Climate change, inter-annual precipitation variability, recurrent droughts, and flash flooding, coupled with increasing water needs, are shaping the co-evolution of socioeconomic and cultural assemblages, water laws and regulations, and equitable drinking water access and allocation worldwide. Recognizing the need for mitigation strategies for drinking water availability in urban areas, the Isotope Hydrology Section of the International Atomic Energy Agency (IAEA) coordinated a state-of-the-art global assessment to evaluate water sources and distribution of drinking water supply in urban centers, an initiative entitled “Use of Isotope Techniques for the Evaluation of Water Sources for Domestic Supply in Urban Areas (2018-2023)”. Here, we report on a) current research trends for studying urban drinking water systems during the last two decades and b) the development, testing, and integration of new methodologies, aiming for a better assessment, mapping, and management of water resources used for drinking water supply in urban settings. Selected examples of water isotope applications (Canada, USA, Costa Rica, Ecuador, Morocco, Botswana, Romania, Slovenia, India, and Nepal) provide context to the insights and recommendations reported and highlight the versatility of water isotopes to underpin seasonal and temporal variations across various environmental and climate scenarios. The study revealed that urban areas depend on a large spectrum of water recharge across mountain ranges, extensive local groundwater extraction, and water transfer from nearby or distant river basins. The latter is reflected in the spatial isotope snapshot variability. High-resolution monitoring (hourly and sub-hourly) isotope sampling revealed large diurnal variations in the wet tropics (Costa Rica) (up to 1.5‰ in δ 18O) and more uniform diurnal variations in urban centers fed by groundwater sources (0.08 ‰ in δ 18O) ([Ljubljana](https://www.google.com/search?client=firefox-b-1-d&sca_esv=f5a20a2e9138d638&sca_upv=1&sxsrf=ADLYWIKR6-DvBtjaWqFYRhn6VgnegOa8kg:1717189104058&q=Ljubljana&stick=H4sIAAAAAAAAAONgVuLQz9U3SMrNNXnEaMwt8PLHPWEprUlrTl5jVOHiCs7IL3fNK8ksqRQS42KDsnikuLjgmngWsXL6ZJUm5WQl5iUCAAFa64FOAAAA&sa=X&ved=2ahUKEwjMrrz047iGAxWyG9AFHSVwCBgQzIcDKAB6BAgTEAE), Slovenia). Similarly, while d-excess was fairly close to the global mean value (+10 ‰) across all urban centers (10-15‰), reservoir-based drinking water systems show significantly lower values (up to ~ -20 ‰) (Arlington, TX, USA and Gaborone, Botswana), as a result of strong evapoconcentration processes. δ 18O time series and depth-integrated sampling highlighted the influence of the catchment damping ratio in the ultimate intake water composition. By introducing new, traceable spatial and temporal tools that span from the water source to the end-user and are linked to the engineered and socio-economic structure of the water distribution system, governmental, regional, or community-based water operators and practitioners could enhance drinking water treatment strategies (including more accurate surface water blending estimations) and improve urban water management and conservation plans in the light of global warming.

Rationale: Many insect species undertake multi-generational migrations in the Afro-tropical and Palearctic ranges, and understanding their migratory connectivity remains challenging due to their small size, short life span and large population sizes. Hydrogen isotope ( δ 2H) can be used to reconstruct the movement of dispersing or migrating insects, but applying δ 2H for provenance requires a robust isotope baseline map (i.e., isoscape) for the Afro-Palearctic. Methods: We analysed the δ 2H in the wings ( δ 2H wing) of 142 resident butterflies from 56 sites across the Afro-Palearctic. The δ 2H wing values were compared to the predicted local growing-season precipitation δ 2H values ( δ 2H GSP) using a linear regression model to develop an insect wing δ 2H isoscape. We used multivariate linear mixed models and high-resolution and time-specific remote sensing climate and environmental data to explore the controls of the residual δ 2H wing variability. Results: A strong linear relationship was found between δ 2H wing and δ 2H GSP values (r 2=0.53). The resulting isoscape showed strong patterns across the Palearctic but limited variation and high uncertainty for the Afro-tropics. Positive residuals of this relationship were correlated with dry conditions for the month preceding sampling whereas negative residuals were correlated with more wet days for the month preceding sampling. High intra-site δ 2H wing variance was associated with lower relative humidity for the month preceding sampling and higher elevation. Conclusion: The δ 2H wing isoscape is applicable to trace butterflies, moths and other terrestrial herbivorous insects that migrate across the Afro-Palearctic range but has limited geolocation potential in the Afro-tropics. The spatial analysis of uncertainty using high-resolution climatic data demonstrated that many African regions with highly variable evaporation rates and relative humidity have δ 2H wing values that are less related to δ 2H GSP values. Increasing geolocation precision will require new modeling approaches using more time-specific environmental data and/or independent geolocation tools.

Water isotope analyses can be used as tracers in water resource sustainability, forensics (e.g. human, ecological, food authenticity and provenance) and climate research, however, limited research has been conducted on water in Mexico. This first national-level tap water survey of coupled H-O isotope ratios is reported from over 50 cities and towns across Mexico. Tap water across the country records a range of 82.4‰ for δ2H and 12.0‰ for δ18O, analyzed using a Picarro L2130-i analyzer. The isotopic values show strong relationships (r > 0.5) with the geography as well as some social and socioeconomic parameters. A Geographic Information System approach is used to develop maps of predicted tap water isotope ratios (isoscapes) for Mexico. These isoscapes will be useful to researchers interested in human-hydrological systems as well as forensic scientists establishing the region of origin or the travel history recorded by human remains.

