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

not-yet-known not-yet-known not-yet-known unknown Land surface models (LSMs) are used to simulate water and energy fluxes between the land surface and atmosphere. These simulations are useful for water resources management, drought and flood prediction, and numerical climate/weather prediction. However, the usefulness of LSMs are dependent by their ability to reproduce states and fluxes realistically. Accurate measurements of water storage are useful to calibrate and validate LSMs outputs. Geological Weighing Lysimeters (GWLs) are instruments that can provide field-scale estimates of integrated total water storage within a soil profile. We use field estimates of total water storage and subsurface storage to critically evaluate two different land surface models: the Modélisation Environnementale communautaire - Surface Hydrology (MESH) which uses the Canadian Land Surface Scheme (CLASS), and the Structure for Unifying Multiple Modeling Alternatives: (SUMMA). These models have differences in how the processes and properties of the land surface are represented. We attempted to parameterize each model in an equivalent manner, to minimize model differences. Both models were able to reproduce observations of total water storage and subsurface storage reasonably well. However, there were inconsistencies in the simulated timing of snowmelt; depth of soil freezing; total evapotranspiration; partitioning of evaporation between soil evaporation and evaporation of intercepted water; and soil drainage. No one model emerged as better overall, though each model had specific strengths and weaknesses that we describe. Insights from this study can be used to improve model physics and performance.

Oxbow lakes are iconic fluvial landforms found in the floodplains of meandering rivers around the world. Their formation is associated with meander cutoff, a process that excises sections of river channel to optimise the downstream transmission of water and sediment. After termination, sedimentary plugs form at either end of the abandoned channel to isolate it from the mainstem. Overbank floods and conveyance through tie channels maintains some hydrological connectivity, but lakes are generally considered to passively infill until they become terrrestrialised. Here, a suite of 64 lakes across two meandering rivers in the Bolivian Amazon are used to demonstrate the hydrological dynamism of oxbow lakes after cutoff by quantifying interannual variations in lake water surface area (WSA) and the mechanisms controlling them. The results suggest that WSA variations are controlled by proximity to the active channel, with the magnitude of these variations being set by mechanisms of connectivity. Lakes connected by tie channels experienced WSA changes up to 3.9 times larger than lakes with no visible connection mechanisms. Incursion lakes displayed similar WSA changes to those with tie channels, while isolated lakes were found furthest from the mainstem and had the smallest range of WSAs. Chute-lakes experienced a wider range of WSAs and were more strongly controlled by mainstem proximity than neck-lakes. An understanding of the processes governing oxbow lake hydrodynamics is important for forecasting nutrient and contaminant fluxes as well as the sensitivity of riparian wetlands to changes in catchment hydrology associated with climate change and flow modification.

Changes in groundwater recharge in areas where irrigated agriculture has increased despite low precipitation and surface water allocation are a big concern. The increasing water demand has raised global drought intensities by 10-500%, worsening hydrological drought. Agriculture accounts for 69% of total groundwater abstraction worldwide, and South America expects to face unprecedented hydrological drought conditions within the next 30 years with uncertain impacts (UNESCO 2022, Wada 2016, Wanders & Wada 2015, Siebert et al. 2015). This study compares coupled and non-coupled versions of PCR-GLOBWB2.0 with MODFLOW regarding model selection and scenario comparison. Natural and human scenarios are presented to understand the effects of groundwater depletion and drought recovery. The scenario comparison evaluation of groundwater fluxes, drought characteristics, and recovery reveals changes in the water cycle due to anthropogenic impacts. This is valuable for identifying drought vulnerability in regions where water management is critical for human consumption and ecosystems.

The increased drought risk, stemming from global warming, has intensified ecological issues and consequently, positioned ecological drought as a significant research topic within the field of eco-hydrology. Nevertheless, due to limitations in the precision of spatiotemporal data, there remains a divergence of opinions regarding the primary water resources for vegetation growth in the northwest region of China, whether it be precipitation or groundwater. Consequently, this study endeavors to construct meteorological drought index, groundwater drought index, and ecological drought index, utilizing precipitation, groundwater storage anomaly, and ecological water deficit, respectively. The maximum correlation coefficient and residual analysis methods were used to analyze the joint and relative impacts of meteorological drought and groundwater drought on ecological drought. The primary findings can be summarized as follows: (1) The area dominated by the joint impact of precipitation and groundwater on the ecological drought variation accounts for about 60%, mainly distributed in arid and semi-arid regions. (2) In spring, summer, autumn, and winter, the average contribution of the joint impact of precipitation and groundwater to the increase in ecological drought variation is between 0.26 and 0.43. (3) In contrast to precipitation variation, ecological drought induced by groundwater scarcity predominantly impacts regions like southern Shaanxi, southeastern Gansu, and southern Qinghai. These areas represent between 12.7% and 21.8% of the total area.

