Abstract Headwater streams comprise nearly 88% of the western U.S. river network and supply most of the region's surface water, making them especially sensitive to warming, snowpack loss, and drought. As surface‐water inputs decline, groundwater increasingly sustains streamflow, elevating the need to understand long‐term trends in base flow and their climatic drivers. We analyzed 75 years of streamflow records (1950–2024) from 115 headwater basins to quantify historical changes, climate controls, and future trajectories of base flow. Using statistical analyses and cluster‐specific Long Short‐Term Memory models trained on downscaled climate data, we assessed both historical behavior and projections under SSP2‐4.5 and SSP5‐8.5. Historical results show widespread base‐flow declines—most pronounced in early summer—driven by warming, reduced snowmelt, and declining antecedent moisture. Antecedent moisture emerged as the dominant positive driver, while snow and temperature exerted regime‐dependent effects. Future projections indicate continued declines of 45%–65% by late century, with earlier seasonal peaks and reduced summer flows. Snowmelt‐dominated and arid basins experience the largest relative reductions, whereas mixed‐regime systems contribute the greatest volumetric losses. These changes pose significant risks to municipal water supply, ecosystems, and wildfire resilience across the region. Collectively, our results highlight the vulnerability of groundwater‐supported streamflow to climate change and demonstrate the value of integrated statistical and machine‐learning approaches for regional hydrologic assessment.

Climate change induced aridity and Euro-American settlement have altered the historical disturbance and flow regimes of large portions of the ponderosa pine forests of northern Arizona. The increased occurrence of high-severity wildfires due to these changes has led to the establishment of various forest restoration programs to protect the region's forests and their watersheds. In 2014, a paired-watershed monitoring project was implemented to compare the impacts of differing levels of forest thinning to watershed hydrology in seven experimental watersheds nested within the Upper Lake Mary (ULM) watershed in Arizona. This study expands the calibration phase of the ULM paired-watershed by synthesizing historic precipitation, surface runoff, groundwater recharge, soil moisture data, and evapotranspiration (ET) data to perform regression analyses and create a holistic water balance for each watershed. The magnitude and timing of seasonal groundwater recharge events were quantified for the first time in this region using a water table fluctuation method. The results showed that recharge did not occur every year and was heavily dependent (P < 0.05) on total winter season precipitation and snowpack duration. On average, recharge composed 9% of the total water budget when present. The results of this study lay the foundation for a greater understanding of how forest restoration alters northern Arizona's forest hydrology and will provide crucial information that should be used in water policy and water resource decision-making as the region plans for future water availability.

Understanding groundwater movement within karst aquifers remains challenging because flow-defining conduit and fracture networks are both complex and inaccessible. In Grand Canyon National Park, dye tracers have been used to establish flow paths for springs that support ecosystems and park operations. Unfortunately, these point-to-point studies are limited when attempting to extrapolate flow paths over thousands of square kilometers. We introduce a mobile lidar-based methodology that resolves groundwater flow-defining structures from actively-discharging stream caves within the aquifer. This methodology enabled efficient collection of centimeter-scale 3D data from over 10 km of remote caves from the Redwall (Mississippian) and Muav (Cambrian) limestones in the North Rim of the Grand Canyon. Our methodology achieved total compounding errors of less than 0.5% and shows strong agreement with traditional cave maps. We find geologic structures exposed within these caves are consistent across kilometers of cave passages, indicating groundwater flow exploits joint sets and bedding dip direction. These patterns suggest that present-day flow paths within the North Rim of Grand Canyon National Park are, in part, a product of regional faulting and uplift. This lidar-derived structural characterization enables karst network flow pattern identification that would be otherwise unavailable from traditional methods.

