Groundwater flowing within mountainous regions is a critical water source for mountain and adjacent low-elevation communities and ecosystems. Despite their importance, quantifying storage changes within these groundwater systems remains a challenge. To evaluate the fate of mountain groundwater and the role of extremely wet winters in replenishing groundwater storage, we use high-precision Global Navigation Satellite Systems surface displacements to constrain changes in groundwater storage within the Sierra Nevada and Cascade mountain ranges, two important mountainous regions spanning much of the westernmost US. We find that groundwater storage has significantly declined over the past two decades, comprising over 90% of the total water storage lost in these regions, associated with recurrent, severe, multi-year droughts. However, extremely wet winters can replenish groundwater by more than twice the average annual flux over short periods, driving these systems from historical lows to above normal conditions. Furthermore, discharge from these groundwater systems is 2-3 times less variable than recharge, indicating they release a relatively constant amount of water year over year, allowing for surplus storage following extremely wet winter periods. These findings show that extreme precipitation (snow + rain) rapidly recharges mountain groundwater and can offset multi-year losses. Moreover, mountain groundwater systems can retain storage gains for at least 1 year post-event, providing a durable source of streamflow, agricultural, and municipal water supply. As extremely wet winters are predicted to become more intense and frequent, we hypothesize they may help offset groundwater loss due to aridification, sustaining ecosystems and buffering against a new, drier climate normal.

Groundwater in mountainous, fractured-bedrock systems is increasingly recognized as a critical component of water storage and transmission, yet characterizing the hydraulic properties of these systems remains challenging due to sparse observations. Here, we use estimates of groundwater storage change in the Sierra Nevada and Cascades mountain ranges over the last two decades derived from Global Navigation Satellite Systems (GNSS) observations of vertical surface displacement to estimate mountain-range-scale effective groundwater hydraulic parameters. We estimate mountain range scale effective hydraulic conductivity and storativity by fitting models of groundwater storage change to GNSS-derived storage estimates during groundwater discharge periods. We estimate mean hydraulic conductivity (8.0 x 10-6 m/s and 5.1 x 10-5 m/s) and storativity (0.063 and 0.15) for the Sierra Nevada and Cascades, respectively, which align with published values based upon their unique bedrock lithologies. The relatively high effective bedrock conductivities within each region indicate that fractured and/or weathered bedrock strongly influence the storage and release properties of these groundwater systems at regional scales. Estimated characteristic drainage lengths of 740 m for the Sierra Nevada are consistent with average channel spacing of both perennial and intermittent streams, while the characteristic drainage length of 984 m for the Cascades is consistent with the average spacing of perennial streams only, indicating differing connections between surface water and groundwater within each region. This new method demonstrates the utility of geodetic observations in studying mountain groundwater and may provide improved constraints on large-scale simulations of mountain groundwater flow, furthering our understanding of these systems.

Abstract Intensification of hydroclimatic volatility increases the potential for large floods to reshape rivers and threaten infrastructure. Fluvial geomorphic resilience, defined as a river's resistance to, and recovery from, flood disturbance, offers a framework for understanding river response to extreme events and guiding management. We assessed resilience at two scales: (1) a detailed geomorphic and hydraulic analysis of the June 2022 flood on East Rosebud Creek, Montana, which was part of broader flooding in the Greater Yellowstone Ecosystem, and (2) a multiriver comparison of 13 flood‐impacted alluvial mountain rivers (East Rosebud Creek plus 12 additional sites across North America and Europe). We quantified resistance using active‐channel width ratios () and recovery using postflood channel evolution () and recovery rate (). East Rosebud Creek showed strong spatial variability in disturbance among two defined segments, with an upstream segment shifting to new forms via widening, coarsening and/or incision, whereas a downstream segment that experienced similar flow strengths adjusted more modestly, within a dynamic geomorphic equilibrium. Slope, confinement and grain size were key influences on flood response in East Rosebud Creek. In the multiriver analysis, multilevel regression showed a positive log‐transformed relationship between normalized flood peak exceedance above the () and mean and a negative linear relationship between mean slope and mean . Multiriver results suggest that slope‐driven influences on vegetation growth shape recovery. Understanding patterns of resilience can improve flood adaptation strategies and inform targeted river management.