Soil water is a crucial factor for the growth of vegetation and sustainable development in water-limited areas. After large-scale vegetation restoration on the Chinese Loess Plateau, understanding the relationship between vegetation and deep soil moisture has become a crucial focus in current research. In this study, we selected artificial forest ( Pinus tabulaeformis, Robinia pseudoacacia, and Platycladus orientalis), apple orchard, secondary forest and farmland as the research objects, and native grassland as the control, using soil drilling techniques, we systematically monitored the soil water content of 0-10 m soil layer over two hydrological years, and explore the effects of different vegetation types on soil water deficiency. The results showed that: (1) The deep soil water various significantly among different vegetation types, which indicating the depth of the influence of vegetation on soil water has reached 10 m. (2) The mean deficit size values of P. tabulaeformis (0.14), R. pseudoacacia (0.17), P. orientalis (0.07), apple orchard (0.15) and secondary forest (0.10) and farmland (0.27) were positive in 0–1 m, indicating that surface soil water had accumulated during more than half of the sampling periods. In the 2–10 m soil layer, the mean deficit size was negative in all vegetation types except in farmland, leading to soil desiccation. The deficit size was found to fluctuate with soil depth. (3) Soil water deficit degree was affected by a combination of soil properties and vegetation growth. Altitude, soil bulk density and canopy density had a significant impact on soil water deficit. Our results indicate that the current afforestation model could lead to the deficiency of deep soil water. Therefore, in planning future vegetation allocation and management, it is imperative to make reasonable vegetation structure according to the available local soil and water resources.

Aquifer-Stream Interactions (ASIs) play a critical role in effective groundwater management, yet their complex dynamics remain poorly understood in channelized lowland perennial streams. This study presents a multi-scale, multi-technique investigation of ASIs along two streams in Shelby County, Tennessee, overlying the confined Memphis aquifer. The goal is to define a suitable methodology for characterizing ASIs in this specific hydrological setting, serving as a starting point for developing a standardized approach. The methodology includes an initial evaluation of various field techniques, followed by extensive surveys using potentiomanometers, electromagnetic induction (EMI), vertical temperature profilers (VTPs), and complementary techniques such as seepage meters, bank tests, and well-data analyses. Results reveal distinct hydrogeomorphic behaviors across and along the streams, challenging the ASI homogeneity notions typically assumed in groundwater models. Nonconnah Creek exhibited streambed colmation and negligible hydraulic gradients, resulting in disconnection from the aquifer during low flows, except for a 300-m losing reach with high downward gradients, potentially degrading the Memphis aquifer. In contrast, the Loosahatchie River displays relatively homogeneous streambed properties and small, upward hydraulic gradients, suggesting uniform ASIs along the surveyed reaches. EMI proved highly effective for mapping streambed sediments quickly, while potentiomanometers accurately measure small head differences critical for understanding ASI dynamics. VTPs were less practical due to extended data-collection times and vulnerability to flooding. This study emphasizes the importance of site-specific, multi-scale investigations using diverse techniques to accurately characterize ASIs in lowland streams, highlighting the confounding influences of geological formations, anthropogenic alterations, and depositional processes on groundwater-surface water interactions. The findings contribute to refining local water balances, informing groundwater management strategies, and underscoring the need for incorporating local-scale field data into regional groundwater models. The proposed methodology serves as a foundation for developing a standardized approach for characterizing ASIs in lowland channelized perennial streams, adaptable for similar stream systems worldwide.

