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Isotope hydrological studies to understand groundwater‐surface water interactions in tropical, high‐elevation catchments are limited. These interactions are important in controlling lake water residence time, aqueous biogeochemistry, and water availability for downstream communities and ecosystems. To better comprehend the complexity of spatio‐temporal variations in the aquifer‐lake domain in tropical volcanic regions, a multi‐tracer approach including water and inorganic carbon stable isotopes (δ2H, δ18O, δ13CDIC), hydrochemistry, and 222Rn was applied in Lake Hule, northern Costa Rica. Seasonal isotope mass balance calculations using lake, stream, precipitation, and groundwater compositions were supplemented with local hydrometeorological information. Evaporation to inflow ratios (E/I) revealed a small variability between the dry (December–April) and wet seasons (May–November), with relatively low evaporation losses, 2.9 ± 1.0 % and 3.2 ± 1.8 %, respectively. Bayesian end‐member analysis indicated that annual inputs from groundwater, precipitation, and runoff represented 61.3 ± 8.1%, 24.4 ± 8.4, and 14.3 ± 5.9% of total lake inflow, respectively. Temporal variations of δ13CDIC confirmed the key role volcanic carbonate buffering plays in this lake and indicated greater CO2 degassing from groundwater sources in the wet season. This tracer‐aided assessment in a volcanic lake maar of northern Costa Rica provides evidence of previously unknown groundwater‐surface water interactions and illustrates the application of isotopic tools for estimating water balances and seasonal variability of groundwater discharge into natural lakes across the volcanic front of Central America. Plain Language Summary: This study analyzed the interactions between groundwater and surface water at Lake Hule, a volcanic maar lake located in northern Costa Rica. The results showed that evaporation losses were relatively similar and low in both dry (December–April) and wet (May–November) seasons and that most of the total inflow to the lake was related to groundwater inputs. The study also revealed the key role of carbon dioxide (CO2) dissolution in the lake water chemistry, with greater volcanic CO2 inputs in the wet season. This study reveals previously unknown groundwater‐surface water interactions regarding groundwater discharge into natural lakes across the volcanic front of Central America. Key Points: Isotope‐based metrics evidenced low evaporation to inflow (E/I) ratios (<5%)The water balance revealed seasonal water residence times of 0.61 (wet) ‐ 1.08 (dry) years under steady‐state conditionsEstimates of groundwater input indicated an annual contribution of 61.3 ± 8.1% of total inflow [ABSTRACT FROM AUTHOR]

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. Recognising 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 centres, 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 δ18O) and more uniform diurnal variations in urban centres fed by groundwater sources (0.08‰ in δ18O) (Ljubljana, Slovenia). Similarly, while d‐excess was fairly close to the global mean value (+10‰) across all urban centres (10‰–15‰), reservoir‐based drinking water systems show lower values (up to ~ −20‰) (Arlington, TX, USA and Gaborone, Botswana), as a result of strong evapoconcentration processes. δ18O 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. [ABSTRACT FROM AUTHOR]

Isotope‐enabled models provide a means to generate robust hydrological simulations. However, daily isotope‐enabled rainfall‐runoff models applied to larger spatial scales (>100 km2) require more input data than conventional non‐isotope models in the form of precipitation isotope time series, which are difficult to generate even with point station measurements. Spatially distributed isotope data can be circumvented by isotope‐enabled climate models. Here, we evaluate the hydrological simulations of the J2000‐isotope enabled hydrological model driven with data from corrected and un‐corrected isotope‐enabled global and regional climate models (isotope‐enabled global spectral model [IsoGSM] and isotope‐enabled regional spectral model [IsoRSM], respectively) compared with 1 year of measured reference station and a yearly average precipitation isotope input for a pilot site, the data‐scarce sub‐humid Eerste River catchment in South Africa. The models driven by all input products performed well for upstream and downstream discharge gauges with Nash Sutcliffe efficiency (NSE) from 0.58 to 0.85 and LogNSE of 0.66 to 0.93. The simulated δ2H stream isotopes using the reference J2000‐iso and J2000‐isoRSM were good for the main river with a stream Kling Gupta efficiency (KGE) of between 0.4–0.9 and the top 100 Monte Carlo simulations varying by around 5‰ for δ2H. For smaller tributaries the model was unable to capture the measured stream isotopes due to biased precipitation isotope inputs. Adjusting the J2000‐iso with a bias corrected IsoRSM improved the stream and groundwater isotope simulation and outperformed the model driven by an average yearly precipitation isotope input. Differences in simulated hydrological processes were only evident between the models when evaluating percolation with unrealistic simulations for the standard J2000 model. While the regional climate model is computationally more intensive than its global counterpart, it provided better stream isotope simulations and improvements to simulated percolation. Our results indicate that isotope‐enabled climate models can provide useful input data in data scarce regions for hydrological models, where improved water management to address climate change impacts is needed. [ABSTRACT FROM AUTHOR]