Groundwater age distributions provide fundamental insights on coupled water and biogeochemical processes in mountain watersheds. Field-based studies have found mixtures of young and old-aged groundwater in mountain catchments underlain by bedrock; yet, the processes that dictate these groundwater age distributions are poorly understood. In this work, we use the coupled ParFlow-CLM integrated hydrologic and EcoSLIM particle tracking models to simulate groundwater age distributions on a lower montane hillslope in the East River Watershed, Colorado (USA). We develop a convolution-based approach to propagate fracture-matrix diffusion processes to the EcoSLIM advection-dominated age distributions. We compare observed 3H and 4He concentrations from two groundwater wells against model predictions that have varying advective transport times and matrix diffusion magnitudes. Based on a Monte Carlo analysis that considers uncertain matrix and fracture parameters, we find that matrix diffusion is needed to jointly predict 3H and 4He observations at both wells. The advection-dominated age distributions lack adequate mixing of young and old-aged water to capture the observed co-occurrence of 3H and 4He. The model scenario that best matches the 3H, 4He, and water level observations when considering both advective flowpath and matrix diffusion mixing processes has a dynamic bedrock groundwater reservoir that is susceptible to considerable storage losses during low-snow periods. This dynamic groundwater system amplifies the need to assimilate deeper bedrock groundwater into watershed hydro-biogeochemical predictions. This work further highlights the importance of considering matrix diffusion when interpreting environmental tracers in bedrock groundwater systems.

During periods of drought, quantifying the intensity of water loss within hydrologic reservoirs, both on and below the surface, is critical to sustain water resources. Drought intensity is typically characterized using drought indices which are driven by meteorologic observations, such as precipitation. These drought indices provide good insight into the quantity of water entering the hydrologic system, however, they are unable to quantify the amount of water retained in a watershed or the amount lost due to runoff and evapotranspiration. We address this by leveraging the sensitivity of three-dimensional Global Positioning System displacements to local and regional hydrologic-storage fluctuations, and produce a new geodetic drought index (GDI), derived from estimated hydrologic-storage deviations, to directly characterize hydrologic storage anomalies. The GDI is derived comparably to the Standardized Precipitation Evapotranspiration Index such that it may be easily incorporated into current drought management workflows. We directly compare the GDI to hydrologic observations within California and find strong associations between specific time scales of the GDI and groundwater well, artificial-reservoir storage, and stream discharge observations. The GDI is most sensitive to groundwater, exhibiting a correlation coefficient of 0.87 at the 3-month time scale. Both artificial-reservoir storage and stream discharge exhibit peak correlation coefficients when considering the 1-month GDI, at 0.81 and 0.47 respectively. No relationship is observed with soil moisture observations. The correlation coefficients decline rapidly away from the optimal time scale, indicating the 1- and 3-month GDI are strong predictors of hydrologic variation within California. In addition to capturing long-term trends, rapid changes in the GDI initiate during clusters of large atmospheric-river events that closely mirror fluctuations in the hydrologic observations. The GDI provides an opportunity to improve hydrologic models for drought-management and to advance our understanding of the water cycle.

