EXAMINING SUBSURFACE RESPONSE TO AN EXTREME PRECIPITATION EVENT USING HYDRUS 1D

North-central Colorado experienced an extreme precipitation event (EPE) in September 2013, during which 80% of the region’s annual average precipitation fell in a few days. Widespread flooding occurred above-ground but the short and long-term subsurface response remains unclear. The objective of the study is to better understand the dynamic subsurface response, namely how the water table and soil water storage responded to a large amount of infiltration in a short period of time and how the hydrologic properties of the subsurface influenced the response. Better understanding of subsurface response to EPEs is expected to increase with the advent of more intense and frequent EPEs in the coming decades. A one-dimensional subsurface flow model, HYDRUS 1D, is built to simulate and examine infiltration of an EPE at a site in the Boulder Creek Watershed, Colorado. Model calibration is conducted with local field data to fit site observations over a six-year period. Rapid water table depth response in field observations is observed, with the modeled subsurface storing water for 18 months, acting as a hydro-buffer during recovery. To examine influence on model results, a sensitivity study of soil hydraulic parameters is conducted. The sensitivity study finds that changes in , an empirical parameter related to pore-size distribution most significantly affects water table depth The implications are that 1D models may provide useful estimates of water table fluctuations and subsurface hydro-buffer capacities in response to EPEs, which could be of use to regions preparing for EPE impacts on water resources.