Balancing groundwater exploitation with ecological preservation requires robust conceptual models, particularly in karst systems where aquifer–lake interactions are complex and highly sensitive to human and climatic pressures. Banyoles Lake (northeast Catalonia, Spain) is a groundwater-fed karstic-tectonic system of exceptional ecological value, yet its hydrogeological functioning remains insufficiently understood. This study revisits its water balance by integrating meteorological, limnological, hydrochemical, and isotopic data into an updated hydrogeological analysis. Results indicate that regional groundwater inflow from the Banyoles–Alta Garrotxa system contributes 11.4 × 106 m3/year, equivalent to 54

Droughts are projected to become more frequent and severe, threatening groundwater resources in Mediterranean regions by altering water quality and hydrological dynamics. This study provides for the first time a field-scale assessment of 85 emerging contaminants (ECs), including 41 pharmaceuticals (PhACs), 10 metabolites and transformation products, and 34 endocrine disrupting compounds (EDCs) plus caffeine as stimulant, under drought conditions in the Onyar River Basin, a representative Mediterranean catchment. Results showed that prolonged drought disrupted stream–aquifer interactions, increasing aquifer vulnerability to multiple pollution sources. The most widely detected PhACs were carbamazepine (0.8–4 ng/L), and sulfamethoxazole (0.6–20 ng/L), while dominant EDCs, were tolyltriazole (3.9–239.54 ng/L) and tris(2-chloroethyl) phosphate (TCEP; 3.3–260 ng/L), and caffeine (6.5–154 ng/L). Concentrations were generally highest in wells near the Onyar River, reflecting river–aquifer interactions, whereas agricultural activities using livestock waste as fertilizer contributed mostly to sulfonamide inputs. During a prolonged drought (2020–2024) the stream-aquifer relationship changed its dynamics, modifying groundwater flow and contaminant transport pathways, increasing aquifer vulnerability to diffuse pollution sources.Indicators, as the Groundwater Ubiquity Score (GUS) and the Persistent-Mobile-Toxic (PMT) classification are valuable tools for predicting contaminant behavior, especially when estimated site-specific sorption parameters are used. Both support groundwater management in Mediterranean catchments, where drought conditions influence the occurrence, spatial distribution and fate of ECs, complicating predictions of contaminant transport and aquifer protection. Therefore, this study underscores that the occurrence of ECs must be interpreted in the context of hydrological conditions prevailing at sampling.

Climate change, land use, and resource exploitation increasingly threaten river hydrology and water quality, leading to scarcity. Non-perennial or intermittent rivers are vulnerable systems characterised by highly variable flows. While recent research has advanced understanding of their natural variability, integrating the complex effects of multiple human impacts at the basin scale remains challenging. Aiming to improve this understanding and supporting effective management, this study uses end-member mixing analysis (EMMA) to identify streamflow sources in an intermittent Mediterranean river network, based on hydrochemical data from 23 sites over 14 monthly surveys and potentiometric data from six shallow wells. The analysis identifies three main contributors to streamflow: headwater runoff, groundwater baseflow from aquifers and hillslopes and treated wastewater. Their relative contributions varied spatially and seasonally. Upstream of urban areas, headwater runoff dominated (71%), while in the central catchment, groundwater contributed 15%-75%. Downstream of industrial areas, contributions were more balanced: 35% runoff, 27% groundwater and 38% wastewater. Alluvial groundwater was more important in small upstream subbasins in terms of maintaining the connectivity of the streamflow, whereas groundwater contribution from deeper aquifers dominated in lower reaches. EMMA results also highlighted discrepancies with point potentiometric data, emphasising the limitations of local measurements for understanding basin-scale hydrology. Under future climate scenarios, reduced headwater runoff and groundwater storage are expected to lower baseflow, increasing reliance on treated-but lower-quality-wastewater to sustain streamflow. This highlights the need for long-term monitoring and management, with a focus on protecting baseflow sources and improving wastewater quality to ensure water sustainability.