The North American monsoon (NAM) is an important source of precipitation across the southwestern United States (US). The approximate northern boundary of this feature crosses the Navajo Nation, in the Four Corners region, where NAM rains have long been important to the livelihoods of Native Americans. Relatively little is known about the characteristics and hydrological significance of the NAM in this region. Here we report a new 4-year record of stable H and O isotope ratios in monsoon-season rainfall and water resources across the Navajo Nation. Monthly precipitation samples collected at 39 sites document a characteristic pattern of 2H- and 18O-enrichment associated with monsoonal precipitation. These changes are weakly correlated with local precipitation intensity, however, and the correlation that does exist is dominated by sub-cloud evaporation effects. In contrast to precipitation amount, monsoon-season isotopic values exhibited limited spatial variability across the region, and after correction for sub-cloud evaporation Navajo Nation values were similar to those from a site in southern Arizona. Airmass back-trajectory analysis suggests that the uniformly high NAM isotope values across the region may reflect 1) a region-wide shift from mid-latitude to low-latitude moisture sources at the onset of the peak monsoon, and 2) substantial land-surface recycling of NAM moisture in upwind regions. Comparison of precipitation isotope data with surface and groundwater values implies that, despite its hydroclimatic significance, monsoon rainfall contributes little to rand subsurface water resources. This highlights the monsoon’s importance for warm-season land-surface ecology and hydrology critical to residents of the Four Corners region.

Water and carbon exchanges between the land and atmosphere reflect key ecohydrologic processes, from global climate change to local watershed dynamics. Environmental stable isotope ratios of H2O and CO2 fluxes have been used to study these processes, yet measurement constraints have limited macroscale surface-atmosphere isotope flux evaluations. Across North American biomes within the US National Ecological Observation Network (NEON), we have worked as a team to translate raw measurements of carbon and water stable isotopes into calibrated daily surface-atmosphere flux isotope ratios for precipitation, evapotranspiration, and net ecosystem carbon exchange. Using information theory metrics, we demonstrate that these isotope observations contain meaningful information about the bulk water and carbon fluxes, with isotope measurements carrying about the same amount of information as wind speed measurements. Decomposition of this multivariate mutual information further shows that: (1) this information is unique, i.e. not carried by other traditional ecosystem measurements; and (2) the information added by isotopes is larger in more arid and cool ecosystems. Combining these isotope fluxes with bulk hydrologic fluxes drawn from a suite of land surface models in a first-order mass balance framework also allows for evaluation of hydrologic model structure and estimated uncertainties in partitioning of fluxes into transpiration, evaporation, overland, and subsurface water fluxes. An inter-model comparison suggests distinct patterns in isotope flux composition associated with disparities in the relative contributions of partitioned fluxes. Our results show that conservative isotope tracers provide novel validation metrics for evaluation of land surface model performance across ecosystems at a continental scale. Broadly, this compilation of datasets - combined with both empirical and process-based isotope modeling - suggests NEON stable isotope observations can improve general understanding of land-surface processes influencing the water and carbon cycles from regional to global scales.

Mechanistic representations of biogeochemical processes in ecosystem models are rapidly advancing, requiring advancements in model evaluation approaches. Here we quantify multiple aspects of model functional performance to evaluate improved process representations in ecosystem models. We compare semi-empirical stomatal models with hydraulic constraints against more mechanistic representations of stomatal and hydraulic functioning at a semi-arid pine site using a suite of metrics and analytical tools. We find that models generally perform similarly under unstressed conditions, but performance diverges under atmospheric and soil drought. The more empirical models better capture synergistic information flows between soil water potential and vapor pressure deficit to transpiration, while the more mechanistic models are overly deterministic. Additionally, both multilayer canopy and big-leaf models were unable to capture the magnitude of canopy temperature divergence from air temperature. Lastly, modeled stable carbon isotope fractionation differed under canopy water stress which illustrates the value of carbon isotopes in helping to characterize ecosystem function and elucidate differences attributable to model structure. This study demonstrates the value of merging underutilized observational data streams with emerging analytical tools to characterize ecosystem function and discriminate among model process representations.

The Jordan River Basin, and its seven sub-catchments of the Central Wasatch Mountains immediately east of Salt Lake City, UT, are home to an array of research infrastructrure that collectively form the Wasatch Environmental Observatory (WEO). Each sub-catchment is comprised of a wildland to urban land use gradient that spans an elevation range of over 2000 m in a linear distance of ~25km. Geology varies across the sub-catchments, ranging from granitic, intrusive to mixed sedimentary rocks in uplands that drain to the alluvial or colluvial sediments of the former Lake Bonneville. Vegetation varies by elevation, aspect, distance to stream channels, and land use.  The sharp elevation gradient results in a range of precipitation from 700 to 1200 mm/yr (roughly 2/3 as snow) and mean annual temperature from 3.5 o to 6.8o C. Spring snowmelt dominates annual discharge. Although climate is relatively similar across the catchments, annual water yield varies spatially by more than a factor of 3, ranging from 0.18 to 0.63. With historical strengths in ecohydrology, water supply, and social-ecological research, current infrastructure supports both basic and applied research in meteorology, climate, atmospheric chemistry, hydrology, ecology, biogeochemistry, resource management, sustainable systems, and urban redesign. Climate and discharge data span over a century for the seven sub-catchments of the larger basin. These data sets, combined with multiple decades of hydrochemistry, isotopes, ecological data sets, social survey data sets, and high-resolution LiDAR topography and vegetation structure, provide a baseline for long-term data collected by NEON, public agencies, and individual research projects. The combination of long-term data with active state of the art observing facilities allows WEO to serve as a unique natural laboratory for addressing research questions facing rapidly growing, seasonally snow-covered, semi-arid regions worldwide and an excellent facility for providing student education and research training.