Dynamic Response of Watershed Subsurface Systems to Extreme Precipitation Events – Implications for the 2013 Colorado Front Range Floods

Abstract:

In September 2013, the Boulder Creek watershed (BCW) was severely affected by a 1000-year precipitation event when 17 inches of rain – nearly the annual average – fell over Boulder, Colorado, USA in just eight days. This rainfall caused an unprecedented flood within the BCW. While the enormous amount of water that fell appeared to have left within days, we hypothesize that a majority of the water that fell during the event was not discharged via surface runoff, but infiltrated into the subsurface.

This study aims to understand how the subsurface responds to extreme precipitation events by quantifying the volume of precipitation that infiltrates into the vadose zone, the change in subsurface water storage, and the spatial and temporal scale of these effects as caused by extreme events. This study focuses on a small drainage basin that lies within the BCW. This site is actively monitored and provides data of importance for hydrologic modeling. The data include groundwater elevation, soil moisture, stream discharge, and precipitation records. The hydrologic structure of the basin consists of a thin, sandy soil layer that caps weathered regolith and saprolite sourced from underlying crystalline basement bedrock.

We use the HYDRUS model to numerically solve the Richards equation for variably saturated flow to simulate 2D groundwater flow both below the groundwater table and in the vadose zone. Average annual precipitation values and groundwater elevation measurements are used to estimate the initial subsurface conditions prior to the event. Atmospheric boundary conditions estimated from meteorological stations are then applied to the top of the model to simulate the extreme event. Preliminary results indicate that the extreme event led to a rise in the groundwater table of up to a meter at low-lying elevations near basin drainages and up to two meters below hillslopes that persisted for over a month after the rain stopped.