Dried soil layer (DSL) is a common phenomenon forms by soil moisture deficiency in the Chinese Loess Plateau (CLP). In order to reveal the temporal stability and elimination degree of DSLs, we obtained the 19 occasions data of soil water content (SWC) at 15 observation sites in a typical slope-gully unit. The available soil moisture (ASM) and DSL were estimated by representative sites which determined through the temporal stability method, and then assessed the reliability of simulation equations between representative locations condition and mean conditions of the study area. Results show that: (1) The temporal dynamics of DSL were characterized with somewhat complexity. The ASM within the DSL (DSL-ASM), ASM within the sandwiched dried soil layer (SDSL-ASM) and quantitative index (QI) varied within the range of 2.75%–3.11%, 2.98%–4.22% and 0.254–0.356, respectively. (2) The possibility of development and recovery for DSL and SDSL in deep layers were less than that in surface layers. The maximum depth of DSL (DSLMD) was significantly and negatively related to the SDRD of DSL-ASM, the maximum depth of SDSL (SDSLMD) was negatively related to the SDRD of SDSL-ASM. (3) The prediction results of ASM above 300 cm depth were more accurate than other layers ( R 2=0.89). The DSL-ASM has a more prediction accurate than SDSL-ASM and QI. On the analysis of the time stability characteristics of ASM and dried soil layers, A2 and C3 can better represent the mean conditions of ASM at the depth of three and four soil layers, respectively. C2, A1 and A1 can better represent the average levels of DSL-ASM, SDSL-ASM and QI, respectively ( R 2=0.43, 0.14 and 0.18). (4) The restoration degrees of DSLs mainly showed no elimination and slight elimination, the DSLs cannot be completely eliminated within a short time. Scientific regulation of soil moisture can alleviate the formation and development of DSLs a certain extent, and provide the possibility for DSLs restoration.

s: In riverine, marsh, coastal, and other environments, vegetation communities are widely distributed and interact with the flow system to produce more complicated flow structures. Four sets of indoor flume vegetation flow experiments were conducted using a typical Beach trough structure in the lower sections of the Yangtze River as an example. The compound channel was divided into the main channel zone, side slope zone, and side beach zone, and simulated vegetation such as reed, sedge, and dwarf grass was used. The emphasis was on the hydrodynamic properties under semi-covered submerged rigid vegetation and semi-covered non-submerged rigid vegetation. This research focuses on the Shiono and Knight equations (SKM model) to clarify the distribution characteristics of cross-section flow velocity and the ”equivalent diameter D” of gradual vegetation in water. We also propose a new secondary flow model using a genetic algorithm and investigate the relationship between the parameters of the vegetation the flow structure and the distribution pattern of the secondary flow coefficient values. Finally, it employs the Taylor method to demonstrate that the proposed ”equivalent diameter D” of the vegetation has some value within a reasonable threshold range. Eventually, the depth-averaged velocity of the compound channel was accurately predicted by combining the experimental data with the novel SKM model. The proposed model can provide technical support for river flooding.

The Yarlung Zangbo River Basin (YZRB), located in the Qinghai-Tibetan Plateau, has been significantly impacted by global warming and greening. Serving as an indicator of coupled vegetation growth and climate variation, the spatiotemporal land surface temperature (LST) has undergone substantial changes in recent decades. In this study, we evaluated the components of the water and energy cycle from 1980 to 2015 using the VIC model, a widely recognized and applied distributed hydrological model, to obtain continuous 35-year daily LST data. The results demonstrated that the VIC model exhibited high adaptability in the YZRB. Then, the fluctuation of LST was examined, and the influence of environmental elements on LST was identified. Our modeling indicated that climate factors were increasing, while human activities remained stable in the YZRB. In YZRB, the greening was witnessed while LST showed an increasing trend. By distinguishing the impacts of climate and human activities on LST, LST was mainly affected by climate with contribution rate at 70.36% from 1980 to 1995. After 1995, LST was mainly affected by human activities, and its contribution rate was 55%. Grassland with medium cover showed the potential of a cooling influence. Among all environmental factors, albedo showed a negative and delayed effect on LST. Temperature, precipitation, and evapotranspiration were positively correlated with LST and displayed relatively synchronous changes. Soil moisture and NDVI were detected as leading positive changes in LST. Our study contributes to clarifying the mechanisms influencing LST in high-altitude and high-latitude regions under global greening, providing fundamental insights for socio-economic development in alpine mountainous regions.

Issues surrounding gender equality are – and should be - front and centre in the water resources community, and other STEM fields. Very necessarily, the focus tends to remain on recruitment and inclusivity offering support for students and early career academics. The leaky pipeline concept used to describe the incremental loss of women from STEM fields with career duration results in a disproportionate loss of women, creating a parallel problem where highly qualified, top tier academics are disproportionately lost from the system after significant financial and personnel investment by institutions is made. Ultimately, the leaky pipeline undermines the extensive investment of the hydrology and other STEM communities in equity, diversity, inclusion, and accessibility (EDIA) recruitment and retention programs by cutting short career ambitions and the trajectories of diverse top performing individuals, resulting in no net benefit of EDIA policy investments. Addressing this critical gender gap requires the attention and support of the hydrology community of practice with specific focus on generating opportunities for advancement, confronting systemic and structural biases, and improving education around allyship. Institutions and professional organizations need to consciously grow diversity in leadership and recognize and outwardly manage the perception of academic excellence around slow research and education that attracts increased diversity. Supporting allyship, reducing competitiveness among community members, and reinforcing collaboration will not only attract, but retain, a higher proportion of diversity in the hydrology community, academia, and STEM professions in general. It is time for the water resources (and other STEM) communities to demand broader accountability and recognition of the barriers to women, implement and reward more diverse definitions of research excellence, and offer allyship training to the community of practice at large.