Study region: This study focuses on Arizona, a dryland state in the southwestern United States with marked variability in climate, elevation, and hydrogeology. Arizona spans two major physiographic regions, the Colorado Plateau and the Basin and Range, each exhibiting distinct hydrologic behavior. Study focus: We quantify long-term base-flow index (BFI) patterns and trends across Arizona and develop a predictive framework for ungauged basins. BFI was calculated at 205 USGS stream gauges using a recursive digital filter applied to multi-decadal streamflow records. Coincident trends in precipitation, temperature, and evapotranspiration were analyzed to assess climate-base-flow relationships. We trained an eXtreme Gradient Boosting (XGBoost) model on hydroclimatic and physiographic variables to estimate long-term BFI from 1991 to 2020 at the 8digit Hydrologic Unit Code (HUC) scale. New hydrological insights for the region: Groundwater discharge accounts for approximately 32 % of streamflow in Arizona, with substantial spatial variability linked to topography, land cover, and climate. High BFI values are found in forested headwaters with spring-fed and snowmelt-driven systems, while low values dominate the state's arid lowlands. Declining BFI trends were most pronounced in monsoon-dominated, warm-dry, and low-slope basins. Precipitation was the strongest climate correlate of BFI trends, underscoring the importance of climate variability for dryland base flow. This integration of observational records and machine learning provides new insights into groundwater-surface water interactions and offers a transferable framework for water resource assessment in data-scarce dryland regions globally.

The Colorado River is a vital water source for the western United States, yet the river is governed by disjointed and outdated policies that have left water management fragmented and water quantities overallocated. Groundwater is an overlooked component of Colorado River Basin (CRB) water supplies, making it vulnerable to overuse from disparities in uncoordinated protective management strategies. In this study, we analyzed state level groundwater policies to reveal the diversity and efficacy of groundwater governance mechanisms. The existing groundwater management plans for each state throughout the basin are fragmented and limited in scope. We found that with policies only covering 22% of the basin, they do not provide adequate protection at the basin scale for the sustainable use of groundwater resources in the face of increasing demands, creating a positive feedback loop that reinforces the scarcity issue. We conclude that a comprehensive management plan that can fully address resource use throughout the CRB is necessary for the sustainable use of groundwater and its contribution to base flow in the Colorado River. We suggest that such a plan could be derived through an interstate compact like the Colorado River Compact that is used for surface water management.

As geoscience and water related enrollment and degrees continue to decline, new methods are needed to recruit and engage students in these interesting and challenging interdisciplinary fields. An existing water-themed distinguished lecture tour was reinvented and reinvigorated to include a workshop to (1) promote interdisciplinary collaboration and (2) increase student engagement in earth sciences. The Geological Society of America has dedicated foundation support for a domestic and international lecture tour to promote the scientific discipline of hydrogeology. In addition to the lecture tour with a standard presentation and meetings with faculty, staff, and students, a half-day, interactive field demonstration of the inventory and assessment techniques for springs ecosystems was conducted at a spring near the lecture site. Local faculty hosts were encouraged to engage students in the planning and implementation of the demonstration workshop. The students were provided additional training to enter the interdisciplinary ecosystem data into a cloud-sourced database and to be actively engaged in producing a publication related to the springs ecosystems inventories and assessments. Of the participants of the demonstration workshops, students at eight of the host institutions became co-authors of the resulting manuscript. With the significant investment of time, resources, and logistics to implement a lecture tour, an additional interactive, hands-on, demonstration workshop is a cost-effective way to increase student engagement and provide interdisciplinary collaboration.

