Observations from natural rain storms and sprinkling experiments at a steep zero‐order catchment in the Oregon Coast Range demonstrate the importance of flow through near‐surface bedrock on runoff generation and pore pressure development in shallow colluvial soils. Sprinkling experiments, involving irrigation of the entire 860 m2 catchment at average intensities of 1.5 and 3.0 mm/h, permitted detailed observation of runoff and the development of subsurface saturation under controlled conditions. A weir installed to collect flow through the colluvium at the base of the catchment recovered runoff equal to one third to one half of the precipitation rate during quasi‐steady irrigation. Three key observations demonstrate that a significant proportion of storm runoff flows through near‐surface bedrock and illustrate the importance of shallow bedrock flow in pore pressure development in the overlying colluvial soil: (1) greater discharge recovery during both the experiments and natural rainfall at a weir installed approximately 15 m downslope of the weir at the base of the catchment, (2) spatially discontinuous patterns of positive pressure head in the colluvium during steady sprinkling, and (3) local development of upward head gradients associated with flow from weathered rock into the overlying colluvium during high‐intensity rainfall. Data from natural storms also show that smaller storms produce no significant runoff or piezometric response and point to a critical intensity‐duration rainfall to overcome vadose zone storage. Together these observations highlight the role of interaction between flow in colluvium and near‐surface bedrock in governing patterns of soil saturation, runoff production, and positive pore pressures.

Abstract. As part of a larger, collaborative study, we conducted field experiments to investigate how rainfall signals propagate through an unsaturated soil profile, leading to a rapid pore pressure response and slope instability. We sprinkler-irrigated an entire, unchanneled headwater basin in the steep, humid Oregon Coast Range, and we drove the system to quasi steady state as indicated by tensiometers, piezometers, and discharge. During initial wetting some of the deeper tensiometers responded before the arrival of an advancing head gradient front. With continued irrigation most tensiometers attained nearzero pressure heads before most piezometers responded fully, and a stable unsaturated flow field preceded the development of a stable saturated flow field. Steady discharge occurred after the last piezometer reached steady state. With the onset of steady discharge the unsaturated zone, saturated zone, and discharge became delicately linked, and a spike increase in rain intensity led to a response in the saturated zone and discharge much faster than could have happened through advection alone. We propose that the rain spike produced a slight pressure wave that traveled relatively rapidly through the unsaturated zone, where it caused a large change in hydraulic conductivity and the rapid effusion of stored soil water. An important control on the hydrologic response of this catchment lies with the soil-water retention curve. In general, below pressure heads of about -0.05 m, soil-water contents change slightly with changes in pressure head, but above -0.05 rn the soil-water content is highly variable. Minor rainstorms upon a wet soil can produce slight changes in pressure head and corresponding large ch•nges in soil-water content, giving rise to the passage of pressure waves in response to increased rain intensity and a relatively rapid response in the unsaturated zone. This rapid unsaturated zone response led to a rapid rise in the saturated zone, and it may be the underlying mechanism enabling short bursts of rain to cause slope instability.

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Abstract. The observation that "old" water dominates storm runoff suggests that release of low-solute water from soils rather than rainwater must cause storm runoff dilution. This inference is supported by sprinkling experiments in an 860-m 2 catchment in the Oregon Coast Range, in which >200 mm of both high and low ionic strength precipitation produced similar concentration-discharge trends. Rainwater chemistry was buffered as it traveled through catchmerit soils: the amount of sprinkling-derived water in the runoff increased during long periods of steady discharge but was not accompanied by a change in runoff solute concentrations. Stored water plays a role in runoff dilution as well. Nearly all runoff from the catchment passes through underlying weathered bedrock rather than perching and discharging only through soil. Bedrock water composition appears to vary through storm events, as the average contact time of water with rock declines with increasing discharge, a behavior at odds with the concept of stable end-members.

The comprehensive physics-based hydrologic-response model InHM was used to simulate 3D variably-saturated flow and solute transport for three controlled sprinkling experiments at the Coos Bay 1 (CB1) experimental catchment in the Oregon Coast Range. The InHM-simulated hydrologic-response was evaluated against observed discharge, pressure head, total head, soil-water content, and deuterium concentration records. Runoff generation, tensiometric/piezometric response in the soil, pore-water pressure generation, and solute (tracer) transport were all simulated well, based on statistical and graphical model performance evaluation. The InHM simulations reported herein indicate that the 3D geometry and hydraulic characteristics of the layered geologic interfaces at CB1 can control the development of saturation and pore-water pressures at the soil-saprolite interface. The weathered bedrock piezometric response and runoff contribution were not simulated well with InHM in this study, most likely as a result of the uncertainty in the weathered bedrock layer geometry and fractured-rock hydraulic properties that preclude accurate fracture flow representation. Sensitivity analyses for the CB1 boundary-value problem indicate that: (i) hysteretic unsaturated flow in the CB1 soil is important for accurate hydrologicresponse simulation, (ii) using an impermeable boundary condition to represent layered geologic interfaces leads to large errors in simulated magnitudes of runoff generation and pore-water pressure development, and (iii) field-based retention curve measurements can dramatically improve variably-saturated hydrologic-response simulation at sites with steep soil-water retention curves. The near-surface CB1 simulations reported herein demonstrate that physics-based models like InHM are useful for characterizing detailed spatio-temporal hydrologic-response, developing processbased concepts, and identifying information shortfalls for the next generation of field experiments. The field-based observations and hydrologic-response simulations from CB1 highlight the challenges in characterizing/simulating fractured bedrock flow at small catchments, which has important consequences for hydrologic response and landslide initiation.

[1] The purpose of this study is to determine if concepts in terrestrial channel network analysis provide insight on intertidal creek network development and to present new metrics for their analysis. We delineated creek network geometry using high-resolution digital images of intertidal marsh near Georgetown, South Carolina. Analyses reveal that intertidal creek networks may be topologically random. Length-area relationships suggest that salt marsh and terrestrial networks have similar scaling properties, although the marsh networks are more elongate than terrestrial networks. To account for recurrent water exchange between creek basins at high tide, we propose that the landscape unit of geomorphic analyses should be the salt marsh island as opposed to salt marsh creek drainage basin area. Using this approach, the relationship between maximum channel length per island and island area is well described by a power function. A similar power relationship exists for cumulative channel length versus island area, giving a nearly unit slope; this implies that marsh islands have a spatially uniform drainage density. Since the island boundaries are easily identified in remote sensing, taking the island as the unit of geomorphic analysis will eliminate discrepancies in delineating basin boundaries and preclude the need for defining basin area in intertidal landscapes. Analyses and results presented here may be used to quantify salt marsh reference condition and provide indicator variables to assess salt marsh disturbance.

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