Geologic features (e.g., fractures and alluvial fans) can play an important role in the locations and volumes of
groundwater discharge and degree of groundwater-surface water (GW-SW) interactions. However, the role of
these features in controlling GW-SW dynamics and streamflow generation processes are not well constrained.
GW-SW interactions and streamflow generation processes are further complicated by variability in precipitation
inputs from summer and fall monsoon rains, as well as declines in snowpack and changing melt dynamics driven
by warming temperatures. Using high spatial and temporal resolution radon and water stable isotope sampling
and a 1D groundwater flux model, we evaluated how groundwater contributions and GW-SW interactions varied
along a stream reach impacted by fractures (fractured-zone) and downstream of the fractured hillslope (non-
fractured zone) in Coal Creek, a Colorado River headwater stream affected by summer monsoons. During early
summer, groundwater contributions from the fractured zone were high, but declined throughout the summer.
Groundwater contributions from the non-fractured zone were constant throughout the summer and became
proportionally more important later in the summer. We hypothesize that groundwater in the non-fractured zone
is dominantly sourced from a high-storage alluvial fan at the base of a tributary that is connected to Coal Creek
throughout the summer and provides consistent groundwater influx. Water isotope data revealed that Coal Creek
responds quickly to incoming precipitation early in the summer, and summer precipitation becomes more
important for streamflow generation later in the summer. We quantified the change in catchment dynamic
storage and found it negatively related to stream water isotope values, and positively related to modeled
groundwater discharge and the ratio of fractured zone to non-fractured zone groundwater. We interpret these
relationships as declining hydrologic connectivity throughout the summer leading to late summer streamflow
supported predominantly by shallow flow paths, with variable response to drying from geologic features based
on their storage. As groundwater becomes more important for sustaining summer flows, quantifying local
geologic controls on groundwater inputs and their response to variable moisture conditions may become critical
for accurate predictions of streamflow.
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Intermittent streamflow generation in a merokarst headwater catchment
Intermittent headwater streams are highly vulnerable to environmental disturbances, but effective management of these water resources requires first understanding the mechanisms that generate streamflow. This study examined mechanisms governing streamflow generation in merokarst terrains, a type of carbonate terrain that covers much of the central United States yet has received relatively little attention in hydrological studies. We used high-frequency sampling of precipitation, stream water, and groundwater during summer 2021 to quantify the contributions to streamflow from different water sources and characterize their short-term dynamics in a 1.2 km 2 merokarst catchment at the Konza Prairie Biological Station (Kansas, USA). Mixing calculations using stable water isotopes and dissolved ions indicate that streamflow is overwhelmingly contributed by groundwater discharge from thin (1–2 m) limestone aquifers, even during wet periods, when soil water and surface runoff are generally expected to be more important. Relationships between hydraulic heads in the aquifers and their contributions to streamflow differed early in the study period compared to later, after a major storm occurred, suggesting there is a critical threshold of groundwater storage that the bedrock needs to attain before fully connecting to the stream. Furthermore, contributions from each limestone unit varied during the study period in response to differences in their hydrogeological properties and/or their stratigraphic position, which in turn impacted both the length of streamflow and its composition. Taken together, we interpret that the subsurface storage threshold and variation in aquifer properties are major controllers of flow intermittency in merokarst headwater catchments.
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- PAR ID:
- 10435455
- Publisher / Repository:
- Environmental Science: Advances
- Date Published:
- Journal Name:
- Environmental Science: Advances
- Volume:
- 2
- Issue:
- 1
- ISSN:
- 2754-7000
- Page Range / eLocation ID:
- 115 to 131
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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