The Senonian aquifer of the Complex Terminal in the Kebili region, southern Tunisia, is a vital but overexploited groundwater resource, mainly used for agricultural and domestic purposes, with limited abstraction for industry and tourism. Overabstraction, driven by increasing demand and extensive unregulated drilling, has led to severe piezometric declines and water quality degradation. Additionally, climate change projections indicate a temperature rise of 2.3–3.8 °C and a precipitation decrease of 14

Groundwater is essential to maintain surface aquatic ecosystems during severe droughts, and provides a safe and sustainable resource for human demand, particularly in Mediterranean areas. Aquifers are affected by different types of impacts caused by of human activities, such as pollution caused by agricultural and livestock activities, and discharges of wastewater treatment plants (WWTP) and sewage leakage; as well as, groundwater overexploitation that affects stream discharge or increases seawater encroachment in coastal areas. This study evaluates the occurrence of nutrients and emerging contaminants, specifically pharmaceutically active compounds (PhACs), in groundwater and surface water as tracers of human activities in the Onyar River Basin (NE Spain). In particular, we analyze the effects of pollution sources and its exploitation on PhACs concentration and stygofauna distribution in the Quaternary and Neogene sedimentary aquifers. Two sampling campaigns were conducted in June-July 2021 (when the highest aquifer exploitation for irrigation occurs) and in February 2023 (before the irrigation season), including 12–14 wells and 5 sampling stream points. Physicochemical parameters were measured in situ and water samples were taken to analyze major ions, nutrients, and PhACs. Stygofauna samples were obtained by filtering at 500 L of pumped groundwater or using a specific net (50 μm mesh sieve), and organisms were fixed with 100

Mediterranean basins face significant water scarcity which requires examining long-term data to evaluate their trends in water availability and quality and assess management options. In this presentation, we explore the historical streamflow changes, the influencing climatic —streamflow, precipitation, temperature, and evapotranspiration (PET and AET)— and land-use factors, and the evolution of surface water quality in the Onyar River (Inner Catalan basins, NE Spain; 295 km2) during the last decades (1960-2020). Results highlight a consistent decline in streamflow, most pronounced over the last two decades, accompanied by an increase in PET, and a probable decrease in groundwater recharge. These changes co-occurred with higher concentrations of river water ammonium and nitrate. We attribute these patterns to changes in land use such as afforestation and intensive fertilization, as well as increased groundwater withdrawal, particularly during irrigation seasons. Additional factors include growing urban water demand and the discharges of treated wastewater back into the river system. Evaluation of the relationship between groundwater and surface water using end-member mixing analysis of hydrochemical data points out an interesting scale-dependence behaviour: groundwater baseflow from alluvial formations was relevant in the smallest subbasins, whereas regional groundwater flow involving deeper aquifers could significantly contribute to stream discharge in the lowest zones of the basin. Since water balance alteration in the future climate scenarios will reduce the contribution of the headwater flow as well as groundwater storage and baseflow generation, reclaimed wastewater shows up as a relevant source to maintain stream runoff, yet its quality is low and might not be properly diluted by rainfall originated runoff. These observations provide a comprehensive overview of the declining water quantity and quality in the Onyar River network, attributing these trends to an interplay of climatic and anthropogenic factors. They urge for integrated water resources management strategies to mitigate the implications of these environmental changes, such as protecting baseflow generating areas as well as controlling reclaimed wastewater quality. Funding: G. Córdoba-Ariza acknowledges funding from Secretariat of Universities and Research from Generalitat de Catalunya and European Social Fund for her FI fellowship (2022 FI_B1 00105).

