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Hydrology plays a crucial role in understanding Earth's intricate water system and addressing water‐related problems, including against the backdrop of ongoing climate change. A retrospective review of the evolution of hydrology up to the current state of research is of great importance for understanding this role. While there have been some quantitative reviews of large numbers of hydrological publications, there still remains a lack of overarching hydrological research assessment, particularly with the focus on hydrological basins as fundamental spatial‐geographic units of hydrological analysis. Large language models, represented by OpenAI's ChatGPT, have demonstrated powerful textual understanding capabilities, making it possible to extract such overarching and basin information from hydrological publications. Here, we considered publications related to hydrology from Web of Science spanning January 1980 to October 2023, and parsed the information from this extensive body of literature by integrating a large language model and geocoding. These techniques enable quantitative analysis of research characteristics across different spatio‐temporal scales, focusing on hotspot topics, collaboration networks, and various basins worldwide. Our study revealed an increase in hydrological research since the 1990 s, with shifts in research priorities from groundwater and nutrients to climate change and ecohydrology. Some basins in North America and Europe have consistently been hotspots for hydrological research. Since the 2010s, there has been a noteworthy increase in interest toward basins in China and South Asia, but attention to many regions with frequent extreme rainfall remains insufficient. Geographical patterns show different preferred research topics for different basins, but climate change has emerged as the most prominent topic across all regions in the last decade. In conclusion, our study provides an effective approach to quantitative analysis of research trends, offering a fresh view on the evolution of hydrology as a research field, its focus on various hydrological basins around the world, and the emergence of overarching and basin‐specific hot topics over time. Key Points: We explore the literature review capability of a large language model combined with geocoding over the whole field of hydrological researchWe quantify the evolution of hydrological research, hotspot topics, collaboration networks, and basins in focus, worldwide over timeHydrological research publication exhibits major and accelerating growth with WRR emerging as the top influential journal over time [ABSTRACT FROM AUTHOR]

Large‐scale covariations of freshwater fluxes and storages on land can critically regulate the balance of green (evapotranspiration) and blue (runoff) water fluxes, and related land‐atmosphere interactions and hydroclimatic hazards. Such large‐scale covariation patterns are not evident from smaller‐scale hydrological studies that have been most common so far, and remain largely unknown for various regions and climates around the world. To contribute to bridging the large‐scale knowledge gaps, we synthesize and decipher hydroclimatic data time series over the period 1980–2010 for 6,405 catchments around the world. From observation‐based data, we identify dominant large‐scale linear covariation patterns between monthly freshwater fluxes and soil moisture (SM) for different world parts and climates. These covariation patterns are also compared with those obtained from reanalysis products and Earth System Models (ESMs). The observation‐based data sets robustly show the strongest large‐scale hydrological relationship to be that between SM and runoff (R), consistently across the study catchments and their different climate characteristics. This predominantly strongest covariation between monthly SM and R is also the most misrepresented by ESMs and reanalysis products, followed by that between monthly precipitation and R. Comparison of observation‐based and ESM results also shows that an ESM may perform well for individual monthly variables, but fail in representing the patterns of large‐scale linear covariations between them. Observation‐based quantification of these patterns, and ESM and reanalysis improvements for their representation are essential for fundamental understanding, and more accurate and reliable modeling and projection of large‐scale hydrological conditions and changes under ongoing global and regional change. Key Points: The closest large‐scale monthly covariations across different regions and worldwide are between soil moisture and runoffMonthly covariations in blue‐water precipitation‐runoff and green‐water precipitation‐evapotranspiration are mostly weakerSoil moisture‐runoff and precipitation‐runoff relationships are the most misrepresented by studied reanalysis products and Earth System Models [ABSTRACT FROM AUTHOR]

