Presentation Title

Determining Spatial and Temporal Controls on Snow Isotopic Signature and Tracing the Snowmelt Pulse as it Moves Through Two Montane Catchments

Presentation Type

Oral Presentation

Category

STEM (science, technology, engineering, mathematics)

Abstract/Artist Statement

Snow accumulation, storage and melt play an essential role in the hydrological processes of snow-dominated mountainous catchments. Low evapotranspiration rates during the winter season make groundwater recharge more sensitive to snow inputs than rainfall. Climate change is projected to globally alter snowpack amount and melt timing and this study aims to help understand how these predicted changes will impact water availability.

In this study we investigate the spatial heterogeneity of stable isotopes of oxygen and hydrogen in snow across two mountainous catchments to estimate the input of snow to local soil, bedrock aquifers, and streams. Stable isotopes are well established as environmental tracers in hydrogeologic systems, owing to the distinct ratios of oxygen and hydrogen isotopes imparted during thermodynamic processes such as phase changes during snowfall, snow metamorphism and snow melt.

Snowpack cores were collected on four sampling runs throughout the snow accumulation and ablation seasons at ten sites in the Lubrecht Experimental Forest near Greenough, MT. Sites were selected to encompass a range of four topographic factors of interest: elevation, aspect, slope angle and hillslope position. Each site was instrumented with a passive capillary sampler that collected a seasonally-integrated bulk snowmelt sample. Soil and bedrock wells at five of the ten sites were sampled after the initial snowmelt pulse and stream samples were taken weekly at the outlets of both catchments.

All samples were analyzed for d2H and d18O and deuterium-excess values using a mass spectrometer. 40 Precipitation samples collected monthly since 2018 provide a local meteoric water line (LMWL) of d2H = 7.29 d 18O – 8.00, with snowpack samples plotting on or above the LMWL and all snowmelt samples plotting slightly below, as expected.

Comparison of sample d18O and deuterium-excess values show that no single factor dominates overall isotopic composition. Visualization of the distribution of the d18O signatures by each topographic variable through all sampling runs shows that spatial trends are weak and affect only isolated parts of the snow cycle. The strongest trends were observed were 1)elevation is negatively correlated with d-excess values in the snowpack and 2) slope angle controls how much enrichment occurs as the snowpack melts with low angle slopes showing the highest enrichment.

Temporal trends in snow isotopic signatures are shown to outweigh spatial trends, with all samples showing enrichment over time, and spatial factors affecting signatures during different phases of snow accumulation and melt.

Mixing models using snowmelt, soil, groundwater and stream isotopic ratios as end-members are used to quantify the source partitioning and timing of these reservoirs across the water year. The models allow us to separate different waters by their unique chemistry and back out how the different water reservoirs interact. Model results allow us to trace the snowmelt pulse temporally as it moves through the catchment and quantify snowmelt contributions to groundwater and streams.

A quantitative understanding of the spatial variability of stable isotopes in complex terrain will help to refine sampling techniques for studies of groundwater recharge from snow. Knowledge of the timing, location and amount of snowmelt input to headwater catchments will help to quantify effects of a changing climate on groundwater resources and help inform water resource management decisions.

