Catchment modelling has undergone tremendous developments during the past decades. In the 1970s, the focus was on simulation of catchment runoff with process descriptions and data inputs being lumped to the catchment scale. Later developments included spatially distributed models allowing data inputs and hydrological processes to be simulated at model grid scale, i.e. much finer than catchment scale. These models were able to explicitly simulate various processes such as soil moisture, evapotranspiration, groundwater and surface runoff. With the advancements in remote sensing technology and availability of high-resolution data, increased attention has in recent years been given to enhancing the capability of catchment models to reproduce spatial patterns and in this way improve our understanding of hydrological processes and the physical realism of catchment models. This development process has involved a wide spectrum of different aspects in the modelling process, reaching from an improved understanding of uncertainties in data, model parameters and model structures to new protocols for good modelling practices in water management. Recognizing the important role of biodiversity and social aspects, hydrologists are now extending the scope of their models to capture the interactions between water, biota and human social systems.

Quantitative knowledge about ecohydrological partitioning across the critical zone in different types of urban green space is important to balance sustainable water needs in cities during future challenges of increasing urbanization and climate warming. We monitored stable water isotopes in liquid precipitation and atmospheric water vapour (δ v) using in-situ cavity ring-down spectroscopy (CRDS) over a two-month period in an urban green space area in Berlin, Germany. Our aim was to better understand the origins of atmospheric moisture and its link to water partitioning under contrasting urban vegetation. δ v was monitored at multiple heights (0.15, 2 and 10 m) in grassland and forest plots. The isotopic composition of δ v above both land uses was highly dynamic and positively correlated with that of rainfall indicating the changing sources of atmospheric moisture. Further, the isotopic composition of δ v was similar across most heights of the 10 m profiles and between the two plots indicating limited aerodynamic mixing. Only the surface at ~0.15 m height above the grassland, δ v showed significant differences, with more enriched values indicative of evaporative fractionation immediately after rainfall events. Further, disequilibrium between δ v and precipitation composition was evident during and right after rainfall events with more positive values (i.e. vapour more enriched than precipitation) in summer and negative values in winter, which probably results from higher evapotranspiration and more convective precipitation events in summer. Our work showed that it is technically feasible to produce continuous, longer-term data on δ v isotope composition in urban areas from in-situ monitoring using CRDS, providing novel insights into water cycling and partitioning across the critical zone of an urban green space. Such data has the potential to better constrain the isotopic interface between the atmosphere and the land surface and to improve ecohydrological models that can resolve evapotranspiration fluxes.

The higher elevation forests of Norfolk Island are regularly immersed in the clouds and scientific and anecdotal evidence suggests that in addition to rainfall, water is likely to be collected as cloud droplets are intercepted by the forest canopy. This water is likely to be important for the local hydrology and ecology, yet it has never been quantified. To address this, a field measurement campaign was established to measure hydrological inputs to the forest floor at two elevated forest sites in the Norfolk Island National Park. Instrumentation included throughfall and stemflow systems and measurements of rainfall in the open in nearby clearings. Sites exhibited very high stem density and basal area by international standards and delivery of water to the forest floor was dominated by stemflow because of the funnelling characteristics of the dominant palm and pine trees. Both sites showed similar hydrological behaviour with stemflow and throughfall of around 48% and 32%, respectively. Stemflow contributions of 48% far exceed observations from the literature which are typically less than 10%. Rainfall rarely occurred in the absence of low-level cloud and some cloud immersion events lasted for many days with hydrologic inputs continuing for extended periods despite rainfall not being observed in the open. Cloud interception accounted for approximately 20% of total water input at both sites which is equivalent to 25% extra water on top of rainfall measured in the open. From an island-wide perspective the calculated extra hydrological input is only small due to the limited spatial extent of elevated forest, however, the additional water is likely to be very important to local hydrological processes and the unique plants, insects and animals which inhabit the higher elevation forests of Norfolk Island.