Soil moisture is an important variable for land-climate and hydrological interactions. To investigate emergent large-scale, long-term interactions between soil moisture and other key hydro-climatic variables (precipitation, actual evapotranspiration, runoff, temperature), we analyze monthly values and anomalies of these variables in 1378 hydrological catchments across Europe over the period 1980–2010. The study distinguishes results for the main European climate regions, and tests how sensitive or robust they are to the use of three alternative observational and re-analysis datasets. Robustly across the European climates and datasets, monthly soil moisture anomalies correlate well with runoff anomalies, and extreme soil moisture and runoff values also largely co-occur. For precipitation, evapotranspiration, and temperature, anomaly correlation and extreme value co-occurrence with soil moisture are overall lower than for runoff. The runoff results indicate a possible new approach to assessing variability and change of large-scale soil moisture conditions by use of long-term time series of monitored catchment-integrating stream discharges. [ABSTRACT FROM AUTHOR]

Considering the potential of wetlands to dampen temporal variability of water flow through the landscape, they are increasingly considered as possible nature‐based solutions to mitigate risks of flooding and drought. In this study, we investigate flow variability by means of a flow dampening factor and use observation data from 1984 to 2013 for 82 Swedish catchments to statistically and comparatively analyze the large‐scale effects on this factor of multiple wetlands and lakes in the landscape. The results show good correlation between large‐scale flow dampening and relative area of lakes and floodplain wetlands within a catchment. An increase in relative area up to around 15% for lakes and 0.5% for floodplain wetlands lowers the temporal standard deviation of runoff (R) to around 10%–15% of that for precipitation (P), compared with a common flow‐variability dampening of around 35% for catchments with lake‐wetland area close to zero. Further increase in these relative areas, or in those of wetland types other than floodplain wetlands, has little or no flow dampening effect. The results indicate that the large‐scale flow dampening effect of lakes and floodplain wetlands is mainly due to their water‐storage capacity and less due to their possible effects on the partitioning of P between R and evapotranspiration. Overall, the results emphasize the importance of accounting for the problem scale and relative water‐storage capacity of wetlands when considering their large‐scale efficiency as possible nature‐based solutions for large‐scale flow‐variability regulation in whole catchments. [ABSTRACT FROM AUTHOR]

Abstract: In response to increasing pressures on water resources, watershed services management programs are implemented throughout the tropics. These programs aim to promote land management activities that enhance the quantity and quality of water available to local communities. The success of these programs hinges on our ability to (a) understand the impacts of watershed interventions on ecohydrology; (b) model these impacts and design efficient management programs; and (c) develop strategies to overcome barriers to practical policy development, including resource limitations or the absence of baseline data. In this paper, we review opportunities in ecohydrological science that will help address these three challenges. The opportunities are grouped into measurement techniques, modelling approaches, and access to resources in our hyperconnected world. We then assess management implications of both the knowledge gaps and the new research developments related to the effect of land management. Overall, we stress the importance of policy‐relevant knowledge for implementing efficient and equitable watershed services programs in the tropics. [ABSTRACT FROM AUTHOR]

