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1. Leveraging Groundwater Dynamics to Improve Predictions of Summer Low‐Flow Discharges

   https://doi.org/10.1029/2023WR035126  

   Johnson, Keira; Harpold, Adrian; Carroll, Rosemary W.; Barnard, Holly; Raleigh, Mark S.; Segura, Catalina; Li, Li; Williams, Kenneth H.; Dong, Wenming; Sullivan, Pamela L. (August 2023, Water Resources Research)

   Abstract Summer streamflow predictions are critical for managing water resources; however, warming‐induced shifts from snow to rain regimes impact low‐flow predictive models. Additionally, reductions in snowpack drive earlier peak flows and lower summer flows across the western United States increasing reliance on groundwater for maintaining summer streamflow. However, it remains poorly understood how groundwater contributions vary interannually. We quantify recession limb groundwater (RLGW), defined as the proportional groundwater contribution to the stream during the period between peak stream flow and low flow, to predict summer low flows across three diverse western US watersheds. We ask (a) how do snow and rain dynamics influence interannual variations of RLGW contributions and summer low flows?; (b) which watershed attributes impact the effectiveness of RLGW as a predictor of summer low flows? Linear models reveal that RLGW is a strong predictor of low flows across all sites and drastically improves low‐flow prediction compared to snow metrics at a rain‐dominated site. Results suggest that strength of RLGW control on summer low flows may be mediated by subsurface storage. Subsurface storage can be divided into dynamic (i.e., variability saturated) and deep (i.e., permanently saturated) components, and we hypothesize that interannual variability in dynamic storage contribution to streamflow drives RLGW variability. In systems with a higher proportion of dynamic storage, RLGW is a better predictor of summer low flow because the stream is more responsive to dynamic storage contributions compared to deep‐storage‐dominated systems. Overall, including RLGW improved low‐flow prediction across diverse watersheds. 

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2. Karst Hydrologic Memory Supplements Streamflow During Dry Periods in Snow‐Dominated, Mountainous Watersheds

   https://doi.org/10.1002/hyp.70019  

   Tennant, Hyrum; Neilson, Bethany T; Hill, Devon; Newell, Dennis L; Evans, James P; Choi, Seohye; McNamara, James P; Ashmead, Nathaniel; Xu, Tianfang (December 2024, Hydrological Processes)

   ABSTRACT Analysis of PRISM and SNOTEL station data paired with USGS streamflow gage data in the western United States shows that, in snow‐dominated mountainous watersheds, streamflow regimes differ between watersheds with karst geology and their non‐karst neighbours. These carbonate aquifers exhibit a spectrum of flow paths encompassing karst conduits, including large fractures or voids that transmit water readily to springs and other surface waters, and matrix flow paths through soils, highly fractured bedrock, or porous media bedrock grains. A well‐connected karst aquifer will discharge a large portion of its accumulated precipitation to surface water via springs and other groundwater flow paths on an annual scale, exhibiting a lagged response to precipitation presenting as a “memory effect” in hydrograph time series. These patterns were observed in the hydrologic records of gaged watersheds with exposed or near‐surface carbonate layers accounting for > 30% of their drainage area. In western snow‐dominated watersheds, where paired streamflow and SNOTEL data are available, analysis of the precipitation and flow time series shows low‐flow volume is strongly related to karst aquifer conditions and winter precipitation when compared to low‐flow volumes present in non‐karst watersheds, which have a complex relationship to multiple driving metrics. Analysis of normalised streamflow and cumulative precipitation in karst watersheds show that low‐flow conditions are highly dependent on the preceding winter precipitation and streamflow in both wet and dry periods. In non‐karst watersheds, increased precipitation primarily impacts high‐flow, spring runoff volumes with no clear relationship to low‐flow periods. When comparing cumulative streamflow and precipitation volumes within each water year and over longer timescales, karst watersheds show the potential filling and draining of large amounts of karst storage, whereas non‐karst watersheds demonstrate a more stable storage regime. Communities in many western US watersheds are dependent on snow‐dominated karst watersheds for their water supply. This analysis, using widely available hydrologic data, can provide insight into the recharge and storage processes within these watersheds, improve our ability to assess current flow regimes, anticipate the impacts of climate change on water availability, and help manage water supplies. 