Excess nitrate and sediment, mobilized by precipitation events and transported into surface waters, is a global water quality challenge. Recent advances in high frequency, in-situ water quality monitoring sensors have created opportunities to investigate constituent concentration dynamics during short-term hydrological changes. In this study, we characterized the event-scale variability of nitrate ( NO 3 -   ) and turbidity (a surrogate for sediment transport) in two large agricultural watersheds of the Upper Mississippi River Basin using hysteresis loop characteristics to determine sources and dominant transport mechanisms. We then applied factor analysis to detect variable groupings and thus determine controls on nitrate and sediment dynamics. We found that NO 3 -   hysteresis behavior was consistent between the two watersheds and demonstrated distal contributions and/or late-event mobilization and flushing that was controlled by the characteristics of the event hydrology (such as, event duration and magnitude of event discharge). In contrast, turbidity hysteresis loops indicated sediment delivery differed between the two watersheds; the smaller watershed with more diverse land use demonstrated consistent early-event flushing or rapidly responding pathways whereas the larger, more agricultural watershed showed variability between dilution vs. flushing as well as delivery pathways between events. This dynamic behavior as well as the magnitude of the hysteretic response was principally related to the time lag between turbidity and discharge peaks for the smaller site, and to the event peak discharge and subsequent stream erosive power at the larger site that switched behavior. This result is critical for watershed water quality management especially in the context of a changing climate and further underscores the utility of high-frequency sensors monitoring data to offer deep insights into hydrological processes controls on contaminant transport and delivery.

This commentary discusses a framework for the benchmarking of hydrological models for different purposes when the datasets for different catchments might involve epistemic uncertainties. The approach might be expected to result in an ensemble of models that might be used in prediction (including models of different types) but also provides for model rejection to be the start of a learning process to improve understanding.

Investigating the response characteristics of various hydrological factors to the construction of water conservancy projects and evaluating their impact on the ecological environment is crucial for ecological protection and restoration in the Loess Plateau, China with a complex environment. In this study, we employed a geomorphology-based hydrological model to simulate the hydrological elements of the Qinhe River Basin in the Loess Plateau. Additionally, we explored the response characteristics of the water cycle and hydrological processes to the construction of reservoirs in the basin. We also examined multiyear changes in peak flood volume and sediment discharge during flood seasons influenced by reservoirs. A thorough evaluation of the simulation results indicated their reliability. The sub-basins hosting reservoirs initially showed an increase in evaporation, followed by a decrease. During the change periods, both runoff and soil water decreased, but remained higher than the mean values for the basin during the same period. The Normalized Difference Vegetation Index of sub-basins associated with five reservoirs was significantly higher than the mean value for the basin during the same period. The peak flood volume and sediment discharge in the basin were characterized by decreasing trends, with the latter showing weak sustainability. The value of each index for a sub-basin associated with a reservoir was higher than the average value for the basin. The construction and operation of reservoirs had a positive impact on the ecology of the basin. Water and soil conservation measures, including sediment regulation and storage using reservoirs, significantly decreased water-related disasters and soil erosion in the basin. This study provides a scientific basis for the design of water conservancy projects and ecological governance in the basin.

