Quantifying annual groundwater recharge and storage in the central Sierra Nevada using naturally occurring³⁵S

Abstract

Identifying aquifer vulnerability to climate change is of vital importance in the Sierra Nevada and other snow-dominated basins where groundwater systems are essential to water supply and ecosystem health. Quantifying the component of new (current year's) snowmelt in groundwater and surface water is useful in evaluating aquifer vulnerability because significant annual recharge may indicate that streamflow will respond rapidly to annual variability in precipitation, followed by more gradual decreases in recharge as recharge declines over decades. Hydrologic models and field-based studies have indicated that young (<1 year) water is an important component of streamflow. The goal of this study was to utilize the short-lived, naturally occurring cosmogenic during baseflow conditions to 14.0 ± 3.4% during high-flow periods of snowmelt. Similar to SCB, the PNS in MVGB groundwater and streamflow was typically <30% with the largest fractions occurring in late spring or early summer following peak streamflow. The consistently low PNS suggests that a significant fraction of annual snowmelt in SCB and MVGB recharges groundwater, and groundwater contributions to streamflow in these systems have the potential to mitigate climate change impacts on runoff. KEYWORDSgroundwater recharge, mountain groundwater, snowmelt infiltration, sulfur-35Groundwater vulnerability to climate change in high-elevation basins has widespread implications for ecosystem health and water supply (Earman & Dettinger, 2011;Earman et al., 2015). In the mountains of the western United States, groundwater is a major component of streamflow, even during peak snowmelt conditions (e.g., Frisbee et al., 2011;Liu et al., 2004). Spatial and temporal changes in snow dynamics, such as declines in snowpack accumulation (Mote 2003;Mote et al., 2005) and earlier onset of snowmelt Knowles et al., 2006;Mote et al., 2005), are of particular concern for Sierra Nevada basins because groundwater recharge is mainly derived from snowpack for most of the southwest (Earman et al., 2006;Winograd et al., 1998). Groundwater recharge in high-elevation basins in the western United States is important for aquifer replenishment (Wilson & Guan, 2004;Manning & Solomon, 2005) and ecosystem health, yet the impact of climate change on groundwater recharge is poorly understood (Earman & Dettinger, 2011;Earman et al., 2015;Viviroli et al., 2011).Understanding climate change impacts on groundwater resources in the Sierra Nevada and other high-elevation basins is difficult because of a weak understanding of direct and indirect effects of climate change on mountain recharge processes (Earman & Dettinger, 2011;Earman et al., 2015). Current forecasts of the effects of climate change vary widely. In snow-dominated basins that are predicted to experience a shift in precipitation from snow to rain, groundwater recharge may decrease because snow is a more efficient recharging mechanism than rain (Earman et al., 2006;Meixner et al., 2016;Winograd et al., 1998 however, no data exist showing t...

Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone

“…Few prior applications of sodium‐22 have been published (Kaste et al, ) and are mostly limited to lake residence time studies (Sakaguchi et al, ). Applications of sulfur‐35 are more widespread, and recent studies have demonstrated its use to derive the residence time of managed aquifer recharge (Clark et al, ; Urióstegui et al, , ) and alpine catchments (Urióstegui et al, ) in conjunction with other age tracers. Tritium and oxygen‐18 combined are powerful tracers for watersheds with similar characteristics, while sodium‐22 and sulfur‐35 would be more beneficial if shorter flow paths dominate the stream flow generation.…”

“…Sulfur‐35 immobilization (e.g., uptake by vegetation or soil microbial processes) has been discussed in prior studies (Böhlke & Michel, ; Michel et al, ) and needs to be investigated further. Analysis of the stable sulfur isotopic composition (sulfur‐34/sulfur‐32) of sulfate in hydrological compartments is promising (Urióstegui et al, ).…”

A review of groundwater in high mountain environments

“…At the same time, the annual pulse of meltwater percolates toward the saturated zone (Hammond, Harpold, Weiss, & Kampf, 2019). In the Sierra Nevada of California, analysis of shortlived cosmogenic sulfur isotopes revealed that less than 15% of freshet streamflow originated from the previous winter's snow pack and that a significant fraction of the annual snowmelt was recharging the groundwater system (Urióstegui et al, 2017). Snowmelt is a more efficient contributor to streamflow (through shallow runoff) than rainfall because the concentrated period of infiltration allows less time for evapotranspiration compared to intermittent precipitation and snowmelt generally occurs in the spring when potential evapotranspiration is lower.…”

“…Baraer et al (2015) found that groundwater contributes 24–80% of dry season stream discharge in four proglacial valleys of the Cordillera Blanca in the northern Peruvian Andes, while Wang et al (2017) found that groundwater contributed 38% of streamflow in a glacierized watershed of the Tianshan Mountains in China. More recent work has increased the spatial and temporal resolution of results, examined fine‐scale groundwater flow paths and made use of new tracers such as chloride isotopes (Shaw et al, 2014), sulfur isotopes (Urióstegui, Bibby, Esser, & Clark, 2017), and dissolved noble gasses (Gleeson et al, 2018).…”

“…Groundwater in high mountain areas is recharged by precipitation in the form of rain and/or snowmelt, and glacier melt. The amount of groundwater recharge is spatially variable and is affected by sharp gradients in climate, vegetation, geology, and topography (Goulden et al, 2012; Smerdon, Allen, Grasby, & Berg, 2009; Smith, Moore, Weiler, & Jost, 2014; Urióstegui et al, 2017). The groundwater recharge that originates in mountains can be important for distal groundwater systems in adjacent lowlands, potentially several hundred kilometers away.…”