Groundwater flowing through the fractured bedrock composing most mountain ranges has been increasingly recognized as a vital source of freshwater for both low-elevation communities and mountain ecosystems, maintaining streamflow and constituting a large portion of recharge to lowland aquifers used to support human activities. Despite the growing awareness of groundwater’s role in mountain hydrology and the potential impacts of climate change on mountain groundwater, it remains a challenge to study the dynamics of mountain aquifers, largely due to the low density of observational wells and challenges in characterizing the mountain block over large areas and depths. Here, we report on a new approach to characterize the flow and hydraulic properties of mountainous aquifers at a mountain range scale. We utilize high-precision Global Navigation Satellite Systems (GNSS) observations of vertical crustal displacement produced by the redistribution of freshwater on or near the Earth’s surface to estimate changes in groundwater storage within the Sierra Nevada and Cascades Range of the western United States with high spatial (10s of kilometer) and temporal (daily) resolution over the past two decades. We find that on average groundwater annual recharge is less than discharge, driving long-term declines in groundwater storage over the last 19 years. Furthermore, we find groundwater recharge to be up to 3x more variable than groundwater discharge in these mountainous areas, suggesting that mountain aquifers release a relatively constant amount of water to streams and adjacent lowland aquifers despite fluctuating recharge conditions. Utilizing identified periods of groundwater discharge, we characterize the hydraulic conductivity, storativity, and flow path length of these groundwater systems using fluid diffusion models in combination with our GNSS-inferred groundwater estimates. Our initial estimates of these parameters reveal relatively high values of bedrock conductivity (~1x10-3-1x10-4 m/s) relative to expected values based upon each region’s bedrock lithology, suggesting that areas with highly fractured bedrock as well as saprolite may exert a strong control on groundwater discharge at the mountain range scale. Furthermore, our results indicate that groundwater flow paths can span lengths on the order of 100s-1000s of meters, supporting the notion that groundwater can flow over extended areas supporting recharge at both a local and regional scales. Our work seeks to provide a new set of tools for hydrologists to investigate these often poorly understood systems.

We are strengthening the application of GPS's capability to estimate change in total water using measurements of elastic displacements of Earth's surface; breaking down total water into its components such as snow, soil moisture, and groundwater; and integrating GRACE gravity data to infer change in total water in groundwater basins.In California's Sierra Nevada, GPS each day tracks the dumping and dissipation of storm water. In Water Year 2023, total water increased abruptly during each of two sequences of snow-dominated atmospheric rivers. Subsurface water, which we take to be total water inferred from GPS minus snow water equivalent, to rise in early January at the time of the first AR sequence, remain constant from late Jan through March (with no increase during the second AR sequence), and rise from April to June as the snowpack melts. Subsurface water increases in the Sierra Nevada by 0.6 m from Oct 2022 to Jun 2023, 45 per cent of cumulative precipitation of 1.4 m. Such a big rise in subsurface water begins to rejuvenate the Sierra Nevada critical zone (Earth's living outer layer between the top of the trees and the bottom of groundwater) and to replenish subsurface water lost during the prior 3 years of drought from 2020 to 2022.Change in total water in California's Central Valley can be determined neither by GRACE alone nor GPS alone. There GPS records primarily Earth's poroelastic response, from which water change is difficult to infer. GRACE cannot distinguish water change in Central Valley from water change in the Sierra Nevada without assuming a hydrology model. We integrate GPS elastic displacements and GRACE gravity to estimate water change in the Central Valley. In the rigorous inversion, GPS determines water change in the Sierra Nevada and Coast Ranges and the remaining water change from GRACE is placed in the Central Valley. We find Central Valley groundwater increased by 0.75 m in the first nine months of Water Year 2023 (the biggest gain ever recorded), replenishing more groundwater than lost during the prior 3 years of drought.