Recently, superparamagnetic silica encapsulated DNA microparticles (SiDNAFe) were designed and in various experiments used as a hydrological tracer. We investigated the effect of bed characteristics on the transport behaviour and especially the mass loss of SiDNAFe in open channel injection experiments. Hereto, a series of laboratory injection experiments were conducted with four channel bed conditions (no sediment, fine river sediment, coarse sand, and goethite-coated coarse sand) and two water qualities (tap water and Meuse water). Breakthrough curves (BTCs) were analysed and modelled. Mass loss of SiDNAFe was accounted for as a first-order decay process included in a 1-D advection and dispersion model with transient storage (OTIS). SiDNAFe BTCs could be adequately described by advection and dispersion with or without a first-order decay process. Mass loss of SiDNAFe increased as a function of the surface roughness of the beds. Retention of SiDNAFe due to surface roughness was 1-2 orders of magnitude greater than gravitational settling rates, as determined in Tang et al. (2022). We speculate this was due to boundary layer kinetic attachment. The dispersive behaviour of SiDNAFe generally mimicked that of NaCl tracer, although SiDNAFe traveled faster on average due to a smaller effective cross-sectional area. No pattern was observed between SiDNAFe mass recovery and water qualities used. DNA concentration data uncertainty was mostly associated with lower SiDNAFe concentrations in the BTCs. This research highlights that riverbeds are important sinks, and the surface roughness affects the fate and transport characteristics of SiDNAFe when in proximity to the water-sediment interface. SiDNAFe possess promising potential as a surrogate for multi-tracing micro-contaminants (e.g., microplastics) in large rivers, which could be a promising tool for enhancing understanding of hydrological processes.

Jarvis-type model with a flexible parameterization of stress functions can improve the descriptions of physiological behaviour for specific vegetation species. However, it is criticized for the empirically formulated multiplicative equation that can deviate from the mutual impact of intercorrelated stress factors, e.g., vapor pressure deficit (VPD) and air temperature ( Ta). This study proposed a modified Jarvis model by adding reduction factors in the stress functions of VPD and Ta to provide a better description of stomatal conductance. The sap flow data of transpiration rate in a beech forest in the mid-latitude of Centre Europe was used to inversely estimate the stomatal conductance, which facilitated the formulation of stress functions. Taking two recommended parameterization strategies for general deciduous broadleaf forest (DBF) led to severe overestimation of transpiration rate with a maximum value of ~2 mm/day in rainless days, which suggested that the beech forest had rather different stomatal response. With the parameterization using boundary analysis, the unmodified and modified Jarvis model provided the better simulation of transpiration with NSE values of 0.75 and 0.77. The results suggested that modelling transpiration can be improved through a more specific parameterization of stomatal conductance, especially for a vegetation species featuring its own stomatal behaviour that differed from its belonged general vegetation type. Particularly, the modified Jarvis model can further improve the description of stomatal conductance and modelling of transpiration in vegetated areas, especially under dry environment conditions with relatively high VPD.

The catchment approach has been traditionally limited to small, experimental catchments where water fluxes can be determined with high accuracy. However, larger catchments where landscape management occurs have emergent drivers of streamflow at scale, and thus may exhibit novel responses to land cover disturbance. We used statistical models of water yield and annual maximum peak streamflow for multiple forested catchments in the low-relief glaciated region of central North America to investigate how forest disturbance may affect water yield and peak flows in similar landscapes. We utilized linear models, linear mixed effects models, and probabilistic flood-frequency analysis, with Bayesian parameter estimation in two case studies in Minnesota, USA: 1) a wildfire comprising ~30% of a 650km 2 wilderness Upper Kawishiwi catchment, and 2) 11 catchments within the St. Louis River Basin ranging from 56 to 8,880 km 2 with a patchwork disturbance regime wherein ~0.25% to 1% of the catchment is harvested or converted to non-forest land use each year. We also assessed for the most likely hydrological recovery year after forest disturbance, and the relative importance of stationary and nonstationary drivers of streamflow. We found forest disturbance correlated with declines in water yield for low-level disturbance regimes, but that water yield increased in response to the large-scale wildfire. Positive and negative associations of forest disturbance with peak flows were observed, generally with low confidence. Hydrologic recovery time ranged from 5 to 12 years for water yield and peak flows following disturbance. Despite these effects of forest disturbance on streamflow, effects of climate variability and stationary catchment size factors were more prominent drivers of streamflow. Basins larger than ~50 km 2 in low-relief glaciated regions were resilient to forest cover change when it comprised <30% of basin area, but climate change may have a larger effect than could be mitigated by land management.