Human activities, such as agriculture and reclaimed water discharges, are the main sources of pharmaceuticals (PhACs) and emerging contaminants (ECs) in groundwater. Their occurrence is expected to be linked to the hydrogeological dynamics of the aquifer, especially during severe drought periods when recharge from human sources dominates; therefore, it is paramount to relate groundwater flow dynamics and pollutant occurrence to properly control and manage water resources quality.This study evaluates the presence of ECs, including PhACs and endocrine disrupting compounds (EDCs) along the Onyar River alluvial aquifer (NE Catalonia, Spain; 295 km2) in two sampling campaigns depicting different hydrological scenarios. The first survey took place in June-July 2021 including the analysis of 45 PhACs in 18 groundwater samples, and the second one in March 2023, after almost two years of a severe drought, focused on 12 groundwater and 10 stream water samples for the determination of 45 PhACs, plus 9 transformation products (TPs) and metabolites, and 32 EDCs. Hydraulic head and hydrochemical data were also collected.Five PhACs were detected in groundwater (acetaminophen, carbamazepine, hydrochlorothiazide, ibuprofen, and venlafaxine) at 0.6 to 57 ng/L in 2021. Sulfamethoxazole was the only antibiotic found. The second campaign (2023) identified eight PhACs in river samples, including trimethoprim, clindamycin, sulfamethoxazole, sulfapyridine, flubendazole, carbamazepine, citalopram, and venlafaxine, ranging from 5.9 to 81.8 ng/L, whereas in groundwater only six PhACs: sulfamethoxazole, sulfamethazine, sulfadiazine, carbamazepine, venlafaxine, and hydrochlorothiazide were identified at 0.5 to 20 ng/L. TPs and metabolites, such as carbamazepine-10,11-epoxy and carbamazepine-2-hydroxy were only found in river samples, while metoprolol acid was present in both river and groundwater samples. Several EDCs were present in both river and groundwater samples, including tolyltriazole, benzotriazole-1H, bisphenol A, caffeine, methylparabens, TCEP, TBEP, TCPP and estrone at concentrations from 0.3 to 142.2 ng/L.Such distinct results are influenced by hydrological factors. As a general interpretation supported by head and chemical data, stream water induced aquifer recharge due to groundwater withdrawal is more intensive during drought periods when the water table is lower, as in 2023. This enhances the transport of ECs introduced by reclaimed water inputs towards the aquifer. Conversely, in periods with a higher water table: June-July 2021, the hydrological setup reverses and pollutants introduced by stream recharge, yet relevant, do not reach wells located far away from the drainage network because of larger groundwater recharge and reduced withdrawal irrigation rates. Therefore, a temporal variability of ECs concentration in groundwater is not a matter of uncertainty, but a consequence of observable and predictable changing hydrological conditions. Ignoring hydrological variability in the interpretation of PhACs and ECs will result in erroneous actions about preventing pollution migration and understanding their actual hazard to human and environmental health.Funding: project EC-FATE, call “MINECO-AEI, PID2022-139911OB-C42” and the Ramon y Cajal contract (RYC2020-030324-I).

Mediterranean basins face significant water scarcity which demands examining long-term data to prospect their trends in water availability and quality. This study focuses on The Onyar River (Inner Catalan basins, NE Spain), to explore its historical streamflow changes, the influencing climatic and land-use factors, and the consequential impacts on water quality. We analyse key hydro-climatic variables—streamflow, precipitation, temperature, and evapotranspiration (both PET and AET)— across a 60-year span (1960 -2020) and their relationship with recent stream water chemistry data (2007-2020). We compare these patterns to the changes in land use, and to the variations in water demand according to urban use, livestock, and crop production estimates. Our findings highlight a consistent decline in streamflow, most pronounced over the last two decades, accompanied by an increase in PET, and a probable decrease in groundwater recharge. Notably, these changes co-occurred with higher concentrations of river water ammonium and nitrate. We attribute these patterns to changes in land use such as afforestation and intensive fertilization, as well as increased groundwater withdrawal, particularly during irrigation seasons. Additional factors include growing urban water demand and the discharges of treated wastewater back into the river system. This study offers a comprehensive overview of the declining water quantity and quality in the Onyar River, attributing these trends to an interplay of climatic and anthropogenic factors. The findings underscore the need for integrated water resource management strategies to mitigate the implications of these changes.

