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Groundwater affects ecosystem services (ES) by altering critical zone ecohydrological and biogeochemical processes. Previous research has demonstrated significant and nonlinear impacts of shallow groundwater on ES regionally, but it remains unclear how groundwater affects ES at the global scale and how such effects respond to environmental factors. Here, we investigated global patterns of groundwater relationships with two ES indicators—net primary productivity (NPP) and soil organic carbon (SOC)—and analyzed underlying factors that mediated groundwater influences. We quantitatively compared multiple high-resolution (∼1 km) global datasets to characterize water table depth (WTD), NPP and SOC, and performed spatial simultaneous autoregressive modeling to test how selected predictors altered WTD-NPP and WTD-SOC relationships. Our results show widespread significant WTD-NPP correlations (61.5% of all basins globally) and WTD-SOC correlations (64.7% of basins globally). Negative WTD-NPP correlations, in which NPP decreased with rising groundwater, were more common than positive correlations (62.4% vs. 37.6%). However, positive WTD-SOC relationships, in which SOC increased with rising groundwater, were slightly more common (53.1%) than negative relationships (46.9%). Climate and land use (e.g., vegetation extent) were dominant factors mediating WTD-NPP and WTD-SOC relationships, whereas topography, soil type and irrigation were also significant factors yet with lesser effects. Climate also significantly constrained WTD-NPP and WTD-SOC relationships, suggesting stronger WTD-NPP and WTD-SOC relationships with increasing temperature. Our results highlight that the relationship of groundwater with ES such as NPP and SOC are spatially extensive at the global scale and are likely to be susceptible to ongoing and future climate and land-use changes.

Non-perennial rivers and streams make up over half the global river network and are becoming more widespread. Transitions from perennial to non-perennial flow are a threshold-type change that can lead to alternative stable states in aquatic ecosystems, but it is unknown whether streamflow itself is stable in either wet (flowing) or dry (no-flow) conditions. Here, we investigated drivers and feedbacks associated with regime shifts between wet and dry conditions in an intermittent reach of the Arkansas River (USA) over the past 23 years. Multiple lines of evidence suggested that these regimes represent alternative stable states, including (a) significant jumps in discharge time series that were not accompanied by jumps in flow drivers such as precipitation and groundwater pumping; (b) a multi-modal state distribution with 92% of months experiencing no-flow conditions for <10% or >90% of days, despite unimodal distributions of precipitation and pumping; and (c) a hysteretic relationship between climate and flow state. Groundwater levels appear to be the primary control over the hydrological regime, as groundwater levels in the alluvial aquifer were higher than the stream stage during wet regimes and lower than the streambed during dry regimes. Groundwater level variation, in turn, was driven by processes occurring at both the regional scale (surface water inflows from upstream, groundwater pumping) and the reach scale (stream–aquifer exchange, diffuse recharge through the soil column). Historical regime shifts were associated with diverse pressures including network disconnection caused by upstream water use, increased flow stability potentially associated with reservoir operations, and anomalous wet and dry climate conditions. In sum, stabilizing feedbacks among upstream inflows, stream–aquifer interactions, climate, vegetation, and pumping appear to create alternative wet and dry stable states at this site. These stabilizing feedbacks suggest that widespread observed shifts from perennial to non-perennial flow will be difficult to reverse.

Non‐perennial streams, which lack year‐round flow, are widespread globally. Identifying the sources of water that sustain flow in non‐perennial streams is necessary to understand their potential impacts on downstream water resources, and guide water policy and management. Here, we used water isotopes (δ18O and δ2H) and two different modeling approaches to investigate the spatiotemporal dynamics of young water fractions (Fyw) in a non‐perennial stream network at Konza Prairie (KS, USA) during the 2021 summer dry‐down season, as well as over several years with varying hydrometeorological conditions. Using a Bayesian model, we found a substantial amount of young water (Fyw: 39.1–62.6%) sustained flows in the headwaters and at the catchment outlet during the 2021 water year, while 2015–2022 young water contributions estimated using sinusoidal models indicated smaller Fyw amounts (15.3% ± 5.7). Both modeling approaches indicate young water releases are highly sensitive to hydrological conditions, with stream water shifting to older sources as the network dries. The shift in water age suggests a shift away from rapid fracture flow toward slower matrix flow that creates a sustained but localized surface water presence during late summer and is reflected in the annual dynamics of water age at the catchment outlet. The substantial proportion of young water highlights the vulnerability of non‐perennial streams to short‐term hydroclimatic change, while the late summer shift to older water reveals a sensitivity to longer‐term changes in groundwater dynamics. Combined, this suggests that local changes may propagate through non‐perennial stream networks to influence downstream water availability and quality. Plain Language Summary Non‐perennial streams, which periodically dry, are common worldwide. Identifying the origin and age of water in non‐perennial streams will help guide water policy and management strategies. We used water isotopes (δ18O and δ2H), a common hydrologic tracer, to identify stream water sources and age during the 2021 summer dry‐down period of a non‐perennial watershed at the Konza Prairie (KS, USA) with two different statistical methods. We found that water sources and flow paths changed as the stream network dried. Approximately half of summer streamflow is young water, meaning it took less than 3 months to travel from precipitation to the stream. However, as the summer progressed, stream water shifted to older sources. We interpret this shift in the water age to indicate a shift in the source of water from rapid flow paths early in the summer to slower flow paths later in the summer, which sustain localized surface water during the driest parts of the year. Taken together, the substantial amount of young water highlights the vulnerability of non‐perennial streams to short‐term weather changes and longer‐term changes in groundwater dynamics that can alter the quantity and quality of water flowing through non‐perennial stream networks to ultimately influence downstream water availability and quality. Key Points Stream isotopic composition was progressively enriched in δ18O and δ2H as the stream network dried Stream isotopic enrichment is caused by evaporative effects and a decrease in surface water connectivity Most streamflow was young water (stored in the subsurface <3 months), with older and more variable water age as the stream network dried