Rapid urbanization and global climate change are likely to exacerbate urban flooding intensity, frequency, and uncertainty. Thus, it is fundamental and crucial to investigate the dominant influencing factors for the mitigation of urban flooding. However, the influence of building patterns on urban flooding remains limited. Taking Beijing, a typical megacity, as a case study area, we quantified the importance of building patterns and their interaction effect at the subwatershed scale using the boosted regression tree (BRT) and geographical detector model (GeoD). The results indicated that (1) the landscape shape index, slope, green space ratio and waterbody ratio were the most important influencing factors determining urban flooding, with a total relative contribution of 67.23%, (2) building metrics had a certain impact on urban flooding, and the sum of the relative contribution can reach 21.03%, (3) with urban flooding density, the landscape shape index, slope, and green space ratio exhibited a combination of negative and positive correlation, and (4) an enhancement effect existed between building metrics, especially the building congestion degree and building density. These findings provide quantitative insights that rational urban morphology planning can improve stormwater management and promote urban sustainability in megacities.

Estimating of soil sorptivity ( S ) and saturated hydraulic conductivity ( K s ) parameters by field infiltration tests are widespread due to the ease of the experimental protocol and data treatment. The analytical equation proposed by Haverkamp et al. (1994) allows the modeling of the cumulative infiltration process, from which the hydraulic parameters can be estimated. This model depends on both initial and final values of the soil hydraulic conductivity, initial soil sorptivity, the volumetric water content increase ( ∆ θ ), and two infiltration constants, the so-called β and γ parameters. However, to reduce the number of unknown variables when inverting experimental data, constant parameters such as β and γ are usually prefixed to 0.6 and 0.75, respectively. In this study, the values of these constants are investigated using numerical infiltration curves for different soil types and initial soil water contents for the van Genuchten-Mualem (vGM) soil hydraulic model. Our approach considers the long-time expansions of the Haverkamp model, the exact soil properties such as S , K s , and initial soil moisture to derive the value of the β and γ parameters for each specific case. We then generated numerically cumulative infiltration curves using Hydrus 3-D software and fitted the long-time expansions to derive the value of the β and γ parameters. The results show that these parameters are influenced by the initial soil water content and the soil type. However, for initially dry soil conditions, some prefixed values can be proposed instead of the currently used values. If an accurate estimate of S and K s is the case, then for coarse-textured soils such as sand and loamy sand, we propose the use of 0.9 for both constants. For the remaining soils, the value of 0.75 can be retained for γ . For β constant, 0.75 and 1.5 values can be considered for, intermediate permeable soils (sandy loam and loam) and low permeable soils (silty loam and silt), respectively. We clarify that the results are based on using the vGM model to describe the hydraulic functions of the soil and that the results may differ, and the assumptions may change for other models.

Bioretention is widely used in urban sustainable stormwater management. However, limited numerical research has been conducted on its performance in cold regions, particularly for winter snowmelt, spring runoff and summer large storms (> 50 mm) for urban flood mitigation. In this study, HYDRUS 1D was used to explore these knowledge gaps. The model was comprehensively calibrated and validated against 2-year hydrologic and water quality data of four bioretention columns with different designs under lab-simulated cold region conditions. The Morris method was used to measure the sensitivity and interaction of the calibrated hydraulic parameters. The model revealed that the effective hydraulic conductivity ( KS) values of the soil media were similar for winter snowmelt and spring runoff when the soil temperature was around -0.5 °C. Preferential flow is likely to occur in soil media during winter or spring of cold regions. The summer modeling showed that the bioretention could substantially reduce peak flow, ponding depth and duration for large storm events (even for 1:100 local storm with 83.4 mm in 4 hours). The water quality modeling confirmed experimental results that the bioretention effectively removed phosphate and ammonium but had leaching issues for chloride and nitrate. Finally, optimization and recommendations of bioretention columns were provided.

