HR: 1340h
AN: H23C-1434    [Abstracts]
TI: Moisture and Energy Conditions Within a Sloping Soil Mass During Drainage: Hewlett and Hibbert 1963 Revisited
AU: * Graham, C B
EM: chris.graham@oregonstate.edu
AF: Oregon State University, 204 Peavy Hall , Corvallis, OR 97331 United States
AU: McDonnell, J J
EM: Jeff.McDonnell@orst.edu
AF: Oregon State University, 204 Peavy Hall , Corvallis, OR 97331 United States
AU: Frey, M
EM: martin.frey@eawag.ch
AF: Swiss Federal Institute for Environmental Science and Technology (EAWAG), šberlandstrasse 133, D�bendorf, 8600 Switzerland
AB: In their seminal 1963 paper, Hewlett and Hibbert (1963, J. Geophysical Res.) described the discharge from an artificial hillslope constructed at the Coweeta catchment with a smooth bed concrete aquiclude filled with uniform soil. The results of this experiment demonstrated the importance of unsaturated soilwater flux on baseflow, and the potential rapid contributions of groundwater to channel storm flow. This experiment underpinned the development of the variable source area concept in the succeeding years and generations of hydrologists trained with Hewlett's artificial hillslope findings in mind. We recently constructed a scaled physical model at Oregon State University to investigate hillslope processes. Unlike the Hewlett and Hibbert model, ours is indoors and represents a 1:11 scale model of a well instrumented and trenched hillslope at the Panola Mountain State Park in Georgia where we have worked for many years. Rainfall is supplied by a rain simulator. Scaled soil depth, bedrock and surface topography were incorporated into the model. In addition, a smooth bed trough was constructed adjacent to the hillslope, to both mirror the Hewlett and Hibbert work and to serve as a control in investigating the effect of surface and bedrock topography on flow processes. While we have shown the effect of subsurface topography at the Panola field site and other locations, this is the first study to examine these effects in a lab context. Despite the simplifying assumptions currently built into the model (homogeneous soil, no bedrock leakage or preferential flowpaths) the system mimics some of the complex behavior seen at the "real" hillslope, including threshold behavior, subsurface flow path variation, and sensitivity to antecedent moisture. Our presentation compares the smooth and rough bed unsaturated drainage in a controlled environment where considerable difference are observed. We then determine the effect of bedrock topography, antecedent moisture, and rainfall spatial variation, intensity and duration on discharge. Numeric modeling simulations using a 3D multiphase Dary-Richards solver are used to provide additional insight internal to the hillslope system. Unlike the Hewlett and Hibbert (1963) observations, our work at the scaled lab hillslope, the natural slope from which it is derived and many of our other trench field sites show clear threshold behavior between applied rainfall and slope-scale runoff. These observations challenge some of the main assertions of Hewlett's early work and highlight the need to quantify soil depth variability for modeling subsurface flow at the hillslope scale.
DE: 1804 Catchment
DE: 1847 Modeling
DE: 1875 Vadose zone
DE: 1894 Instruments and techniques: modeling
SC: Hydrology [H]
MN: Fall Meeting 2005