This study investigates the occurrence, transport, and risks associated to antibiotic residues, antibiotic resistance genes (ARGs) and antibiotic resistant Escherichia coli (AR-E. coli) in eleven natural springs in an agroecosystem environment with intense livestock production, where groundwater nitrate concentration usually sets above 50 mg L-1. Out of 23 multiple-class antibiotics monitored, tetracycline and sulfonamide residues were the most ubiquitous, and they were detected at concentrations ranging from ng L-1 to μg L-1. Five ARGs were monitored, conferring resistance to the antibiotic classes of major use in livestock production. Thus, genes conferring resistance to sulfonamides (sul1 and sul2) and tetracyclines (tetW) as well as a gene proxy for anthropogenic pollution (intI1) were present in most springs. sul1 was the most abundant, with absolute concentrations ranging from 4 × 102 to 5.6 × 106 gene copies L-1 water. AR-E. coli showing resistance to sulfonamides and tetracyclines was also detected, with a prevalence up to approximately 40 % in some sites but with poor correlations with the concentration of antibiotic residues and ARGs. The occurrence of antibiotics, ARGs and AR-E. coli was characterized by large seasonal variations which were mostly associated to both hydrological factors and reactive transport processes. Finally, a risk assessment approach pointed out towards low risk for both the groundwater environment and human health, when spring water is used for direct human consumption, associated with the occurrence of antibiotics, ARGs and AR-E. coli. However, long-term effects cannot be neglected, and proper actions must be taken to preserve groundwater quality.

The mineral water of Vilajuïga village in Alt Empordà (NE Catalonia, Spain) owes its uniqueness to an emanation of geogenic CO 2 that modifies groundwater hydrochemistry to produce a differentiated HCO 3 –Na- and CO 2 -rich groundwater among the usual Ca–HCO 3 type found in this region. A hydrogeological conceptual model attributes its occurrence to the intersection of two faults: La Valleta and Garriguella-Roses. The former provides a thrust of metamorphic over igneous rocks, formed during the Paleozoic, over a layer of ampelitic shale that, from a hydrogeological perspective, acts as a confining layer. The Garriguella-Roses normal fault, which originated during the Neogene, permits the degassing of geogenic CO 2 that is attributed to volcanic activity occurring in the Neogene. Groundwater mixing from the metamorphic and igneous rock units plus the local occurrence of CO 2 creates a HCO 3 –Na water that still holds free-CO 2 in solution. Interaction with the gas phase is restricted at the intersection of the two faults. Radiocarbon dating, after correcting for geogenic dead carbon, estimates an age of 8,000 years BP . The low tritium content (0.7 TU) indicates that Vilajuïga water is a mix of “older” groundwater recharged in the metamorphic rocks of the Albera range and “younger” groundwater from the igneous rocks of the Rodes range, over a recharge area of 45 km 2 and a maximum elevation of 600 m. Given its origin as rare groundwater in the southern slope of the Eastern Pyrenees, purposeful monitoring is necessary to evaluate the groundwater vulnerability and anticipate impacts from nearby wells and climate-change effects.

The mineral water of Vilajuïga village in Alt Empordà (NE Catalonia, Spain) owes its uniqueness to an emanation of geogenic CO2 that modifies groundwater hydrochemistry to produce a differentiated HCO3–Na- and CO2-rich groundwater among the usual Ca–HCO3 type found in this region. A hydrogeological conceptual model attributes its occurrence to the intersection of two faults: La Valleta and Garriguella-Roses. The former provides a thrust of metamorphic over igneous rocks, formed during the Paleozoic, over a layer of ampelitic shale that, from a hydrogeological perspective, acts as a confining layer. The Garriguella-Roses normal fault, which originated during the Neogene, permits the degassing of geogenic CO2 that is attributed to volcanic activity occurring in the Neogene. Groundwater mixing from the metamorphic and igneous rock units plus the local occurrence of CO2 creates a HCO3–Na water that still holds free-CO2 in solution. Interaction with the gas phase is restricted at the intersection of the two faults. Radiocarbon dating, after correcting for geogenic dead carbon, estimates an age of 8,000 years BP. The low tritium content (0.7 TU) indicates that Vilajuïga water is a mix of “older” groundwater recharged in the metamorphic rocks of the Albera range and “younger” groundwater from the igneous rocks of the Rodes range, over a recharge area of 45 km2 and a maximum elevation of 600 m. Given its origin as rare groundwater in the southern slope of the Eastern Pyrenees, purposeful monitoring is necessary to evaluate the groundwater vulnerability and anticipate impacts from nearby wells and climate-change effects.