Fracture-released radiogenic noble gas nuclides are used to identify locations and constrain the volume of new fracture creation during subsurface detonations. Real-time, in situ noble gases and reactive gases were monitored using a field-deployed mass spectrometer and automated sampling system in a multilevel borehole array. Released gases were measured after two different detonations having distinct energy, pressure, and gas volume characteristics. Explosive-derived gases (N2O, CO2) and excess radiogenic 4He and 40Ar above atmospheric background are used to identify locations of gas transport and new fracture creation after each detonation. Fracture-released radiogenic 4He is used to constrain the volume of newly created fractures with a model of helium release from fracturing. Explosive by-product gas was observed in multiple locations both near and distal to the shot locations for both detonations. Radiogenic 4He and 40Ar release from rock damage was observed in locations near the detonation after the second, more powerful detonation. Observed 4He response is consistent with a model of diffusive release from newly created fractures. Volume of new fractures estimated from the 4He release ranges from 1 to 5 m2 with apertures ranging from 0.1 to 1 mu ${\upmu }$m. Our results provide evidence that radiogenic noble gases released during fracture creation can be identified at the field scale in real time and used to identify timing and location of fracture creation during deformation events. This technique could be useful in subsurface science and engineering problems where the location and amount of newly created rock fracturing is of interest including fault rupture, mine safety, subsurface detonation monitoring and reservoir stimulation.

Anthropologically, drought intensity is measured not by how strongly the rain falls over a few days but by how dry the land becomes over a specific period of time. The duration and intensity of this drying period, affects hydrologic pools (i.e. rivers, lakes, and groundwater) uniquely based on the characteristics of their respective drainage basins. Contrarily, drought management techniques currently rely heavily on meteorologically derived drought indices (e.g., the Standardized Precipitation Evapotranspiration Index), which offer valuable insights into the amount of water entering the system but provide no information about water retention levels. As such, currently only GPS-based drought indices provide direct characterization of hydrologic drought with both high spatial resolution, and daily temporal resolution. To assist in the retention of hydrologic resources, we present an update on the status of the United States GPS-Based Drought Index (US-GDI). Our methodology advances those presented by Young et al, 2024. We leverage the availability of the data provided by the Nevada Geodetic Laboratory, and produce a framework which provides rapid US-GDI hydrologic drought assessment solutions with a latency of ~48 hours. Final solutions are expected within 10-14 days. Solutions for the full study period are calculated daily, with hydrologic load estimates, GDI evaluations between one day and 48 months, and step offsets in the vertical component updated daily. To assess the sensitivity of the US-GDI to hydrologic resources, we present an analysis of the correlation between US-GDI timescales and to stream discharge, surface-reservoir storage/elevations, and groundwater across specific hydrologic units across the United States. To facilitate the distribution of the results, we introduce a webpage which provides direct access to all solutions provided by the US-GDI (including both hydrologic loading estimates, and GDI time scale solution. The US-GDI represents an opportunity to significantly improve hydrologic resource preservation and maintenance during periods of sustained hydrologic drought.

We investigated the contribution of bedrock groundwater to streamflow as a function of catchment scale in a headwater stream. Synoptic surveys were conducted during hydrologically important periods of the year using multiple environmental tracers in stream water, soil water, and bedrock groundwater, along a first-order montane stream, in west-central Montana. Sampled analytes included 222Rn, used to constrain total subsurface flux, and major and minor elements, used in end-member mixing analysis (EMMA) to identify the contributions of soil and bedrock groundwater to the stream. Partitioning between soil-derived and bedrock-derived groundwater was then analyzed as a function of the incremental and accumulated sub-catchment sizes. Radon results indicated that subsurface water contributions accounted for the majority of streamflow at all surveyed times. EMMA results revealed that the bedrock groundwater contribution to streamflow varied between 26% during peak snowmelt and 44% during late summer. Streamflow generation was dominated by soil groundwater contribution along the entire reach, but the bedrock groundwater contribution increased consistently with accumulated sub-catchment size. However, groundwater contributions were not well-correlated with incremental sub-catchment size. The scale at which increased bedrock groundwater discharge can be correlated with sub-catchment size appears to be >1 km2 for our study. Our results are consistent with a conceptual model where streamflow is predominantly generated by a 3D subsurface nested flow system. Local subsurface heterogeneities control the stream source at local scales but begin to average out at scales >2 km2. Our study indicates that, while soil groundwater is the dominant source, bedrock groundwater remains an important and predictable contributor to streamflow throughout the year, even in a snow-dominated, mountainous headwater catchment.

