DWCZ-MEF-Data from Warix et al. (2023), Local topography and hydraulic conductivity influence riparian groundwater age and groundwater-surface water connection

The western United States is experiencing increasing rain-to-snow ratios due to climate change, and the resulting effects on groundwater–surface water connections remain uncertain. We examined how watershed topography and streambed hydraulic conductivity influence groundwater age and stream discharge at eight sites along a headwater stream within the Manitou Experimental Forest, Colorado, USA.

To do so, we measured:

• continuous stream and groundwater discharge, water level, and specific conductivity from April to November 2021;
• biweekly stream and groundwater chemistry;
• groundwater chlorofluorocarbons (CFCs) and tritium in spring and fall;
• streambed hydraulic conductivity; and
• local slope.

We used the chemistry data to calculate fluorite saturation states that informed an end-member mixing analysis of streamflow sources. We then combined CFC and tritium data to estimate the age composition of riparian groundwater.

Our results suggest that future stream drying is more probable where local slope is steep and streambed hydraulic conductivity is high. In these areas, groundwater source shifted seasonally — as indicated by age increases — and we observed a high fraction of groundwater in streamflow, primarily interflow from adjacent hillslopes. In contrast, where local slope is flatter and streambed hydraulic conductivity is low, streamflow is more likely to persist, with seasonally constant groundwater age buffered by storage in alluvial sediments.

Paired measurements of groundwater age and streamflow, together with characterization of watershed topography and subsurface properties, identify likely controls on future stream-drying patterns.