This study quantifies the separate and combined effects of spatial and temporal variability for waterborne solute transport through catchments. The questions addressed regard whether, when and why the different types of variability may dominate catchment-scale transport. We utilize a versatile numerical solute transport code with a particle-based Monte Carlo time-domain random walk method to simulate waterborne transport through a generic catchment. Simulations are concretized and exemplified using data on spatiotemporal flow-transport variability from direct stream discharge observations and independently calculated advective solute travel time distributions for catchments within the water management district Northern Baltic Proper in Mid-Eastern Sweden. A main conclusion is that the projections of catchment mass loading based on spatial variability alone are robust estimates of long-term average solute transport development. This is especially true when annually aggregated mass load rather than finer temporal resolution of mass flux is considered. Temporal variability yields short-term fluctuations around the long-term average solute breakthrough development, and earlier or later arrival than the latter, depending on the timing and duration of solute input relative to the temporal flow variability. The exact temporal characteristics of future solute breakthrough are thus fundamentally uncertain, but their statistical expectation may be well quantified by accounting only for spatial variability. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The multimodel ensemble of the Coupled Model Intercomparison Project, Phase 5 ( CMIP5) synthesizes the latest research in global climate modeling. The freshwater system on land, particularly runoff, has so far been of relatively low priority in global climate models, despite the societal and ecosystem importance of freshwater changes, and the science and policy needs for such model output on drainage basin scales. Here we investigate the implications of CMIP5 multimodel ensemble output data for the freshwater system across a set of drainage basins in the Northern Hemisphere. Results of individual models vary widely, with even ensemble mean results differing greatly from observations and implying unrealistic long-term systematic changes in water storage and level within entire basins. The CMIP5 projections of basin-scale freshwater fluxes differ considerably more from observations and among models for the warm temperate study basins than for the Arctic and cold temperate study basins. In general, the results call for concerted research efforts and model developments for improving the understanding and modeling of the freshwater system and its change drivers. Specifically, more attention to basin-scale water flux analyses should be a priority for climate model development, and an important focus for relevant model-based advice for adaptation to climate change. [ABSTRACT FROM AUTHOR]

The article focuses on the scale dependence and the effects of model resolution on the measurements of advective travel times and their distributions in the catchment areas in Norrstroöm and Forsmark, Sweden. It states that the advective travel times of the drainage basin in Norrstroöm and the two catchment show clear scale dependence. It says that the local dispersion and molecular diffusion in mobile water contributes to the spreading of solute in travel times.

This study has investigated and outlined the possible quantification and mapping of the distributions of advective solute travel times through hydrological catchments. These distributions are essential for understanding how local water flow and solute transport and attenuation processes affect the catchment-scale transport of solute, for instance with regard to biogeochemical cycling, contamination persistence and water quality. The spatial and statistical distributions of advective travel times have been quantified based on reported hydrological flow and mass-transport modeling results for two coastal Swedish catchments. The results show that the combined travel time distributions for the groundwater-stream network continuum in these catchments depend largely on the groundwater system and model representation, in particular regarding the spatial variability of groundwater hydraulic parameters (conductivity, porosity and gradient), and the possible contributions of slower/deeper groundwater flow components. Model assumptions about the spatial variability of groundwater hydraulic properties can thus greatly affect model results of catchment-scale solute spreading. The importance of advective travel time variability for the total mass delivery of naturally attenuated solute (tracer, nutrient, pollutant) from a catchment to its downstream water recipient depends on the product of catchment-average physical travel time and attenuation rate. [ABSTRACT FROM AUTHOR]

This study develops a general quantification framework for consistent intermodel and intercatchment comparison of the nutrient and pollutant mass loading from multiple sources in a catchment area to downstream surface and coastal waters. The framework accounts for the wide spectrum of different transport pathways and travel times through the subsurface (soil, groundwater, sediment) and the linked surface (streams, lakes, wetlands) water systems of a catchment. The account is based on key flow partitioning and mass delivery fractions, which can be quantified differently by different flow and transport and reaction models. The framework application is exemplified for two Swedish catchment cases with regard to the transport of phosphorus and of a generic attenuating solute. The results show essential differences in model quantifications of transport pathways and temporal spreading, with important implications for our understanding of cause and effect in the catchment-scale nutrient and pollutant loading to downstream waters. [ABSTRACT FROM AUTHOR]

Summary: This paper investigates the propagation of quantifiable probability and quantification uncertainty of water pollution from local pollutant sources at and below the land surface, through the groundwater system, to downstream surface water recipients. Methodologically, the study shows how the risk and uncertainty of surface water pollution within a catchment may be assessed by a combined methodology of a Lagrangian stochastic advective-reactive modelling approach, which accounts for the quantifiable pollutant transport randomness, and a scenario analysis approach, which accounts for different quantification uncertainties. The results show that, in general, unambiguous risk assessment requires at least a reliable order-of-magnitude quantification of the prevailing relation between the average rate of physical pollutant transport from source to recipient and the average rate of pollutant attenuation. If this average relation can be reliably estimated to fall within two identified, relatively wide open value ranges, the assessment of pollution risk to surface waters from localised sources at or below the soil surface may be unambiguous even under otherwise large quantification uncertainty. For a relatively narrow, closed value range of this average rate relation, however, risk assessment must either rely on conservative assumptions, or else be based on a more detailed and resource demanding quantification of pollutant transport. [Copyright &y& Elsevier]