Reductions in streamflow caused by groundwater pumping, known as “streamflow depletion,” link the hydrologic process of stream‐aquifer interactions to human modifications of the water cycle. Isolating the impacts of groundwater pumping on streamflow is challenging because other climate and human activities concurrently impact streamflow, making it difficult to separate individual drivers of hydrologic change. In addition, there can be lags between when pumping occurs and when streamflow is affected. However, accurate quantification of streamflow depletion is critical to integrated groundwater and surface water management decision making. Here, we highlight research priorities to help advance fundamental hydrologic science and better serve the decision‐making process. Key priorities include (a) linking streamflow depletion to decision‐relevant outcomes such as ecosystem function and water users to align with partner needs; (b) enhancing partner trust and applicability of streamflow depletion methods through benchmarking and coupled model development; and (c) improving links between streamflow depletion quantification and decision‐making processes. Catalyzing research efforts around the common goal of enhancing our streamflow depletion decision‐support capabilities will require disciplinary advances within the water science community and a commitment to transdisciplinary collaboration with diverse water‐connected disciplines, professions, governments, organizations, and communities. Plain Language Summary Pumping water from a well can reduce flow in surrounding streams, a phenomenon called “streamflow depletion.” It is important for water managers to know when, where, and how much streamflow depletion is occurring because it can affect the amount of water available for ecosystems and other water users. However, estimating streamflow depletion is challenging because weather and other factors affect streamflow, in addition to pumping. Here, we discuss important topics related to streamflow depletion that need further research. Most importantly, scientists need to move beyond estimating changes in flow caused by pumping, and also develop improved approaches to estimate the impacts of these streamflow changes on ecosystems and water users. Additionally, it will be important to develop improved tools for estimating streamflow depletion and linking those estimates to water management decisions. Making these advances will require basic scientific research and collaboration between hydrologists and other fields; these efforts should be prioritized because streamflow depletion is occurring at a rapid pace around the world. Key Points Changes in streamflow caused by groundwater pumping (“streamflow depletion”) are a link between basic and applied hydrologic science Streamflow depletion science is critical to support decision making and requires advances in hydrology and transdisciplinary collaboration We identify key priorities for streamflow depletion research to improve hydrological process understanding and support water management

Streamflow duration is important for aquatic ecosystems and assigning stream protection status. This study predicts streamflow duration, represented as the fraction of time with flow each year, using a combination of sensor data and crowd‐sourced visual observations for a study area in northern Colorado, USA. We used 11 stream stage sensors and 177 visual monitoring points to examine how frequently streams should be sampled to compute flow fractions accurately. This showed that the number of visual observations needed to compute accurate flow fractions increases with decreasing flow duration. We then developed random forest models to predict mean annual flow fractions using climate, topographic, and land cover predictors and found that snow persistence, summer precipitation, and drainage area were important predictors. Model performance was best when using sites with ≥10 visual observations. Our model predicts that almost all (98%) of streams in the study region are non‐perennial, about 10% more than the amount of non‐perennial streams in the National Hydrography Dataset. Stream type maps are sensitive to the time period of data collection and to thresholds used to represent perennial versus non‐perennial flow. To improve maps of non‐perennial streams, we recommend moving beyond categorical classification of streams to a continuous variable like flow fraction. These efforts can be best supported with frequent observations in time that span streams with a wide range of flow fractions and drainage area attributes. Plain Language Summary Most small streams in the world are not monitored, so we know little about when they are flowing or dry. Yet, the amount of time streams flow can determine whether they are protected by water quality legislation and streamside management plans. In this study we used visual observations of stream flow/no flow and stream sensors to develop a model that predicts the fraction of time that streams flow. At a study area in northern Colorado, volunteer observers documented stream flow/no flow at 177 stream segments, and we placed sensors in 11 headwater streams at different elevations. We found that streams needed to be visited approximately weekly to determine how long they flow each year. Streams that rarely flow needed to be visited more often than those that flow most of the time. Our model shows that most (98%) of the streams in the study area do not flow continuously. The amount of time that streams flow is sensitive to changing climate and water demands. Ongoing monitoring of these streams will help us track and predict the range of flow conditions that are possible throughout the vast networks of small streams that feed larger rivers and lakes. Key Points Predicted April–September fraction of time with flow using sensors, crowd‐sourced observations, and statistical models in Colorado streams Snow persistence, summer precipitation, and drainage area are dominant predictors of flow fractions in the Northern Colorado study area Developing a reliable model of flow fraction requires sampling diverse streams that span the full spectrum of flow fractions (0–1)

