Runoff generation in a steep, soil‐mantled landscape

Abstract

[1] Scale and slope dependence of hydrologic response are investigated for two channel network source areas (unchanneled valleys) in the Oregon Coast Range. Observations of response to both natural and applied precipitation reveal that runoff occurred as subsurface flow in which water passed through partially saturated soil, into the shallow fractured bedrock, to emerge as subsurface partial source areas near the channel head. The two dominant approaches to modeling subsurface flow in steep topography, routing of Darcy or fracture flow and the hydrologic similarity approximation of TOPMODEL, respectively predict either a strong slope dependence or no slope dependence to timescales of subsurface runoff generation. Compilation of data from our Coos Bay study sites with observations reported previously elsewhere indicates weak area dependence but no slope dependence in the lag-to-peak and discharge recession constants. This finding supports the interpretation that patterns of antecedent soil moisture and vadose zone characteristics control response times of runoff generation by subsurface storm flow. As slope should influence lateral flow routing once subsurface saturation develops, we conclude that the hydrologic response of steep catchments appears to be insensitive to slope because the controlling timescale is that of the vertical unsaturated flow.

A paradigm shift in stormflow predictions for active tectonic regions with large-magnitude storms: generalisation of catchment observations by hydraulic sensitivity analysis and insight into soil-layer evolution

“…Both recession outflow rates with the macropore effect for L = 20 m and 100 m are similar to the recession of the runoff rate in KT in Fig. 7 regardless of a difference in the horizontal slope length, suggesting that the vertical water movement may play an important role in producing the stormflow recession characteristics as estimated from the observations (Montgomery and Dietrich, 2002).…”

“…6; these values may also differ from those predicted by a distributed runoff model in which the 4464 Makoto Tani: A paradigm shift in stormflow predictions stormflow component is represented by only a downslopeflow component (Ishihara and Takasao, 1964;Troch et al, 2003). The dependence of stormflow responses on unsaturated vertical flow has often been discussed from on-site observations (Montgomery and Dietrich, 2002) and from theoretical considerations (Tani, 1985a;Kosugi, 1999). As a result, parameterisation of catchment properties must consider the historical evolution of the soil layer for distributed runoff models of active tectonic regions.…”

“…Although the dependences deductively introduced from the surface topography as usually used in distributed runoff models might be often regarded doubtful by on-site observations in active tectonic regions (Montgomery and Dietrich, 2002;Weiler et al, 2006;McDonnell et al, 2007), only a few studies have focused on intercomparison (Negley and Eshleman, 2006;Uchida et al, 2006;. Since many observations of stormflow responses have already been obtained, a higher priority should be given to an inductive detection of the dependences on catchment properties by the intercomparison of these data to apply the results to runoffmodel parameterisations.…”

“…However, some hillslope observations, especially in active tectonic regions, do not indicate the dominant effects of topography. This incompatible observational result is attributable to the dominant function of underground pathways, including 4454 Makoto Tani: A paradigm shift in stormflow predictions weathered bedrock (Montgomery and Dietrich, 2002;Kosugi et al, 2006;Katsuyama et al, 2010;Gabrielli et al, 2012). Kosugi et al (2011) demonstrated that a localised bedrock aquifer distribution not following the catchment topography produced a unique triple-peak hydrograph response in a headwater catchment.…”

“…This study was motivated by the question of why stormflow responses were not simply predicted by distributed runoff models from the catchment properties (Montgomery and Dietrich, 2002;McDonnell et al, 2007). As noted previously, this may be a result of underground water movement, but how this movement controls stormflow has not been evaluated.…”