Continental freshwater transports and loads excess nutrients and pollutants from various land surface sources into downstream inland and coastal water environments. This study shows that even small, hydrologically unmonitored near-coastal catchment areas may generate large nutrient and pollutant mass loading to the sea of a magnitude similar to or greater than monitored river loads. Systematic near-coastal gaps in the monitoring of freshwater discharges to the sea may therefore mislead the quantification of coastal mass loading significantly. A methodology is presented for quantifying the mass load contributions of all the different unmonitored pathways of hydrological mass transport to the coast, including unmonitored river parts, whole unmonitored streams, and submarine groundwater discharge. This can be used for guiding future efforts to improve monitoring so that it includes the essential hydrological pathways of nutrient and pollutant loading to the sea. [ABSTRACT FROM AUTHOR]

We analyze and compare simulations and controlling processes of the past 60 years and possible future short- and long-term development of phosphorus and nitrogen loading from the Swedish Norrström drainage basin to the Baltic Sea under different inland source management scenarios. Results indicate that both point and agricultural source inputs may need to be decreased by at least 40% in order to reach a long-term sustainable 30% reduction of anthropogenic coastal nitrogen loading, as required by national environmental goals. A corresponding 20% anthropogenic phosphorus load reduction goal may be reached in the short term by analogous combined 40% source input reduction, but appears impossible to maintain as a long-term achievement by inland source abatement only. In general, realistic quantification of the slow subsurface nutrient transport and accumulation-release dynamics may be essential for accurately predicting and managing nutrient loading to surface and coastal waters. [ABSTRACT FROM AUTHOR]

This study uses controlled numerical experimentation to comparatively simulate and investigate solute transport and concentration responses and patterns in the Baltic Sea for various solute releases from the land through two different coastal cases. These cases are the Swedish Kalmar County coast and the Polish coast of the Vistula River outlet. For equivalent solute releases, the coastal flow conditions and their interactions with main marine currents determine the local coastal solute spreading, while the overall spreading over the Baltic Sea is similar for the two coastal cases, despite their large local differences. For nutrient-proportional solute release scenarios, the highly-populated Vistula catchment yields much greater total, but smaller per-capita nutrient impacts, in the Baltic Sea than the Kalmar County catchment. To be as low as from the Vistula catchment, the per-capita nutrient contribution from Kalmar County would have to be reduced much more than required on average per Swedish inhabitant by the Baltic Sea Action Plan. This highlights an unfairness issue in the per-capita distribution of nutrient load allowance among the Baltic countries, which needs to be considered and handled in further research and international efforts aimed to combat the Baltic Sea eutrophication. [ABSTRACT FROM AUTHOR]