Persistent declines in groundwater storage observed in mountainous regions of the western US over the past two decades are expected to continue, driven by increasingly variable winter temperatures and snowpack accumulation (Carroll et al., 2024; Hall et al., 2024), threatening human and ecosystem health. However, brief but extreme periods of precipitation associated with frequent and intense atmospheric river events deposit significant amounts of water in the mountains of the western US, acting as potentially significant sources of groundwater recharge in an increasingly arid environment. Here, we provide high-resolution estimates of groundwater storage within the western US by removing estimates of water stored in winter snowpack, the soil column, and artificial reservoirs from Global Navigation Satellite Systems (GNSS) inferred estimates of terrestrial water storage (TWS) between January 2006 and June 2024. We find long-term declines in water storage within mountainous regions of the western US such as the Sierra Nevada and Cascades (approx. 355 mmand 105 mm of equivalent water thickness, respectively) align with estimates derived from GRACE/GRACE-FO and watershed mass balance models, corroborating observed aridification within mountainous regions over the past two decades. Despite these declines, we find periods of extreme precipitation, such as winters 2011, 2017, and 2023, can provide more than twice the average annual recharge of mountain groundwater (Fig.1). Furthermore, we find the state of groundwater in many mountainous regions of the west following winter 2023 were driven from record lows in autumn 2022 to above or near normal conditions and have been maintained over the past year despite moderate winter conditions in 2024, indicating that extreme precipitation events can maintain mountain groundwater storage over prolonged periods. As the strength and frequency of atmospheric river events are predicted to increase due to anthropogenic warming (Gershunov et al., 2019; Nellikkattil et al., 2023), we hypothesize that mountain groundwater storage may be maintained by extreme precipitation events in the coming decades.

Coal mining and reclamation can have a profound influence on hydrogeologic systems, with clear consequences for groundwater quality, yet their long-term influence on downgradient water quality over time following reclamation is less well documented. Geochemical trends were evaluated in water quality downgradient of a fully reclaimed landscape at the former Big Sky Mine in the Rosebud Creek watershed (southeastern Montana, USA), over a 3-year period (2020–2022), including bond release in 2022. Within 6 km downgradient from the reclaimed area, sulfate concentrations decreased from approximately 3500 to 1800 mg l−1 within the Miller Coulee alluvial aquifer. Major ions, δ34SSO4 values, and residence time tracers suggest that the observed decreases in sulfate concentration result from a combination of dilution by mixed-age inflows and incomplete transit of the high salinity plume from the mine boundary. Both bedrock and alluvial aquifers of the Rosebud Creek corridor contained contributions of millennia-old regional groundwater, which may serve to mitigate mine-derived high salinity waters. Rosebud Creek, which traverses the outflow zone of Miller Coulee in the study area, exhibited high sulfate concentrations during low flows and consistent downgradient increases in sulfate concentration. The possibility of plume dynamics in Miller Coulee suggests that the greatest water quality impacts may not yet have reached Rosebud Creek.

