Near-surface hydrologic response for a steep, unchanneled catchment near Coos Bay, Oregon: 2. Physics-based simulations

Abstract

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 hydrologic-response 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 process-based 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.