USE OF AN INSTRUMENTED PLANAR EXPERIMENTAL PLOT TO PROVIDE GUIDANCE FOR PHYSICALLY-BASED DISTRIBUTED HYDROLOGIC MODELING

Physically-based distributed hydrologic models are designed to represent the physical mechanisms that determine pathways of water through a watershed. Such models simulate, to varying degrees of detail, runoff along the ground surface, infiltration and water movement through the unsaturated zone, groundwater flow, evapotranspiration, and stream flow. These processes are most often studied individually rather then holistically; few data sets exist that allow simultaneous testing of how all of these hydrologic processes interact. In this study, we comprehensively examine structure and scale relationships in physically-based hydrologic models at a spatial scale intermediate between those of typical laboratory or watershed studies. Hydrologic simulations are based on a complete set of hydrologic measurements collected at a 24 m² hydrologic test plot in Seattle, Washington. To mimic a gently sloping, shallow soil hill slope, the plot has a 5% grade and is grass-covered, containing 0.3m of till-based soil above an impermeable base. Hydrologic measurements collected at the site include precipitation, surface runoff, subsurface runoff, incoming short and longwave radiation, net radiation, air temperature, humidity, wind speed, soil water contents, soil temperatures, and soil heat fluxes. Hydrologic processes in the plots are simulated using the numerical models, HYDRUS 2D and HYDRUS 1D for variably saturated flow, and MODHMS for fully-coupled groundwater and surface water flow. We examine how different model scales and structures affect hydrologic process representation, in particular soil water contents and discharge. Results provide guidance for selecting ‘physically-based' model parameter values and insight into the level of complexity required for accurate simulation of water pathways in a distributed model.