This article discusses the use of spatial and temporal visualization techniques to understand the production and transport of dissolved organic carbon (DOC) in a high-elevation Andean catchment. The authors introduce a new model called STARR-DOC, which simulates the dynamics of DOC alongside water movement. The study focuses on the Zhurucay catchment and provides visualizations of water movement and DOC production in different soil types. The findings suggest that the production of DOC is influenced by soil water content, with the valley bottom areas exhibiting faster response times compared to the hillslopes. The visualization helps to identify areas within the catchment that have higher or lower DOC production and transport under different meteorological conditions. [Extracted from the article]

Productive agricultural supply chains require the support of functional ecosystems, but intense agricultural practices change local hydrological systems (e.g. river diversion). In this study, the impact of farm dams was assessed for the Verlorenvlei catchment, a sensitive ecosystem currently under a state of hydrological change in South Africa. We developed a new module for the Jena Adaptable Modelling System (JAMS)/J2000 rainfall–runoff model to assess the streamflow impact from the points of abstraction, losses during storage and irrigation. The model achieved a satisfactory streamflow calibration with efficiencies Nash Sutcliffe Efficiency (NSE, logNSE) of 0.52 and 0.51. The irrigated area reduced simulated streamflow by 12 to 19%. The results from the study agree with remote sensed evapotranspiration, measured lake surface water levels and streamflow, but uncertainty remains in the total simulated dam evaporation. While many catchments lack the data required for a detailed irrigation impact assessment, this approach considers total water use, dam storage to area relationships and general farming practices. [ABSTRACT FROM AUTHOR]

Wetlands process large amounts of carbon (C) that can be exported laterally to streams and rivers. However, our understanding of wetland inputs to streams remains unclear, particularly in tropical systems. Here we estimated the contribution of seasonal wetlands to the C pool of a lowland headwater stream in the Australian tropics. We measured dissolved organic and inorganic C (DOC and DIC) and dissolved gases (carbon dioxide—CO2, methane—CH4) during the wet season along the mainstem and in wetland drains connected to the stream. We also recorded hourly measurements of dissolved CO2 along a 'stream–wetland drain–stream' continuum, and used a hydrological model combined with a simple mass balance approach to assess the water, DIC and DOC sources to the stream. Seasonal wetlands contributed ∼15% and ∼16% of the DOC and DIC loads during our synoptic sampling, slightly higher than the percent area (∼9%) they occupy in the catchment. The riparian forest (75% of the DOC load) and groundwater inflows (58% of the DIC load) were identified as the main sources of stream DOC and DIC. Seasonal wetlands also contributed marginally to stream CO2 and CH4. Importantly, the rates of stream CO2 emission (1.86 g C s−1) and DOC mineralization (0.33 g C s−1) were much lower than the downstream export of DIC (6.39 g C s−1) and DOC (2.66 g g C s−1). This work highlights the need for further research on the role of riparian corridors as producers and conduits of terrestrial C to tropical streams. Plain Language Summary: Streams and rivers play a vital role in carrying carbon to oceans. This carbon can originate from biological processes in the water or from external sources like rocks, forest and wetland soils. The proportion of carbon from each source depends on factors such as the local geology, climate, and landscape. In this study, we measured how much of the carbon transported by an Australian tropical stream was sourced from the wetlands in the catchment. We found that seasonal wetlands contributed ∼15% of the carbon measured in the stream. We conclude that the main sources of carbon to the stream were the riparian forest, and rock‐derived carbon carried by groundwater. Key Points: Synoptic sampling of DIC, DOC and dissolved C gases along a tropical stream and connected wetlandsSeasonal wetlands contributed ∼15% and 16% of the stream DOC and DIC loadsThe riparian forest and groundwater inflows were likely the main sources of DOC and DIC to the stream, respectively [ABSTRACT FROM AUTHOR]

Realistic projections of the future climate and how this translates to water availability is crucial for sustainable water resource management. However, data availability constrains the capacity to simulate streamflow and corresponding hydrological processes. Developing more robust hydrological models and methods that can circumvent the need for large amounts of hydro‐climatic data is crucial to support water‐related decisions, particularly in developing countries. In this study, we use natural isotope tracers in addition to hydro‐climate data within a newly developed version of the spatially‐distributed J2000iso as an isotope‐enabled rainfall‐runoff model simulating both water and stable isotope (δ2H) fluxes. We pilot the model for the humid tropical San Carlos catchment (2500 km2) in northeastern Costa Rica, which has limited time series, but spatially distributed data. The added benefit of simulating stable isotopes was assessed by comparing different amounts of observation data using three model calibration strategies (i) three streamflow gauges, (ii) three gauges with stream isotopes and (iii) isotopes only. The J2000iso achieved a streamflow Kling–Gupta efficiency (KGE) of 0.55–0.70 across all the models and gauges, but differences in hydrological process simulations emerged when including stable water isotopes in the rainfall‐runoff calibration. Hydrological process simulation varied between the standard J2000 rainfall‐runoff model with a high simulated surface runoff proportion of 37% as opposed to the isotope version with 84%–89% simulated baseflow or interflow. The model solutions that used only isotope data for calibration exhibited differences in simulated interflow, baseflow and model performance but captured bulk water balances with a reasonable match between the simulated and observed hydrographs. We conclude that J2000iso has shown the potential to support water balance modelling for ungauged catchments using stable isotope, satellite and global reanalysis data sets. [ABSTRACT FROM AUTHOR]