Every application of soil erosion models brings the need of proper parametrization, i.e., finding physically or conceptually plausible parameter values that allow a model to reproduce measured values. No universal approach for model parametrization, calibration and validation exists, as it depends on the model, spatial and temporal resolution and the nature of the datasets used. We explored some existing options for parametrization, calibration and validation for erosion modelling exemplary with a specific dataset and modelling approach. A modified version of the Morgan-Morgan-Finney (MMF) model was selected, representing a balanced position between physically-based and empirical modelling approaches. The resulting calculator for soil erosion (CASE) model works in a spatially distributed way on the timescale of individual rainfall events. A dataset of 142 high-intensity rainfall experiments in Central Europe (AT, HU, IT, CZ), covering various slopes, soil types and experimental designs was used for calibration and validation with a modified Monte-Carlo approach. Subsequently, model parameter values were compared to parameter values obtained by alternative methods (measurements, pedotransfer functions, literature data). The model reproduced runoff and soil loss of the dataset in the validation setting with R 2 adj of 0.89 and 0.76, respectively. Satisfactory agreement for the water phase was found, with calibrated saturated hydraulic conductivity (k sat) values falling within the interquartile range of k sat predicted with 14 different PTFs, or being within one order of magnitude. The chosen approach also well reflected specific experimental setups contained in the dataset dealing with the effects of consecutive rainfall and different soil water conditions. For the sediment phase of the tested model agreement between calibrated cohesion, literature values and field measurements were only partially in line. For future applications of similar model applications or datasets, the obtained parameter combinations as well as the explored methods for deriving them may provide guidance.

Dew is closely related to the micro-use of water and to large-scale hydrological processes. Dew formation on grasslands plays a particularly vital role in maintaining the ecohydrological cycle, however, its characteristics and sources were rarely reported. Here, stable isotope for dew, ambient water vapor, soil water, plant water, creek water, and precipitation were tracked to determine the characteristics of dew from ecohydrological processes in the meadow. The structural equation model was used to investigate how environmental factors affect dew formation. The Mix SIAR model in R was used to determine the sources of dew, and explore the dew transport route of six species of the graminoid-Kobresia meadow in an alpine graminoid-Kobresia meadow in northern Qinghai-Tibet Plateau. Our results showed that the annual amount was about 37.92 ± 1.03 mm, acounting for 7.13% of precipitation. Both atmospheric pressure and temperature showed significant positive effects on dew formation, while wind speed had a negative effect. Evapotranspiration indirectly affected dew formation. The contribution rates of soil water, plant water, and ambient water vapor to dew formation were 48.20±5.46%, 38.30±5.07%, and 13.50±1.82%, respectively. The proportion of dew utilization by graminoid and Kobresia species showed no significant species differences, the mean value was 10.5±3.8%. Our statistical analysis determines the role of dew in an alpine graminoid-Kobresia meadow in the northern Qinghai-Tibet Plateau, which provides an improved understanding of dew formation based on a stable isotope technology.

Increased urbanization, coupled with the projected impacts of climatic change, mandates further evaluation of the impact of urban development on water flow paths to guide sustainable land use planning. Though the general urbanization impacts of increased storm runoff peaks and reduced baseflows are well known; how the complex, non-stationary interaction of the dominant water fluxes within dynamic urban water stores sustain streamflow regimes over longer periods of time are less well quantified. In particular, there is a challenge in how hydrological modelling should integrate the juxtaposition of rapid and slower flow pathways of the urban ‘karst’ landscape and different approaches need evaluation. In this context, we utilized hydrological and water stable isotope datasets within a modelling framework that combined the commonly used HEC urban runoff model along with a simple hydrological tracer module and transit time modelling to evaluate the spatial and temporal variation of water flow paths and ages within a heavily urbanized 217km 2 catchment in Berlin, Germany. Deeper groundwater was the primary flow component within less urbanized regions of the catchments, with increased direct runoff and shallow subsurface contributions in more urbanized areas near the catchment outlet. The addition of wastewater effluent in the mid-reaches of the catchment was the dominant water supply to the lower stream, and sustained baseflows during the summer months. Water ages from each modelling approach imitated flow contributions and opportunity for mixing with subsurface storage; with older water and lower young water contributions in less urbanized sub-catchments and younger water and higher young water contributions in more urbanized regions. The results form a first step towards more integrated modelling tools for similar peri-urban catchments, given the potential limitations of more simple model frameworks. The results have broader implications for assessing the uncertainty in evaluating urban impacts on hydrological function under environmental change.