Spring ecosystems provide vital services to humans and wildlife in northern Arizona. Management of springs presents unique challenges due to the diversity of stakeholders and the multiple uses of springs. We employ two methods of data collection to understand the perceptions of springs ecosystem management: interviews and focus groups with stakeholders of springs in Coconino National Forest (CNF) and Kaibab National Forest (KNF) and a national survey aimed towards public perceptions. We analyze the human perceptions of springs to understand variations between stakeholders and how perceptions of stakeholders and the public vary. Our results indicate differences between human perceptions and current management practices. The main competing interests of springs management are cattle grazing, recreation, and Indigenous Nations' cultural significance. The survey respondents, representative of public perceptions, indicate springs management for Indigenous Nations' cultural significance is important. We see positive correlation between concern for threats to springs from grazing and management to prioritize cultural significance indicating respondents prefer springs to be managed for cultural significance. Cattle grazing and other high impact human uses are viewed less favorably than other management scenarios. Stakeholders' perceptions varied and non-Indigenous stakeholders suggested management should be based on multiple uses and management focused on specific attributes of springs valuable to humans. Our results of the survey respondents’ perceptions for management of Indigenous culture and some support from stakeholders show a joint management concept might be beneficial for springs management in the CNF and KNF. Using the information we collected on the perceptions of spring ecosystems management, we discuss mechanisms for springs management to increase the ability for springs to provide cultural significance values to Indigenous Nations by potentially limiting high impact and degrading uses of springs. We provide results and examples of how all stakeholders can have the ability to benefit from the resources springs provide.

EDITORIAL article Front. Earth Sci., 07 March 2023Sec. Hydrosphere Volume 11 - 2023 | https://doi.org/10.3389/feart.2023.1165061

Better characterization of the water resources of the Grand Canyon National Park (GRCA) and World Heritage Site, USA, will inform management decisions regarding a proposed water supply intake from Bright Angel Creek (BAC) and will inform the associated forecasts on water security related to climate change and the impact on spring-dependent species. Characterization of the water supply for GRCA was improved through multiyear hydrograph analyses at five springs discharging from the Redwall-Muav (R) aquifer: Roaring Springs, Emmett Spring, At Last Spring, Tapeats Spring, and Abyss River Spring. Comparison of snowmelt response timing and mean recession coefficients among the five springs show no significant differences, indicating similar timing of snowmelt-related discharge and flow regimes across 50 km of horizontal and 1,000 m of vertical distance through the aquifer system. The mean water volumes discharged during snowmelt and the mean annual discharge are significantly different between Roaring Springs, Emmett Spring, Tapeats Spring, and Abyss River Spring. Linear regression analyses indicate no annual trends in any of the evaluated aquifer characteristics for any spring, except for more stable baseflow between recessions at Tapeats Spring. Water budgets calculated for Roaring Springs are imbalanced, suggesting missing outflow components, a need to refine the recharge region through dye trace studies, and inaccurate precipitation and evapotranspiration data, requiring model improvement. Continued hydrograph comparisons, trend analyses, and water budgets for additional years and springs will be useful for future groundwater modeling and for forecasting impacts on the R aquifer.

Springs are ecosystems influenced by the exposure of groundwater at the Earth's surface. Springs are abundant and have played important, highly interactive ecological, cultural, and socio-economic roles in arid, mesic, and subaqueous environments throughout human evolution and history. However, springs also are widely regarded as being highly threatened by human impacts. Cantonati et al. (2020a) recommended increased global awareness of springs, including basic mapping, inventory and assessment of the distribution and ecological integrity of springs. We conducted a preliminary global analysis on the ecological integrity of springs by reviewing information on the distribution, ecohydrogeology, associated species, kinds and intensity of human uses, and level of ecological impairment of spring ecosystems. We reviewed information on an estimated 250,000 spring ecosystems among 78 countries across much of the world. Available literature on spring ecological integrity is sparse, widely scattered, and spatially erratic, with major gaps in knowledge. We report large differences in the quality and extent of information among countries and continents, with only moderate data availability even among developed countries, and limited information across most of the developing world. Among countries with available data, ecological impairment of springs is everywhere rampant, sometimes exceeding 90% in developed regions. Impairment among Holarctic nations is generally negatively related to distance from human development, elevation, and latitude, but such patterns are less evident in Africa, Australia, and South America. Declining trends in ecosystem condition, compounding threat factors, and spring-dependent population declines, extirpation, and extinctions of plants, invertebrates, fish, and herpetofauna are widely reported. Overall, available information indicates a global crisis in spring ecosystem integrity, with levels of ecosystem impairment ranging from Vulnerable to fully Collapsed. The threats to aquifers and the ecological integrity of springs vary spatially. Many springs are impaired by local impacts due to flow diversion, geomorphic alteration, land use practices, recreation impacts, and the introduction of non-native species. These threats can be reduced through education, rehabilitation of geomorphology and habitat quality, and species reintroductions if the supporting aquifer remains relatively intact. However, springs also are widely threatened by regional to global factors, including groundwater extraction and pollution, as well as climate change. Such coarse-scale, pre-emergence impacts negatively affect the sustainability of spring ecosystems and the aquifers that support them. Improving understanding and stewardship of springs will require much additional systematic inventory and assessment, improved information management, and reconsideration of basic conservation concepts (e.g., habitat connectivity), as well as cultural and socio-economic valuation. Substantial societal recognition, discussion, and policy reform are needed within and among nations to better protect and sustainably rehabilitate springs, their supporting aquifers, and the spring-dependent human and biotic populations that depend upon them.