Personal Statement

Groundwater is an overlooked and poorly understood component of water resource management because it is not as visible or measurable as surface water due to the paucity of observation wells available to researchers. Communities in arid and semi-arid climates depend heavily on groundwater resources to supply drinking, agricultural and industrial water. Demand for groundwater is increasing as populations grow and climate change alters the timing and amount of recharge to groundwater systems. In snow-dominated, mountainous terrain like Western Montana, valley communities depend heavily on groundwater that is sourced from snowmelt in the surrounding mountains. For example: the underlying aquifer is the sole source of water for residents in the Missoula Valley, and is composed predominantly of snowmelt from the surrounding mountain ranges. For such a vital component of the hydrologic system, snowmelt recharge processes remain poorly understood. One way that we can open the ‘black-box’ of these subsurface processes is by using stable isotope ratios of hydrogen and oxygen in water as environmental tracers to quantify the timing and magnitude of snowmelt recharge in mountainous catchments. Stable isotopes of water have been widely used in the hydrogeologic community to understand snow-groundwater interactions, but wide knowledge gaps persist regarding the variation of these tracers in both time and space. Without knowledge of how landscape factors and timing affect snow stable isotope signatures, quantification of snow inputs and timing is not accurate. This work aims to build on current research within hydrogeology to better understand snowmelt recharge processes vital to many communities globally. By developing an understanding of the spatiotemporal variation of snowpack and snowmelt isotopic signatures, we can determine by what degree different landscape characteristics affect these tracers. This knowledge will aid in producing better estimates of snow-derived groundwater availability, allowing hydrogeologists and hydrologists to make better management decisions. With climate change affecting weather patterns and reducing snowpacks across the globe, this is an important component of water resource planning.

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Mar 4th, 9:20 AM Mar 4th, 9:35 AM

Determining Spatial and Temporal Controls on Snow Isotopic Signature and Tracing the Snowmelt Pulse as it Moves Through Two Montane Catchments

UC 332

Snow accumulation, storage and melt play an essential role in the hydrological processes of snow-dominated mountainous catchments. Low evapotranspiration rates during the winter season make groundwater recharge more sensitive to snow inputs than rainfall. Climate change is projected to globally alter snowpack amount and melt timing and this study aims to help understand how these predicted changes will impact water availability.

In this study we investigate the spatial heterogeneity of stable isotopes of oxygen and hydrogen in snow across two mountainous catchments to estimate the input of snow to local soil, bedrock aquifers, and streams. Stable isotopes are well established as environmental tracers in hydrogeologic systems, owing to the distinct ratios of oxygen and hydrogen isotopes imparted during thermodynamic processes such as phase changes during snowfall, snow metamorphism and snow melt.

Snowpack cores were collected on four sampling runs throughout the snow accumulation and ablation seasons at ten sites in the Lubrecht Experimental Forest near Greenough, MT. Sites were selected to encompass a range of four topographic factors of interest: elevation, aspect, slope angle and hillslope position. Each site was instrumented with a passive capillary sampler that collected a seasonally-integrated bulk snowmelt sample. Soil and bedrock wells at five of the ten sites were sampled after the initial snowmelt pulse and stream samples were taken weekly at the outlets of both catchments.

All samples were analyzed for d2H and d18O and deuterium-excess values using a mass spectrometer. 40 Precipitation samples collected monthly since 2018 provide a local meteoric water line (LMWL) of d2H = 7.29 d 18O – 8.00, with snowpack samples plotting on or above the LMWL and all snowmelt samples plotting slightly below, as expected.

Comparison of sample d18O and deuterium-excess values show that no single factor dominates overall isotopic composition. Visualization of the distribution of the d18O signatures by each topographic variable through all sampling runs shows that spatial trends are weak and affect only isolated parts of the snow cycle. The strongest trends were observed were 1)elevation is negatively correlated with d-excess values in the snowpack and 2) slope angle controls how much enrichment occurs as the snowpack melts with low angle slopes showing the highest enrichment.

Temporal trends in snow isotopic signatures are shown to outweigh spatial trends, with all samples showing enrichment over time, and spatial factors affecting signatures during different phases of snow accumulation and melt.

Mixing models using snowmelt, soil, groundwater and stream isotopic ratios as end-members are used to quantify the source partitioning and timing of these reservoirs across the water year. The models allow us to separate different waters by their unique chemistry and back out how the different water reservoirs interact. Model results allow us to trace the snowmelt pulse temporally as it moves through the catchment and quantify snowmelt contributions to groundwater and streams.

A quantitative understanding of the spatial variability of stable isotopes in complex terrain will help to refine sampling techniques for studies of groundwater recharge from snow. Knowledge of the timing, location and amount of snowmelt input to headwater catchments will help to quantify effects of a changing climate on groundwater resources and help inform water resource management decisions.