Coupling Field Data and a Flow Model to Characterize the Role of Groundwater in a Montane, Semi-Arid, Headwater Catchment, Gordon Gulch, Colorado

Abstract

Thesis directed by Professors Suzanne P. Anderson and Shemin Ge 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.

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

“…Well 1 is at a horizontal distance of 150 m away from the small ephemeral stream and 12 m higher in elevation above the streambed. The flow record of the nearest stream gauge has not shown evidence of stream influence on Well 1 (Anderson & Ragar, 2021a; Henning, 2016; Salberg, 2021). In contrast to Well 1, Well 2 is influenced by nearby streamflow, whereas Well 6 is affected by lateral flow and upslope infiltration (Anderson & Ragar, 2021b; Henning, 2016; Salberg, 2021).…”

“…The water table position is found where the pressure head is zero. To reflect the depth of the average water table at the site (Salberg, 2021), the modeled water table was initialized at a 10‐m depth.…”

“…For model spin‐up we use the daily average precipitation minus evapotranspiration as the recharge boundary condition on the model top. To account for evapotranspiration, we examined previous studies that estimated potential evapotranspiration for the Gordon Gulch basin and found that potential evapotranspiration values may range from 31 to >100% of the annual average precipitation (Hale, 2022; Langston et al., 2015; Salberg, 2021). Most recently, Salberg (2021) calculated monthly total evapotranspiration loss for a catchment‐scale water budget of Gordon Gulch.…”

“…To account for evapotranspiration, we examined previous studies that estimated potential evapotranspiration for the Gordon Gulch basin and found that potential evapotranspiration values may range from 31 to >100% of the annual average precipitation (Hale, 2022; Langston et al., 2015; Salberg, 2021). Most recently, Salberg (2021) calculated monthly total evapotranspiration loss for a catchment‐scale water budget of Gordon Gulch. They suggested that ∼435 mm of the average 580 mm annual precipitation of the Gordon Gulch drainage basin was lost to evapotranspiration, a ∼75% average.…”

The Role of Groundwater Flow in a Montane, Semi-Arid, Headwater Catchment

“…The wells are arrayed along a transect across the catchment (Figures 1 and 2 ) at positions on the north-facing slope (well 1), the valley bottom (well 2), and the south-facing slope (well 6). The wells were equipped with Solinst Levelogger Junior non-vented pressure transducers operating at ten-minute intervals (Anderson and Ragar, 2021e); atmospheric pressure corrections were made with a Solinst Barologger that was hung in a tree near the wells (Salberg, 2021). Well construction details for the monitoring wells are presented in Table 2 .…”