Spatiotemporal dynamics of water sources in a mountain river basin inferred through δ²H and δ¹⁸O of water

Abstract

In mountainous river basins of the Pacific Northwest, climate models predict that winter warming will result in increased precipitation falling as rain and decreased snowpack. A detailed understanding of the spatial and temporal dynamics of water sources across river networks will help illuminate climate change impacts on river flow regimes. Because the stable isotopic composition of precipitation varies geographically, variation in surface water isotope ratios indicates the volume-weighted integration of upstream source water. We measured the stable isotope ratios of surface water samples collected in the Snoqualmie River basin in western Washington over June and September 2017 and the 2018 water year. We used ordinary least squares regression and geostatistical Spatial Stream Network models to relate surface water isotope ratios to mean watershed elevation (MWE) across seasons. Geologic and discharge data was integrated with water isotopes to create a conceptual model of streamflow generation for the Snoqualmie River. We found that surface water stable isotope ratios were lowest in the spring and highest in the dry, Mediterranean summer, but related strongly to MWE throughout the year. Low isotope ratios in spring reflect the input of snowmelt into high elevation tributaries. High summer isotope ratios suggest that groundwater is sourced from low elevation areas and recharged by winter precipitation. Overall, our results suggest that baseflow in the Snoqualmie River may be relatively resilient to predicted warming and subsequent changes to snowpack in the Pacific Northwest.

Empirical stream thermal sensitivity cluster on the landscape according to geology and climate

“…Our results show that annual patterns in thermal sensitivity are diverse and mediated by the underlying geology and climate across two Pacific Northwest river basins. Climate change is decreasing snowpack in the region, resulting in earlier runoff and extended summer baseflow (Elsner et al, 2010;Wu et al, 2012), and may decrease groundwater discharge depending on the sources and timing of recharge (Brooks et al, 2012;McGill et al, 2021). For many of our study sites, thermal sensitives were highest in late summer during the hottest, lowest flow portion of the year.…”

“…An investigation of the underlying geology across the Snoqualmie and Wenatchee basins supports our conclusion that low thermal sensitivities are indicative of groundwater inputs. The lowland portion of the Snoqualmie watershed contains a deep, permeable, and productive glacial aquifer that is presumed to be the source of summer baseflow to much of the river (Bethel, 2004;McGill et al, 2021;Turney et al, 1995). Glacial and interglacial deposits in the valley contain several geohydrologic units with differing aquifer potentials (Bethel, 2004); however, most deposits can form small but useable aquifers that could help to sustain baseflow in summer months (Turney et al, 1995;Soulsby et al, 2004;Blumstock et al, 2015).…”

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

“…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.…”

Daily stream temperature predictions for free-flowing streams in the Pacific Northwest, USA

“…Many statistical models use air temperature as a proxy to represent physical energy exchanges driven by solar radiation and thermal conduction (e.g., [31]). However, as with the example of groundwater above, the relationship between air temperature and stream temperature can change across seasons and years as it is mediated by snowmelt and river discharge [24,32,33]. For example,…”