Multiple change pressures from human activities and from climate variability and change affect water systems on land and at the coast and sea. Changes in these water systems in turn affect and feedback to multiple societal and ecosystem conditions and risks, e.g., for: water security and safety; food and energy security; human, animal and ecosystem health; and drought, flood and general climate-change mitigation/adaptation. Compound events with high societal and ecosystem impacts may result from various combinations of these water-related pressures and interactions. As a starting contribution to the European COST Action DAMOCLES, this study addresses the questions of: Which water-related compound events are most relevant for stakeholders? How much is currently known, by scientists and various stakeholders, about the multiple underlying variables and their inter-linkages for these events? What novel approaches can be used to ensure continuous stakeholder engagement?Using a "source-to-sea water-link" case study in the Baltic region, specifically the relatively well-studied Swedish Norrström drainage basin and its adjacent and surrounding coastal areas and associated marine basin of the Baltic Sea, we apply a participatory approach to conceptual model building, involving relevant stakeholders for the land, coast and sea aspects of linked water-related pressures, changes and impacts, and their interactions. Use of this participatory approach can increase understanding of how stakeholders from different sectors view and think about these interactions, and facilitate collaborative exploration of cross-sectoral cooperation opportunities for research, applications and innovations regarding water-related high-impact compound events. As a main part of the study and its approach application, six workshops have been held with stakeholders from different land, coast and sea sectors. In these, mind maps and causal loop diagrams have been co-created by the involved stakeholders and scientists for key land-coast-sea interactions under climate change and socio-economic developments to identify associated connections among sectors with potential high social and environmental impacts. A main aim of this co-creation process is to identify and eventually model and quantify the main dynamic cause-effect links for these interactions and impacts. We here report and analyze the resulting mind maps and causal loop diagrams, and their similarities and differences, as co-created in the sector workshops based on their different focus themes and stakeholder perspectives. In synthesis, we also discuss the causal loop diagram implications for further systems modelling and quantification of key water-related compound events and their impacts. [ABSTRACT FROM AUTHOR]

We develop a data-driven approach to robustly assess freshwater changes due to climate change and/or human irrigation developments by use of the overarching constraints of catchment water balance. This is applied to and tested in the high-uncertainty case of Greece for five nested catchments of different scales across the country and for freshwater changes from an early period (1930–1949) with small human influences on climate and irrigation to a recent period (1990–2009) with expected greater such influences. The results show more or less equal contributions from climatic decrease in precipitation and from human irrigation development to a considerable total decrease in runoff (R) over Greece. This is on average −75 ± 10 mm/year and is greatest for the Ionian catchment in the west (−119 ± 18 mm/year) and the Peloponnese catchment in the south (−91 ± 16 mm/year). For evapotranspiration (ET), a climate-driven decrease component and an irrigation-driven increase component have led to a net total increase of ET over Greece. This is on average 26 ± 7 mm/year and is greatest for the Mainland catchment (29 ± 7 mm/year) and the Aegean catchment in the east (28 ± 6 mm/year). Overall, the resulting uncertainties in the water-balance constrained estimates of R and ET changes are smaller than the input data uncertainties. [ABSTRACT FROM AUTHOR]

A data-driven screening methodology is developed for estimating nutrient input and retention-delivery in catchments with measured water discharges and nutrient concentrations along the river network. The methodology is applied to the Sava River Catchment (SRC), a major transboundary catchment in southeast Europe, with seven monitoring stations along the main river, defining seven nested catchments and seven incremental subcatchments that are analysed and compared in this study. For the relatively large nested catchments (> 40,000 km 2 ), characteristic regional values emerge for nutrient input per unit area of around 30 T/yr/km 2 for dissolved inorganic nitrogen (DIN) and 2 T/yr/km 2 for total phosphorus (TP). For the smaller nested catchments and incremental subcatchments, corresponding values fluctuate and indicate hotspot areas with total nutrient inputs of 158 T/yr/km 2 for DIN and 13 T/yr/km 2 for TP. The delivered fraction of total nutrient input mass (termed delivery factor) and associated nutrient loads per area are scale-dependent, exhibiting power-law decay with increasing catchment area, with exponents of around 0.2–0.3 for DIN and 0.3–0.5 for TP. For the largest of the nested catchments in the SRC, the delivery factor is around 0.08 for DIN and 0.03 for TP. Overall, the nutrient data for nested catchments within the SRC show consistency with previously reported data for multiple nested catchments within the Baltic Sea Drainage Basin, identifying close nutrient relationships to driving hydro-climatic conditions (runoff for nutrient loads) and socio-economic conditions (population density and farmland share for nutrient concentrations). [ABSTRACT FROM AUTHOR]

