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

Abstract

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 (δ 18 O 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 δ 18 O 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 δ 18 O 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 δ 18 O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ 18 O 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 midsummer 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.

Hydrological dynamics of snowmelt induced streamflow in a high mountain catchment of the Pyrenees under contrasting snow accumulation and duration years

“…These uncertainties can be reduced in future research by obtaining sublimation estimates from eddy‐covariance towers and employing more sophisticated simulations of the catchment's hydrological cycle that offer a more accurate representation of soil characteristics and groundwater processes. In addition, despite the considerable number of water samples taken in this study, the application of hydrograph separation based on water isotopes is complicated by the lack of detailed control of isotopic variations in individual precipitation events, water stored in soils and groundwater, and distributed samples of snowpack isotopy (Kamensky, 1998; Lee et al, 2010; Leuthold et al, 2021; Schmieder et al, 2016). Such monitoring should also be considered in further research.…”

“…During the melting season, rain provides a large streamflow contribution, and the meltwater contribution is often difficult to infer. Stable water isotopes (generally δ 2 H and δ 18 O) have helped to better understand the contribution of snowmelt to streamflow and the residence time of melting water in the catchments (Leuthold et al, 2021; McGill et al, 2021; Penna et al, 2017), thanks to the more depleted isotopic values of the melted snow compared to streamflow (McGill et al, 2021; Vystavna et al, 2021). However, a full separation of the contribution of each component is difficult to obtain, since it requires a very intense spatially and temporally isotopic sampling of each component.…”

Hydrological dynamics of snowmelt induced streamflow in a high mountain catchment of the Pyrenees under contrasted snow accumulation and duration years

“…The mean isotopic values of the recorded series show strong interannual differences, exhibiting higher (lower) values during the snow poor (rich) seasons. However, the application of hydrograph separation based on water isotopes is complicated by the lack of detailed control of isotopic variations in individual precipitation events, water stored in soils and groundwater, and distributed samples of snowpack isotopy (Kamensky, 1998;Lee et al, 2010;Leuthold et al, 2021;Schmieder et al, 2016). Such monitoring should be considered in further research.…”

“…During the melting season, rain provides a large streamflow contribution, and the meltwater contribution is often difficult to infer. Stable water isotopes (generally δ 2 H and δ 18 O) have resulted extremely useful to better understand the contribution of snowmelt to streamflow and the residence time of melting water in the catchments (Leuthold et al, 2021;McGill et al, 2021;Penna et al, 2017), thanks to the more depleted values of snow isotopy compared to streamflow (McGill et al, 2021;Vystavna et al, 2021). However, a full separation of the contribution of each component is difficult to obtain, since it requires a very intense spatially and temporally isotopic sampling of each component.…”

Water isotopic composition traces source and dynamics of water supply in a semi‐arid agricultural landscape

“…To allow assessment of meaningful deviation of environmental waters from the LMWL, the analytical uncertainty estimates of 0.6‰ for δ 18 O and 0.9‰ for δ 2 H were used in a Monte Carlo propagation of uncertainty to the LMWL, following the approach of Leuthold et al (2021). The Monte Carlo ensemble consisted of 5000 realizations to propagate analytical uncertainty to the slope and intercept of the LMWL calculated from precipitation sample results.…”