Complex stormflow behavior in hillslopes - temporal and spatial attributes (Invited)
Abstract
Rapid flow pathways in vegetated hillslopes can occur via several mechanisms: (1) preferential flow in soils; (2) ‘biomat flow’; (3) infiltration-excess overland flow; and (4) saturation overland flow. Field studies in Japan have noted that self-organization processes may manifest the connectivity of relatively short macropores from hillslopes to stream channels (Sidle et al. 2000, 2001). This preferential flow connectivity depends on soil wetness and runoff exhibits threshold behavior. Finite element solutions of the Richards equation applied to a short hillslope segment containing a variety of disconnected macropores confirms field results of increasing macropore connectivity and flow as rainfall inputs and soil wetness increase. Hillslope soil moisture flow bypasses macropores in drier regions of the soil profile, while flows converge on macropores in wet regions. The effect of moisture dependent connectivity of macropores was evident in observations of rapid subsurface stormflow from organic horizons of a forested hillslope in British Columbia, where the subsurface stormflow response was more than two orders of magnitude higher during wet antecedent conditions compared to dry conditions. This type of rapid flow in organic horizons is similar to ‘biomat flow’ observed in degraded forest (Japanese cypress) soils in Japan where water flowed below the surface within the loose litter and root-permeated zone. Companion studies showed that substantial overland flow (presumably infiltration-excess) occurred from small cypress plots, but at the hillslope scale, overland flow was 2 to 10 times lower than at the plot scale, and the differences in runoff between plot and hillslope scales were greater during major storms. Connectivity of overland flow paths was enhanced by bursts of high rain intensity and drier antecedent conditions where localized hydrophobic conditions may appear. Much of this ‘overland flow’ may actually occur in the upper root-permeated soil horizons, even under poor cover conditions (i.e., ‘biomat’ flow). In the steep, incised Japan sites, saturation overland flow only occurred near the base of hillslopes during the latter portion of the largest storms. All of these rapid flow paths exhibit some kind of threshold behavior but complexity of response varies. Preferential flow shows strong evidence of self-organization with increasing water content, while the complex behavior of overland flow is more manifested in drier conditions and during heavy rainfall.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H31H..08S
- Keywords:
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- 1804 HYDROLOGY / Catchment;
- 1826 HYDROLOGY / Geomorphology: hillslope;
- 1839 HYDROLOGY / Hydrologic scaling;
- 1875 HYDROLOGY / Vadose zone