Up until now, the cosmogenic radioisotope 39Ar has not been used for surface exposure or burial dating of minerals due to its low concentration in rocks and the large sample size requirements for its detection by low-level counting. The novel analytical method Atom Trap Trace Analysis (ATTA) – already well established for gas samples from groundwater, ocean water or ice cores – can measure the isotopic ratio of 39Ar to stable Ar in the range of 10-16 on just a few ml STP of argon and therefore opens up new possibilities for applying 39Ar. This talk will report on the initial steps taken towards using 39Ar as a geochronometer. Calculations of production rates of 39Ar in typical continental rocks, exposed to cosmic radiation at the Earth surface, show that sample sizes of the order of 100 g of rock should yield a sufficient number of 39Ar atoms (order 103 to 104) for detection by ATTA. However, the amount of 40Ar in such samples – and therefore the total extractable Ar amount - is much lower than what is typically extracted from ice and water samples, which contain atmosphere-derived Ar. The 40Ar content in rock stems from 40K-decay and depends on the rock formation age and the potassium content. Dilution with 39Ar-free Ar results in sufficient total argon volumes for the standard ATTA analysis. Gas extractions from heated rock samples indeed show 39Ar isotope abundances 2-3 orders of magnitude above the atmospheric ratio, well within the measurement range of ATTA after dilution. In order to check the feasibility of exposure dating of rocks, several samples were taken for comparison from boulders of glacier moraines, previously dated with 10Be, in the Jamtal valley in Austria. Additionally, 10Be-dated samples from other moraine sites are to be analysed for 39Ar for further validation of 39Ar as a tool for exposure dating. As of now it remains to be seen whether reliable agreement between the dating methods can be achieved. Due to its relatively short half-life of 268 years, 39Ar would be a useful addition in multi-tracer studies on geologic processes within the last two millennia.

Buried bedrock valleys in North America are often filled with coarse-grained deposits that form productive buried-valley aquifers, providing a valuable water source for human use. However, the role of buried-valley aquifers in the hydrology of watersheds, including baseflow generation and supporting ecosystems, is not widely recognized. This study demonstrates that the presence of a buried-valley aquifer influences near-surface hydrology, specifically the interaction of a creek coaligned with the bedrock valley thalweg. We combine synoptic sampling of environmental tracers and geochemistry of the creek during low flow conditions and regional groundwater data to determine downstream variation in groundwater interaction. Modelled downstream variation suggests that the greatest amount of groundwater discharge occurs where the buried-valley aquifer is present beneath, but not necessarily in direct contact with the creek. Further, a shift in baseflow source to the creek occurs depending on the depth to bedrock, indicated by a change in water isotopic and chemical composition similar to that of groundwater located in the sediments above bedrock to a signature similar to groundwater in the underlying bedrock formations. This study demonstrates that buried-valley aquifers can focus groundwater flow to a modern-day creek valley, providing a unique source of baseflow in the Canadian Prairies and midwestern United States. The interaction between buried-valley aquifers and streams has implications for water allocation and land use planning, whereby a critical baseflow source may also be a target aquifer to develop with good quality water.

Drought intensity is commonly characterized using meteorologicly-based metrics that struggle to provide insight into water deficits within deeper hydrologic systems. In contrast, Global Positioning System (GPS) displacements are sensitive to both local and regional hydrologic-storage fluctuations. While a few studies have leveraged this sensitivity to produce geodetic drought indices, hydrologic drought characterization using GPS is not commonly accounted for in drought assessment and management. To motivate this application, we produce a new geodetic drought index (GDI) and quantify its ability to characterize hydrologic drought conditions in key surface and sub-surface hydrologic reservoirs across California. In northern California, the GDI exhibits a strong regional association with reservoir storage at the 1-month time scale (correlation coefficient: 0.83) and groundwater levels at the 3-month time scale (correlation coefficient: 0.87), along with moderate associations with stream discharge at the daily (instantaneous) time scale (correlation coefficient: 0.50). Groundwater in southern California is best characterized with a 12-month GDI (correlation coefficient: 0.77), and reservoir storage is optimized with the 3-month GDI (correlation coefficient: 0.72). Differences between northern and southern California reveal that the GDI is sensitive to unique aquifer and drainage basin characteristics. In addition to capturing long-term hydrologic trends, rapid changes in the GDI initiate during clusters of large atmospheric river events that closely mirror fluctuations in traditional hydrologic and meteorological observations. We show that GPS-based hydrologic drought indices provide a significant opportunity to improve drought assessment, in California and beyond, by improving our understanding of the hydrologic cycle.

