HR: 10:20h
AN: H52B-01    [Abstracts]
TI: Interactions Between Overland and Subsurface Runoff on Steep Forest Slopes: is Hortonian Overland Flow Important?
AU: * Sidle, R C
EM: sidle@slope.dpri.kyoto-u.ac.jp
AF: Geohazards Division, DPRI, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan
AU: Gomi, T
EM: gomi@slope.dpri.kyoto-u.ac.jp
AF: Japan Science and Technology Agency, DPRI, Kyoto University, Gokasho, Uji, Kyoto, 611- 0011, Japan
AU: Terajima, T
EM: terajima@faculty.chiba-u.jp
AF: Dept. Earth Science, Chiba University, 1-33 Yayoicho, Inage, Chiba, 263-8522, Japan
AU: Hirano, T
EM: 06ud6208@graduate.chiba-u.jp
AF: Dept. Earth Science, Chiba University, 1-33 Yayoicho, Inage, Chiba, 263-8522, Japan
AB: The prevailing notion that Hortonian overland flow is insignificant on temperate forest hillslopes has been challenged in poorly managed Japanese cypress (hinoki) plantations, due to overstocked stands and poor ground cover. Small (0.5 x 2 m) and hillslope-scale (8 x 25.5 m) runoff plots were established in forested sites of thinned and unthinned hinoki stands and mixed deciduous forests in Mie, central Japan. Additionally, small runoff plots, TDR probes, and recording tensiometers were installed in a forest site near Tokyo to assess the interactions of surface and subsurface flow paths. Runoff coefficients during larger storms (> 50 mm total rainfall) at Mie typically exceeded 0.10 for small plots, whereas hillslope scale plots rarely had runoff coefficients > 0.10. The contrast between plot sizes is more pronounced and consistent for the thinned hinoki forest with good ground cover and the deciduous forest site. In both of these sites, runoff coefficients from small plots were > 0.1 for total rainfall > 30 mm. In contrast, runoff coefficients from large plots for almost all storms were < 0.1, and larger storms had much lower runoff coefficients compared to smaller storms. These data suggest that much of the Hortonian overland flow re-infiltrates into soil organic horizon or matrix where it moves downslope as subsurface flow. Questions still remain whether overland flow measured in plots is really Hortonian. Field observations during major storms at similar sites indicate that runoff plots collect some shallow subsurface and preferential flow in the organic-rich biomat. Detailed soil water measurements in a deciduous forest near Tokyo revealed that 47% of rainfall (168 mm) in a large storm that infiltrated into the soil moved laterally as biomat flow; the remainder of the infiltrated rainwater percolated deeper into the mineral soil. Of the 9.3% of the rainfall that was measured as runoff, a portion appears to be biomat flow. Biomat flow represents a significant, albeit transient flow path that buffers against Hortonian overland flow even for thin litter layers. Water flowing through such near-surface biomats will be more rapid than subsurface runoff in the mineral soil, but much slower than true Hortonian overland flow and will have ample opportunity for vertical infiltration.
DE: 1830 Groundwater/surface water interaction
DE: 1839 Hydrologic scaling
DE: 1850 Overland flow
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Joint Assembly