less than 1 to more than 1000 m; in type of tracer; and in vertical or horizontal flow direction. One factor that significantly affects transport speed is the supply of water that generates the flow. Continuous application of water at the land surface, such as ponding or steady irrigation, as opposed to a sporadic supply such as natural rainfall, causes preferential flow that is faster and that varies less among studies. For continuously supplied water, nearly all observations of maximum transport speed fall between 1 and 100 m/d, suggesting that a value in that range could serve as a guideline for expected transport speed under comparable conditions. For sporadically applied water, if transport speeds are adjusted by a factor equal to the fraction of time during which water is applied, the adjusted values also fall in a range of about 1 to 100 m/d. Results suggest that in some common modes of preferential flow, certain mechanisms tend to compensate for factors that would normally cause substantial differences in flow rate. For example, an increase in water content within an unsaturated fracture may not affect the speed of solute transport in thick films if the greater water content is accommodated not by changing film thickness but by covering a larger portion of fracture-wall surface. The generalization that certain transport speeds are predictable to a large extent from water input conditions may improve the reliability and ease of predicting worst-case contaminant travel times and other quantities of hydrologic importance.
UR: http://wwwrcamnl.wr.usgs.gov/uzf/
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: Fall Meeting 2005