When investigating future nitrate (NO3-) concentrations in groundwater, climate change has a major role as it determines the future water budget and, in turn, the conditions in the aquifer which will finally have a decisive effect on NO3- concentrations. In this study, the different effects on water balance and NO3- concentration under three projected climate scenarios - RCP 2.6, RCP 4.5, and RCP 8.5 - are analysed in a water protection area in the Lower Rhine Embayment in Germany. Recharge values were calculated from downscaled precipitation and temperature data for the 21st century in a water budget that considers land use in the evapotranspiration term. Nitrate concentration evolution is estimated using recharge and expected fertilization rates with a lumped-parameter model. In order to be able to map the NO3- concentration, the investigation area is divided into 1000 × 1000 m cells. Each cell is assigned a specific NO3- input and a NO3- degradation capacity. Results show significant variations in NO3- development projected with the different climate scenarios due to different temperatures and consequently actual ET, and precipitation. Nevertheless, nitrate concentrations clearly increase in all projections. The total NO3- mass increases most strongly with RCP 8.5 until 2099 (by 89% compared to 2020) and least with RCP 4.5 (by 50%). Further projections show a 20% reduction in agricultural NO3- input can reduce NO3- concentrations, but insufficiently to comply with drinking water guidelines in all regions and aquifers. The model indicates that NO3- input loads should be defined according to future recharge variations governed by climate change. Consequently, a time-varying fertilization rate specific for each region, with their own turnover time and degradation rate, must be estimated to meet pollution environmental goals.

The occurrence of veterinary antibiotics and hydro-chemical parameters in eleven natural springs in a livestock production area is evaluated, jointly with the characterization of their DOM fingerprint by Orbitrap HRMS. Tetracycline and sulfonamide antibiotics were ubiquitous in all sites, and they were detected at low ng L-1 concentrations, except for doxycycline, that was present at mu g L(-1 )in one location. DOM analysis revealed that most molecular formulas were CHO compounds (49 %-68 %), with a remarkable percentage containing nitrogen and sulphur (16 %-23 % and 11 %-24 %, respectively). Major DOM components were phenolic and highly unsaturated compounds (similar to 90 %), typical for soil-derived organic matter, while approximately 11 % were unsaturated aliphatic, suggesting that springs may be susceptible to anthropogenic contamination sources. Comparing the DOM fingerprint among sites, the spring showing the most different profile was the one with surface water interaction and characterized by having lower CHO and higher CHOS formulas and aliphatic compounds. Correlations between antibiotics and DOM showed that tetracyclines positively correlate with unsaturated oxygen-rich substances, while sulfonamides relate with aliphatic and unsaturated oxygen-poor compounds. This indicates that the fate of different antibiotics will be controlled by the type of DOM present in groundwater.

In this paper, we present a smart implementation of a self-powered device able to analyze the ionic conductivity of low volume liquid samples. The device sensing core consists of a stack of two single use paper-based batteries that operate as conductivity sensors. To operate the system in a self-powered mode a resistive element is connected to the battery stack so its output voltage is directly related to the conductivity of the electrolyte to be analyzed. The output voltage signal is managed by a flexible hybrid circuit fabricated using inkjet printing that requires a minimal number of discrete off-the-shelf electronic components to function. The energy delivered by the battery allows displaying the result to the user by means of screen-printed electrochromic displays. The sensing battery has been calibrated and tested using NaCl samples at different conductivities. As a practical application, a final device able to assess the suitability for crop irrigation of water samples has been designed, fabricated and validated with real water samples coming from different sources. However, its applicability expands to conductivity measurement of any biological sample.

The EU Nitrate Directive has been ruling for almost 30 years, nevertheless nitrate concentration in the Lombardy Plain did not decrease. Together with failures of management implementation, a possible cause for such field observations is that management actions were taken without adequately considering the actual hydrogeological dynamics. To consider this aspect, the paper presents a groundwater flow and transport numerical model of a specific area of the Lombardy Plain. The aim of this model is to demonstrate how modelling, as a management tool, can be useful in the governance process. The groundwater model, using well-known MODFLOW-MT3D codes, is based on existing hydrogeological information, while a nitrogen mass balance has been performed at municipal scale to determine the agricultural N surplus to the subsurface. The model adequately reproduces head levels and nitrate concentrations in observation wells for a 10-year simulation period, showing that 4.5% of the N annual input remains stored in the system. The model indicates the efficiency of rivers and springs to export N out from the system at an estimated rate of 77.5% of the annual N inputs. Back to governance, the model shows that management data at municipal level (e.g. irrigation rates, groundwater withdrawal, N net recharge) provide a satisfactory scale for successfully reproducing nitrate evolution. Hence those variables that can be object of debate during a governance process can be treated as input data to the numerical model. Therefore, backcasting exercises can be conducted to check whether the model outcome fits with the expected results of specific management actions. The model highlights how the N mass balance evolves, providing clues on which factors can be managed to reduce nitrate concentrations and meet the Directive's requirements. Numerical groundwater models, as an option to address water management issues, ultimately contribute to solve the information and capacity governance gaps.