Forest restoration is necessary for maintaining healthy watersheds and the ecological spatial networks that provide environmental goods and services. Consideration of the dollar value of these provided benefits in restoration planning is essential to the efficient use of limited resources available to project implementation. Nonmarket valuation is a methodology of economics commonly used to estimate monetary values for environmental goods and services that are not typically bought or sold in a traditional market. Valuation studies are prolific within the restoration literature; however, the use of nonmarket values as decision support is not well represented. We introduce a method using Geographic Information Systems (GIS) to spatially analyze the results from a nonmarket valuation study that estimated dollar values for the attributes of forest restoration characteristic of a semi-arid watershed in the Southwest United States. Map layers were created for the five attributes valued by the study and represent areas in the watershed that are designated as critical habitats, determined to influence surface water quality, prone to high-severity wildfire, representative of culturally significant areas, and contribute to aquifer recharge. A series of overlay analyses were performed to create a composite benefit map that spatially displays nonmarket values throughout the watershed. The per acre benefit values range from USD 0 to USD 104 where all five attributes are present.

1 School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ, United States, 2 Arizona Geological Survey, University of Arizona, Tucson, AZ, United States, 3 JE Fuller Hydrology and Geomorphology, Inc., Flagstaff, AZ, United States, 4 Arizona Geological Survey, University of Arizona, Tucson, AZ, United States, City of Flagstaff, Flagstaff, AZ, United States, US Department of Agriculture, Forest Service, Rocky Mountain Research Station, Moscow, ID, United States

Thinning of semi-arid forests to reduce wildfire risk is believed to improve forest health by increasing soil moisture. Increased snowpack, reduced transpiration and reduced rainfall interception are frequently cited mechanisms by which reduced canopy density may increase soil moisture. However, the relative importance of these factors has not been rigorously evaluated in field studies. We measured snow depth, snow water equivalent (SWE) and the spatial and temporal variation in soil moisture at four experimental paired treatment-control thinning sites in high elevation ponderosa pine forest northern Arizona, USA. We compared snow and soil moisture measurements with forest structure metrics derived from aerial imagery and 3-dimensional lidar data to determine the relationship between vegetation structure, snow and soil moisture throughout the annual hydrologic cycle. Soil moisture was consistently and significantly higher in thinned forest plots, even though the treatments were performed 8-11 years before this study. However, we did not find evidence that SWE was higher in thinned forests across a range of snow conditions. Regression tree analysis of soil moisture and vegetation structure data provided some evidence that localized differences in transpiration and interception of precipitation influence the spatial pattern of soil moisture at points in the annual hydrologic cycle when the system is becoming increasingly water limited. However, vegetation structure explained a relatively low amount of the spatial variance (R-2 < 0.23) in soil moisture. Continuous measurements of soil moisture in depth profiles showed stronger attenuation of soil moisture peaks in thinned sites, suggesting differences in infiltration dynamics may explain the difference in soil moisture between treatments as opposed to overlying vegetation alone. Our results show limited support for commonly cited relationships between vegetation structure, snow and soil moisture and indicate that future research is needed to understand how reduction in tree density alters soil hydraulic properties.