Dissertation - University Access Only
Doctor of Philosophy (PhD)
Department / School
Robert A. Kohl
Macropores, structural features, and profile discontinuities have been suggested as the primary contributing factors in the development of preferential flow paths in soils. The application of a partial vacuum at the ponded surface of undisturbed soil cores is one method that has been suggested for use in partitioning the total flow by effective pore diameter. The hypothesis that negative head pressures, produced by a partial vacuum, could be used to partition flow was tested using undisturbed soil cores. Reductions in flow rate via an increase in the partial vacuum, from -5 cm to -20 cm of water, were observed. These results were attributed to the exclusion of flow from a threshold pore diameter and an experimentally observed gradient effect, predicted by the Poiseuille equation. The negative head procedure was limited in its ability to partition flow because the gradient effect could not be separated from the total flow rate reduction at a given negative pressure head setting. Interruptions in the application of water to greenhouse cores and field plots, that were near field capacity prior to water application, always produced lower infiltration rates after water application was restarted. This observation formed the basis for the testing of a second hypothesis. It was assumed that air entrapment in the mesopore network was the operative mechanism producing flow rate reductions. These reductions remained stable over experimental periods of up to 50 hours. An artificial meso-macropore pore network, constructed of polyvinyl tubing, indicated that air could be trapped in mesopores even when macropores were flowing full. Carbon dioxide was also used to saturate the atmosphere above soil cores as a means of verifying air entrapment as the flow limiting mechanism. Flow rates were reduced after interrupting the water application but the rate reduction was not as great for the CO2 atmosphere as it was for air alone. Due to the air entrapment phenomenon it was determined that the antecedent moisture content must be evaluated in terms of wetting history in order to access the degree to which trapped air would affect the determination of "saturated" hydraulic conductivity. Cores and field plots allowed to free drain for a period of approximately one week (8v 30 percent) produced the highest infiltration rates. Cores with initial volumetric moisture contents less than 15 percent and interrupted infiltration runs produced the lowest infiltration rates. Volumetric moisture contents, derived from neutron probe values, suggested that saturated hydraulic conductivity was never achieved in the undisturbed soil cores. This was the case for all runs and the full range of observed flow rates at initial volumetric moisture contents from 7 to 33 percent, even though steady state conditions appeared to have been reached. Total pore space in the upper mantle of the soil profile was approximately 10 percent greater than the volumetric moisture content even after 50 hours of continuous ponding.
Library of Congress Subject Headings
South Dakota State University
Wangemann, Stephen George, "Elucidation of Preferential Flow Paths in Soils" (1994). Electronic Theses and Dissertations. 29.