Summary: Phosphorous (P) transport and retention is investigated here based on data from a 3-year field study of five drainage ditch reaches at Färingsö, west of Stockholm, Sweden. Results show that P retention rate in the drainage reaches greatly depends on and decreases with increasing water flow and watercourse depth. This supports previous simulation indications and implies that changing drainage reach depth and flow conditions may considerably change P loading to downstream waters. In deeper drainage reaches with tile-drained fields in their catchment areas, which are common in Nordic lowlands, particulate P amounted to about 80% of total P, with about half of this fraction being bio-available. As much as 70% of that particulate P was retained in a reach section that was flow- and depth-restricted by heavy vegetation. Retained P in sediments was found to be mostly bound to Fe and/or Al. Some of that PAl–Fe fraction may after dissolution become more permanently retained by re-binding to Ca. [Copyright &y& Elsevier]

Climate-driven increase in the frequency of extreme hydrological events is expected to impose greater strain on the built environment and major transport infrastructure, such as roads and railways. This study develops a data-driven spatial-statistical approach to quantifying and mapping the probability of flooding at critical road-stream intersection locations, where water flow and sediment transport may accumulate and cause serious road damage. The approach is based on novel integration of key watershed and road characteristics, including also measures of sediment connectivity. The approach is concretely applied to and quantified for two specific study case examples in southwest Sweden, with documented road flooding effects of recorded extreme rainfall. The novel contributions of this study in combining a sediment connectivity account with that of soil type, land use, spatial precipitation-runoff variability and road drainage in catchments, and in extending the connectivity measure use for different types of catchments, improve the accuracy of model results for road flood probability. [ABSTRACT FROM AUTHOR]

Summary Climatic changes can profoundly alter hydrological conditions in river basins. Lake Baikal is the deepest and largest freshwater reservoir on Earth, and has a unique ecosystem with numerous endemic animal and plant species. We here identify long-term historical (1938–2009) and projected future hydro-climatic trends in the Selenga River Basin, which is the largest sub-basin (>60% inflow) of Lake Baikal. Our analysis is based on long-term river monitoring and historical hydro-climatic observation data, as well as ensemble mean and 22 individual model results of the Coupled Model Intercomparison Project, Phase 5 (CMIP5). Study of the latter considers a historical period (from 1961) and projections for 2010–2039 and 2070–2099. Observations show almost twice as fast warming as the global average during the period 1938–2009. Decreased intra-annual variability of river discharge over this period indicates basin-scale permafrost degradation. CMIP5 ensemble projections show further future warming, implying continued permafrost thaw. Modelling of runoff change, however, is highly uncertain, with many models (64%) and their ensemble mean failing to reproduce historical behaviour, and with indicated future increase being small relative to the large differences among individual model results. [ABSTRACT FROM AUTHOR]

This study simulates and quantifies the exchange and the pathways of deep and shallow groundwater flow and solute transport under different climate and permafrost conditions, considering the example field case of the coastal Forsmark catchment in Sweden. A number of simulation scenarios for different climate and permafrost condition combinations have been performed with the three-dimensional groundwater flow and transport model MIKE SHE. Results show generally decreasing vertical groundwater flow with depth, and smaller vertical flow under permafrost conditions than under unfrozen conditions. Also the overall pattern of both the vertical and the horizontal groundwater flow, and the water exchange between the deep and shallow groundwater systems, change dramatically in the presence of permafrost relative to unfrozen conditions. However, although the vertical groundwater flow decreases significantly in the presence of permafrost, there is still an exchange of water between the unfrozen groundwater system below the permafrost and the shallow groundwater in the active layer, via taliks. 'Through taliks' tend to prevail in areas that constitute groundwater discharge zones under unfrozen conditions, which then mostly shift to net recharge zones (through taliks with net downward flow) under permafrost conditions. [ABSTRACT FROM AUTHOR]