Soil-Layering and Hydraulic Property Effects On Unsaturated Zone Storage Dynamics and Simulated Runoff Response in Small Catchments.

In small rangeland and forested catchments, hillslope and channelized runoff provide the driving force behind erosion and sediment transport processes. Improved quantitative understanding of the complex hydrologic-response dynamics in the variably saturated near-surface is therefore needed to inform land and resources management decisions. Our objective is to examine the influence of soil layering and hydraulic properties on spatially and temporally variable runoff response for a range of environmental conditions. We employ a heuristic, simulation-based approach with the Integrated Hydrology Model (InHM), a comprehensive physics-based model. InHM couples 3D variably-saturated subsurface flow with 2D overland flow, and has been used successfully to capture observations of integrated and distributed hydrologic-response dynamics at several well-characterized experimental catchments. Previous simulations for the C3 catchment (Oregon) located in steep, forested terrain, and the R-5 (Oklahoma) and Tarrawarra (Australia) catchments located in gently-sloping rangelands, provide the starting points for developing alternative simulation scenarios designed to improve conceptual understanding. Simulation results illustrate how soil layering and hydraulic properties influence storage thresholds and runoff generation processes for these contrasting catchments. Alternative simulation scenarios for C3 demonstrate how saturation overland flow and subsurface stormflow are competing mechanisms controlled by the rate of lateral drainage within variably saturated soil layers. Alternative simulation scenarios for R-5 and Tarrawarra demonstrate how the saturation excess and infiltration excess mechanisms represent end members of a continuum of near-surface processes that generate overland flow. Overall, the simulation results quantitatively confirm that even for idealized systems with homogeneous layering, runoff response processes are more complex than simpler, single-process models would suggest.