Groundwater is critical in sustaining streamflow, especially in headwater catchments, because of its ability to supply baseflow. In water-limited arid and semi-arid mountain environments, the need to characterize groundwater recharge and discharge has grown in tandem with demands to manage current and future water resources. However, studying groundwater in complex terrain is challenging due to limited field measurements. Nearly a decade of monitoring in Gordon Gulch in the Colorado Front Range provides an opportunity to study such an environment. The field data is used to parameterize and calibrate a groundwater flow model (MODFLOW-NWT). Model results reveal that groundwater is recharged primarily during one to two recharge periods each year, driven by spring snowmelt coupled with rain or by intense/prolonged summer rain. Gordon Gulch is a net gaining stream, with greater fluxes from groundwater to stream in lower Gordon Gulch and during springtime. Groundwater is discharged to the stream via long, deep flowpaths sourced from upper Gordon Gulch and from hillslopes, and via short, shallow flowpaths in lower Gordon Gulch. Modelled groundwater accounts for approximately 16 to 34% of baseflow in the stream. Using Gordon Gulch as a case study, this model and data analysis contribute to a larger effort to understand and constrain the mechanisms driving groundwater recharge and groundwater-stream exchanges in semi-arid, headwater catchments.

Groundwater is critical in sustaining streamflow, especially in mountain catchments, because of its ability to supply baseflow in the absence of precipitation. In water-limited arid and semi-arid mountain environments, the need to characterize groundwater recharge and discharge has grown in tandem with demands to effectively manage current and future water resources. However, studying groundwater is challenging in complex terrain due to limited field measurements. Nearly a decade of monitoring data collection at Gordon Gulch in the Colorado Front Range provides a unique opportunity to study such an environment. The field data is used to parameterize and calibrate a groundwater flow model (MODFLOW-NWT). Model results reveal spatial and temporal patterns in groundwater recharge and discharge to the stream. Groundwater is recharged primarily by one to two recharge events each year, driven by spring snowmelt and rain. The majority of groundwater recharge occurs in upper Gordon Gulch and is stored in saprolite and weathered bedrock. Groundwater is discharged to the stream via long, deep flowpaths sourced from upper Gordon Gulch and short, shallow flowpaths from soil and saprolite in lower Gordon Gulch. Using Gordon Gulch as a case study, this model and data analysis contribute to a larger effort to understand and constrain the mechanisms driving groundwater recharge and groundwater-stream exchanges in semi-arid, montane environments.