Irrigated agriculture has led to aquifer depletion in many places, necessitating effective groundwater conservation policies to slow unsustainable water use. However, the success of conservation policies is contingent on complex interactions between agronomic production, aquifer response, and risk management tools, all of which shape agricultural water diversions. This study uses an agent‐based model to evaluate how uniform pumping restrictions and crop insurance, a popular risk management tool, jointly influence farmer decisions in the Sheridan‐6 Local Enhanced Management Area (SD‐6 LEMA) collaboratively regulated region of western Kansas. Calibrated against observed data under widespread crop insurance coverage and LEMA regulation, the model captures farmers' adaptive decision‐making shaped by prior‐year satisfaction. Scenario‐based analysis reveals that pumping restrictions reduce groundwater withdrawals by up to 19.9% during drought years and modestly promote crop diversification. Crop insurance indirectly supports diversification by stabilizing behavior following the 2012 drought and 2013 policy change, though the direct impact of crop insurance on groundwater diversions is limited (1.4% reduction). Behavioral differences—not direct financial incentives—drive divergence between insured and uninsured farmers: insured farmers maintain stable strategies, while uninsured farmers shift in response to environmental variability and peer influence. Stricter irrigation restrictions sustain aquifer levels but have limited additional effects on cropping patterns when crop insurance is present. During droughts, crop insurance buffers profit losses under strict water limits by nearly half. Overall, integrating groundwater conservation with crop insurance enhances aquifer sustainability and farm‐level economic resilience. Findings offer insights into designing coordinated water governance and agricultural policies amid growing climate uncertainty. Plain Language Summary Groundwater is essential for farming, especially in dry regions. But heavy irrigation has caused many aquifers to shrink. To protect these water sources, local governments and farming communities are adopting groundwater conservation policies. At the same time, farmers use crop insurance to reduce financial risks from bad weather or poor prices. In this study, we looked at how groundwater pumping limits and crop insurance affect farmer decisions. We focused on western Kansas, where a group of farmers agreed to reduce groundwater diversions. Using an agent‐based model that simulates how farmers make decisions, we tested different scenarios to see how these policies and tools interact. We found that water limits reduced pumping by nearly 20% during droughts. Crop insurance didn't directly save much water, but it helped farmers stick to their plans by reducing the financial stress of droughts and new policies. Insured farmers were more consistent in what they grew, while uninsured farmers changed more often in response to weather and neighbor behavior. When used together, pumping limits and crop insurance helped protect groundwater while supporting farm incomes during tough times. These results can inform smarter policies that balance environmental sustainability and farmer wellbeing‐especially as climate change increases uncertainty. Key Points An agent‐based model assesses combined groundwater pumping limits and crop insurance effects Pumping limits cut groundwater diversions; crop insurance stabilizes income without hindering conservation goals Crop insurance promotes cropping strategy consistency; uninsured farmers' behavior shifts more with policy and drought

International audience

Humans and ecosystems are deeply connected to, and through, the hydrological cycle. However, impacts of hydrological change on social and ecological systems are infrequently evaluated together at the global scale. Here, we focus on the potential for social and ecological impacts from freshwater stress and storage loss. We find basins with existing freshwater stress are drying (losing storage) disproportionately, exacerbating the challenges facing the water stressed versus non-stressed basins of the world. We map the global gradient in social-ecological vulnerability to freshwater stress and storage loss and identify hotspot basins for prioritization ( n  = 168). These most-vulnerable basins encompass over 1.5 billion people, 17% of global food crop production, 13% of global gross domestic product, and hundreds of significant wetlands. There are thus substantial social and ecological benefits to reducing vulnerability in hotspot basins, which can be achieved through hydro-diplomacy, social adaptive capacity building, and integrated water resources management practices. This work identifies the world’s most vulnerable basins to social and ecological impacts from freshwater stress and storage loss: a set of 168 hotspot basins for global prioritization that encompass 1.5 billion people, 17% of global food crops, 13% of global GDP, and hundreds of significant wetlands.

Groundwater, Earth’s largest nonfrozen freshwater reservoir, is vital for water supply security. Groundwater models help to manage complex domestic, agricultural, and industrial water demands while preserving ecosystem health under climate change. The community-driven groundwater model portal (GroMoPo) hosts groundwater model metadata to analyse biases and distribution of groundwater models. Over 450 models are currently featured on GroMoPo, with most models from high-GDP countries at local-to-regional scales. The GroMoPo initiative addresses current knowledge gaps and facilitates future collaboration and data sharing.