Restoring riparian shade helps maintain healthy stream ecosystems and computer models are helpful in guiding restoration. The effects of meandering and orientation on shade to direct and diffuse lighting was investigated using a simplified model of riparian vegetation. Previous studies have shown that straight streams oriented east-west (EW) experience higher lighting than those oriented north-south (NS). Meandering decreased reach-averaged direct lighting in EW valley streams but increased lighting in NS valley streams. When meander amplitude equalled 50% of meander wavelength, lighting of streams in EW and NS-aligned valleys was similar. Meandering and valley orientation had little effect on diffuse lighting. In EW valleys lighting was highest at stream bends and a strategy to reduce lighting quickly would be to prioritise planting of tall vegetation on bends. In NS valleys lighting was lowest at bends, suggesting the opposite planting strategy. Shade exceeding 70%, a criterion for preventing nuisance aquatic plant growths and stressfully high water temperatures, occurs in ‘typical’ meandering streams on cloudless, mid-summer days at mid-latitudes once H/ W exceeds about 2. The model over-simplifies riparian vegetation and the effects of canopy shape and overhang merit further investigation.

Water sources carry chemicals that can have a significant impact on the water environment of a river network, and understanding the contribution of different water sources to the river network can help to manage the pollution of the river network at its source. Hydrological connectivity of a river network affects the self-purification capacity and flood prevention capacity of the river. Thus an isotope tracer approach was applied to figure out the contribution rate of different water bodies to a river network and hydrological connectivity was quantified by introducing retention rate. Changzhou city was selected as the study area because it is an urbanized city with the characteristics of plain river network and it is faced with poor hydrological connectivity due to artificial constructions (dams and pumps) and human activity (urbanization). River water, well water (shallow groundwater), lake water and rainfall were collected during the flood season and nonflood season, and hydrogen and oxygen isotopes were determined. The temporal and spatial variations in hydrogen and oxygen isotopes in different water bodies and the state of the water cycle in different water bodies were analyzed. IsoSource and MixSIAR models were established to analyze the contribution rate of river network water sources in the study area, and their effectiveness was compared. Results of MixSIAR model were selected to evaluate the hydrological connectivity of the river network in the study area, providing a method to quantify the hydrological connectivity of specific river of the river network in Changzhou. This method could also be applied to other urban plain river network area to study its river connectivity.

We discuss a study that aimed to understand the genesis and inflow conditions of abstracted water by intake with induced infiltration located at a mountain river. A simple approach based on a combination of two research techniques was used: two-component water mixing modelling and studies of the variability of concentrations of environmental tracers in a dynamic test. This approach is versatile, easy to apply and modify, and can be good method for controlling surface/groundwater interactions. We used the gas tracers because gas exchange and dissolution of gases in infiltrating rainwater gives it a unique gas signature that is largely retained in groundwater. We focus on understanding river/aquifer interactions at the scale of reach of an intake. To understand these issues, a two-day field hydrogeological experiment based on a pumping test of increasing intensity was conducted. At each pumping stage, groundwater and river samples were collected to determine the concentration of noble gases, CFCs, SF6, stable isotope content, and chemical composition of the water. The study results indicate a short pressure propagation time between the intake and the river, which results in inflow of water already at low water abstract by the intake. Despite limited river water inflow, there was a continuous hydraulic contact river/aquifer, largely independent of changes in pumping intensity. As pumping intensity increased, the river/aquifer hydraulic system tended towards stabilised conditions of mixing and inflow to the intake. In general, a small proportion of river water (up to 26%) was found in the exchange flux between the river and groundwater.