Geologic features (e.g., fractures and alluvial fans) can play an important role in the locations and volumes of groundwater discharge and degree of groundwater-surface water (GW-SW) interactions. However, the role of these features in controlling GW-SW dynamics and streamflow generation processes are not well constrained. GW-SW interactions and streamflow generation processes are further complicated by variability in precipitation inputs from summer and fall monsoon rains, as well as declines in snowpack and changing melt dynamics driven by warming temperatures. Using high spatial and temporal resolution radon and water stable isotope sampling and a 1D groundwater flux model, we evaluated how groundwater contributions and GW-SW interactions varied along a stream reach impacted by fractures (fractured-zone) and below the fractured hillslope (non-fractured zone) in Coal Creek, a Colorado River headwater stream affected by summer monsoons. During early summer, groundwater contributions from the fractured zone dominated, but declined throughout the summer. Groundwater contributions from the non-fractured zone were constant throughout the summer and became proportionally more important later in the summer. We hypothesize that groundwater in the non-fractured zone is dominantly sourced from a high-storage alluvial fan at the base of a tributary that is connected to Coal Creek throughout the summer and provides consistent groundwater influx. Water isotope data revealed that Coal Creek responds quickly to incoming precipitation early in the summer, and summer precipitation becomes more important for streamflow generation later in the summer. We quantified the change in catchment dynamic storage and found it negatively related to stream water isotope values, and positively related to modeled groundwater discharge and the ratio of fractured zone to non-fractured zone groundwater. We interpret these relationships as declining hydrologic connectivity throughout the summer leading to late summer streamflow supported predominantly by shallow flow paths, with variable response to drying from geologic features based on their storage. As groundwater becomes more important for sustaining summer flows, quantifying local geologic controls on groundwater inputs and their response to variable moisture conditions may become critical for accurate predictions of streamflow.

Discharge of deeply sourced groundwater to streams is difficult to locate and quantify, particularly where both discrete and diffuse discharge points exist, but diffuse discharge is one of the primary controls on solute budgets in mountainous watersheds. The noble gas helium is a unique identifier of deep groundwater discharge because groundwater with long residence times is commonly enriched in helium. In this study, a portable mass spectrometer was used to measure longitudinal variation in dissolved helium concentrations in two mountainous rivers at high spatial resolution not feasible with traditional sampling techniques. Helium profiles were then simulated using a mass-balance model to quantify longitudinal variation in groundwater discharge to the receiving rivers. Results indicate helium concentrations were enriched by multiple orders of magnitude above atmospheric equilibrium in both rivers and that this persisted for up to 18 km below observed pulse inputs in the Colorado River. Helium mass-balance models match observed longitudinal patterns with the exception of sharp initial increases in helium observed in the rivers. Increased longitudinal groundwater discharge rates correspond to mapped geologic structures in both watersheds that likely transport deep geothermal water. Models show variable sensitivity to spatial assignment of input variables representing the groundwater source, illustrating the importance of collecting data from discrete groundwater discharges where possible. The methodology shows promise for field experiments designed to assess air-water exchange rates and to quantify total groundwater discharge from a combination of discrete and diffuse sources.

Storage-discharge relationships and dynamic changes in storage connectivity remain key unknowns in understanding and predicting watershed behavior. In this study, we use Global Positioning System measurements of load-induced Earth surface displacement as a proxy for total water storage change in four climatologically diverse mountain watersheds in the western United States. Comparing total water storage estimates with stream-connected storage derived from hydrograph analysis, we find that each of the investigated watersheds exhibits a characteristic seasonal pattern of connection and disconnection between total and stream-connected storage. We investigate how the degree and timing of watershed-scale connectivity is related to the timing of precipitation and seasonal changes in dominant hydrologic processes. Our results show that elastic deformation of the Earth due to water loading is a powerful new tool for elucidating dynamic storage connectivity and watershed discharge response across scales in space and time.