Modeling Groundwater-Surface Water Interactions in a Semi-Arid, Montane Environment, Gordon Gulch, Colorado

Abstract:

Groundwater is a vital support to streamflow and ecosystems in semi-arid, montane catchments, which are environments considered particularly sensitive to climate change. Despite its importance, the relationship between groundwater and streamflow is not well understood. We seek to model groundwater-surface water interactions to examine how a changing climate may impact these systems. Our study area, Gordon Gulch, is a semi-arid, montane, forested, 2.6 km² catchment within the Colorado Front Range, ranging in elevation from 2450 to 2750 meters. Gordon Gulch is located within the Boulder Creek Critical Zone Observatory (BcCZO) and has been actively monitored and studied for over a decade. To assess how groundwater discharge to surface water varies temporally and spatially, we develop a numerical model using MODFLOW 6, coupled with MT3D. We use the wealth of available data from the BcCZO including groundwater levels, precipitation, streamflow, water quality, and shallow seismic refraction surveys. The data show biannual hydrologic maximums in groundwater elevation and streamflow in the spring and late summer. The model computes the distribution of hydrologic heads and flux rates between groundwater and surface water within the catchment. Model results show a rapid subsurface response (on the scale of hours to days) to water inputs, resulting in a prolonged increase in streamflow and a sustained increase in groundwater level. Spatially, we observe that groundwater level and its interaction with surface water is strongly influenced by topography. Our model offers a framework for constraining the mechanisms driving groundwater-surface water exchanges and illuminates how subsurface storage and streams in montane catchments may react to a changing climate.