Examining subsurface response to an extreme precipitation event using HYDRUS-1D

Abstract

North-central Colorado experienced an extreme precipitation event (EPE) in September 2013, during which the equivalent of 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 influence 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 using HYDRUS-1D, was built to simulate and examine infiltration of an EPE at a site in the Boulder Creek Watershed, Colorado. Model calibration was conducted with local field data to fit site observations over a 6-yr period. A rapid water table depth response in field observations was observed, with the modeled subsurface storing water for 18 mo acting as a hydro-buffer during recovery. To examine influence on model results, a sensitivity study of soil hydraulic parameters was conducted. The sensitivity study found that changes in n, an empirical parameter related to pore-size distribution, most significantly affects water table depth. The implications are that one-dimensional 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 effect on water resources.