Despite the increasing public concern about the frequent occurrence of pharmaceuticals in the water bodies, their transport and fate are not yet well known; in particular in groundwater. In this laboratory study, batch experiments were conducted to investigate the sorption behaviour of selected pharmaceuticals. The choice of compounds was motivated by their chemical properties as well as by their occurrence at the relevant field sites. It included: antipyrine, atenolol, caffeine, carbamazepine, ciprofloxacin, diclofenac, ketoprofen, ofloxacin, and sulfamethoxazole. Sorption behaviour has already been investigated for some of these compounds (e.g. carbamazepine), but for the others (e.g. antipyrine, ketoprofen), extensive studies are missing so far. For the experiments, artificial and actual aquifer materials from complementary field sites were selected: technical coarse quartz sand and sediments from alluvial Vistrenque Aquifer, France (sandy loam), and fluvio-deltaic Baix Fluvià Aquifer, Spain (sandy clay loam, clay, and medium sand). In these field sites occurrence of pharmaceuticals in groundwater was previously stated, and the presented laboratory experiments were complementary to the field investigations. Five concentration steps for determining the sorption isotherms were investigated. Correlation analysis showed dependencies of K-coefficients of individual compounds and sediment properties; however, no clear, universal patterns for all compounds were found. Batch experiments suggest that sorption behaviour was governed by compound-specific properties rather than by sediment properties. These results contribute to the understanding sorption behaviour of pharmaceuticals in heterogeneous sediments, although some inconsistencies were revealed between laboratory scale results and field scale observations.

Despite the European Nitrate Directive (ND) being issued almost 30 years ago, groundwater nitrate contamination is still a serious threat to ecosystems and human health. In one of the areas most affected by nitrates, the Lombardy Plain (Italy), the effectiveness of the ND and the capacity of governance to support its application correctly was assessed using a socio-hydrogeological approach. Nitrate trends over 11 years show that most regions present steady or increasing concentrations, highlighting how contamination can affect previously impaired situations and supposedly resistant and resilient aquifers. Stakeholder network analysis reveals that the governance framework does not support knowledge dissemination and changes in farmers’ attitudes, hindering water quality improvements. Nitrogen input needs to be reduced and manure relocation monitored. The local governance scale has a key role in enhancing ND dissemination. Reports to the EU Commission should integrate multi- or interdisciplinary evaluation of trends, including governance dynamics, alongside hydrochemical information.

The spatial distribution of antibiotics in alluvial aquifers presents a large variability caused by the joint action of several factors including hydrology, land use, and groundwater properties. In this study, the influences of these factors on the spatial variability of antibiotics is evaluated based on an extensive database of 47 wells located in the Baix Fluvià alluvial aquifer (NE Catalonia). Statistical methods such as redundancy and variation partitioning (VP) analyses, which are not commonly used in hydrogeological studies, are herein tested and used to estimate the effects of environmental factors on the observed antibiotic occurrence. Using VP, the total explained variation of the antibiotic distribution only reaches 18% of the total variability, meaning that the whole set of explanatory parameters is insufficient or inadequate to describe the occurrence of antibiotics and their concentration. The results point out that groundwater properties are the most representative parameters, while those related to antibiotic sources and aquifer susceptibility have lower influences. Omitting solute transport parameters that actually govern antibiotic fate (i.e., sorption coefficients and degradation rates) from the statistical analysis limited the success of the VP results. VP thus highlights the importance of researching antibiotic transport in groundwater by determining the reactive properties of these pollutants above other hydrogeochemical variables. We conclude that the present capacity to predict antibiotic existence at a specific location—for instance, a supply well—based on field data is still poor and unrepresentative, being an impediment for groundwater pollution management.

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come.