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3. Controls on Discharge and Drying in an Intermittent Grassland Stream: Temporal and Network Variability

   https://doi.org/10.1002/eco.70108  

   Raihan, M A; Dodds, W K; Zipper, S; Moore, T L (September 2025, Ecohydrology)

   ABSTRACT Intermittent streams are prevalent worldwide, yet the understanding of drivers of their changing flow patterns remains incomplete. We examined hydrological changes spanning four decades (1982–2020) in Kings Creek, an intermittent grassland stream within the Konza Prairie Biological Station in Kansas, USA. We analysed streamflow data from a US Geological Survey gauge on Kings Creek and three upstream Long Term Ecological Reasearch (LTER) sub‐watersheds with annual, biennial or quadrennial burn frequencies and linked trajectories of woody encroachment to increased evapotranspiration and changes in streamflow. Riparian woody cover doubled in the annually and biannually burned sub‐watersheds and sevenfold in the quadrennially burned watersheds. We observed significant decreases (84%) in daily discharge and number of annual flow days (55%) at the downstream USGS Kings Creek gauge, with similar changes in the LTER sub‐watersheds. The changing riparian cover, propelled by the regional expansion of woody plants, contributed to decreased streamflow by amplifying actual evapotranspiration (ET). Seasonal assessments underscored the critical influence of late summer conditions (July–September), under which increases in ET were linked to rising temperatures and increased evapotranspiration by riparian cover. Our results highlight the significant hydrological impacts of woody encroachment in grasslands and emphasize the importance of long‐term ecohydrological monitoring in unravelling the interplay between climate and vegetation as controls on the hyper‐variable flow patterns in this intermittent stream. Predicting and managing hydrological impacts on the flow of intermittent grassland rivers and streams worldwide requires accounting for the effects of accelerating woody encroachment. 

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4. Insights Into Heterogeneous Streamflow Generation Processes and Water Contribution in Forested Headwaters

   https://doi.org/10.1002/hyp.70241  

   Ortega, Jaime; Segura, Catalina; Brooks, J Renée; Sullivan, Pamela L (August 2025, Hydrological Processes)

   Understanding how diverse headwater streams contribute water downstream is critical for accurate modelling of seasonal flow dynamics in larger systems. This study investigated how headwater catchments, with diverse subsurface storage, influence downstream flows within Lookout Creek—a 62 km², 5th‐order catchment in the rain‐snow transition zone in western Oregon, USA. We analysed one year of hydrometric and water stable isotope data collected at 10 stream locations, complemented by a decade of precipitation isotopic data. As expected, isotopic data revealed that most of the streamflow was sourced from large fall and winter storms. Generally, stream isotope ratios decrease with elevation. However, some streams had higher isotopic values than expected, reflecting the influence of isotopically heavy storms and relatively low storage. Other streams that tended to have low flow variability in response to precipitation inputs had lower isotopic values, indicating higher elevation water sources than their topographic watershed boundaries. Both hydrometric data and water isotope‐based end‐member mixing models suggest storage differences among headwater catchments influenced the seasonal water contributions from tributaries. Most notably, the contributions of Cold and Longer Creeks, which occupy less than 10% of the Lookout Creek drainage area, sustain up to 50% of the streamflow in the summer. These catchments have high storage and high groundwater contributions, as evidenced by flat flow duration curves. Finally, our data suggest that geologic variability and geomorphic complexity (presence of earthflows and landslides) can be indicators of storage that dramatically influence water movement through the critical zone, the variation in streamflow, and the response of streams to precipitation events. Heterogeneity in headwater catchment storage is key to understanding flow dynamics in mountainous regions and the response of streams to changes in climate and other disturbances. 

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5. Shifting groundwater fluxes in bedrock fractures: Evidence from stream water radon and water isotopes

   https://doi.org/10.1016/j.jhydrol.2024.131202  

   Johnson, Keira; Christensen, John N; Payton_Gardner, W; Sprenger, Matthias; Li, Li; Williams, Kenneth H; Carroll, Rosemary WH; Thiros, Nicholas; Brown, Wendy; Beutler, Curtis; et al (May 2024, Journal of Hydrology)

   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 downstream of 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 were high, 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. 

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