ESTIMATING UNSATURATED-ZONE HYDROLOGIC PARAMETERS USING SOIL MOISTURE, WATER TABLE, AND PRECIPITATION DATA

Dynamic interconnections between infiltration, groundwater, and streamflow remain elusive. In particular, understanding the response of the watershed subsurface system to extreme rainfall events is a key part of untangling those interconnections. Between September 11 and 17, 2013, the Colorado Front Range experienced a record breaking rainfall event that caused widespread flooding. The Upper Boulder Creek Watershed is a sub-watershed near the headwaters of Boulder Creek. The Watershed is instrumented with numerous groundwater monitoring wells, soil moisture measurement sites, streamflow gauge stations, and meteorological stations.

A first order water budget for the extreme rainfall event of September 2013 suggests that a significant portion of the rainfall, ~70%, may have infiltrated into the subsurface. Monitoring shows that groundwater levels and soil moisture responded quickly to the event, but peak values of groundwater levels were delayed relative to the precipitation. In addition, the elevated moisture and groundwater levels remained long past the extreme rainfall event. Here, we use the time series of soil moisture, groundwater level, and precipitation in inverse modeling using HYDRUS-1D to infer vertical profiles of hydrologic properties in the unsaturated zone. We determine the permeability structures in both the unsaturated and saturated zones and permeability-moisture function in the unsaturated zone. Such vertical property profiles will allow for spatial upscaling and extrapolation of these properties for the watershed model.