Seasonal connections between meteoric water and streamflow generation along a mountain headwater stream

• Published in: Hydrological processes Vol. 35; no. 2
• Main Authors: Leuthold, Sam J., Ewing, Stephanie A., Payn, Robert A., Miller, Florence R., Custer, Stephan G.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.02.2021; Wiley Subscription Services, Inc
• Subjects: altitude; Atmospheric precipitations; Catchments; climate; Climate change; Composition; Connectivity; deuterium; Elevation; Groundwater; Groundwater storage; Headwater catchments; Headwaters; humans; Hydrogen; Hydrology; Isotope composition; Isotopes; isotopic lapse rate; Lapse rate; Meteoric water; mineralization; Moisture content; Montana; mountainous watersheds; Mountains; nitrate flushing; nitrates; nitrogen; Oxygen; Precipitation; proximal area contributions; Rain; Rainfall; Rivers; Runoff; Seasonal rainfall; Seasons; Snowmelt; Soil; Soil water; Soil water storage; Stream discharge; Stream flow; Stream water; streamflow generation; streams; Summer; Summer precipitation; Surface water; Transpiration; water isotope; Water management; Water sources; Water storage; Watersheds; Wetlands; winter; Montana
• DOI: 10.1002/hyp.14029
• ISSN: 0885-6087

In snowmelt‐driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ18O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high‐relief mountainous catchments. We characterized δ18O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ18O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow‐derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ18O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ18O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late‐winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid‐summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions. Assessments of seasonal stable water isotope trends in a mountain watershed indicate increased connectivity between areas proximal to streams and streamflow during the winter period. The use of a longitudinal sampling scheme provided a mechanism for relating changes in elevation with hillslope‐stream connectivity, taking advantage of the isotopic lapse rate of precipitation. Correlations between elevation and δD are strongest during February and March, and weak during July and August, indicating increased transmission of meteoric water during winter and spring, and decreased later in the year.