Changes in rainfall patterns due to climate change may accelerate the runoff of sulfur (S) and nitrogen (N), which are anthropogenically emitted as air pollutants and deposited and accumulated in forest soils. In this study, intensive observations were conducted in a forested catchment on the Sea of Japan side to clarify changes in stream water quality and runoff processes during rainfall events. The pH, electrical conductivity, and sulfate (SO 4 2−) concentration in the stream water decreased with increasing flow rate, whereas the nitrate (NO 3 −) concentration increased. The SO 4 2− concentration was negatively correlated with the flow rate, and the NO 3 − concentration was positively correlated with the cumulative precipitation amount of each event and remarkably increased when it exceeded 10 mm. Hydrograph separations using the water isotopic parameter (deuterium excess, d-excess = δ 2H – 8 × δ 18O) showed that most of the stream flow during the rain events was derived from pre-storm water, whereas the portion derived from rainwater increased at rainfall intensities stronger than 10 mm h −1. The S isotope ratio (δ 34S) in the stream water fluctuated minimally compared to the d-excess value during rainfall events, suggesting relatively minor contributions of precipitation SO 4 2− to stream water via direct inflow. The SO 4 2− well homogenized in soil layers mainly contributed to the discharge during the rainfall events. Future climate change may further accelerate S and N runoff from forest catchments and disrupt material cycles in the ecosystem if warming causes more intense rainfall.

Stable isotopic methods in hydroclimate monitoring are powerful for improving water resources management, but applications are limited, especially in semi-arid regions where such management is needed most. In this study, we show that we can address shortcomings related to lack of a seasonal signal using stable water isotopic signatures measured over the eastern San Francisco Bay Area of California during two contrasting events. We use hydrometric data from a gauged watershed in the study area and isotopic signatures of rain sampled at more than 20 locations during two contrasting storm events (Winter Storm Olive in February, 2023 and a warmer atmospheric river event in March 2023), and apply a solute transport model with a travel-time approach to examine predicted watershed responses and potential water tracing applications. The observed range in δ 18O in the rain samples is similar for both storms, about 5‰. However, the distributions do not overlap – the mean air temperature during Olive was about 2 0C, and the mean δ 18O of the rain samples is -12‰, while the AR event had a mean temperature of about 9 0C and a mean δ 18O of -6‰, close to the long-term average δ 18O measured in local precipitation. In the model results, event size exerts a strong control on the relative amounts of runoff vs pre-event water in the stream, while uncertainty in stream hydrograph separation is related to the degree of contrast between precipitation/runoff and pre-event water. Key to flood prediction, adaptation and mitigation, especially in coastal urban areas, is knowledge of the contributing water sources and timing of flows in streams and other features susceptible to flooding. The strong contrast in stable isotopes between these two events over the same area, illustrates the potential to use stable isotope signatures to track the transport and mixing of event water through natural and engineered watersheds.

We used imagery from remote sensing (FORCE Time Series Analysis submodule (combining Landsat and Sentinel-2 imagery)) to derive spatially distributed times series (8 years) of NDWI data to infer patterns of floodplain inundation and river-floodplain connectivity in two contrasting polders in the Lower Oder Valley National Park. The upstream Polder A (14.4 km 2) was extensively flooded for prolonged periods most winters. Wavelet analysis showed that this strong seasonality was primarily driven by winter water levels in the river Oder that could enter and leave the polder through two opened flood gates. Subsequent drainage was slow and aided by a pumping station. Inundation of the downstream Polder 10 (17.7km 2) was lower and had less marked seasonality. This reflected the impact of flood attenuation by storage in Polder A upstream, but also the greater connectivity (via 10 flood gates) to the Oder and a functional network of channels which facilitated rapid drainage after flood peaks. In Polder A, secondary periods of transient inundation could also occur in response to local intense summer rainfall. Wavelet analysis also showed that groundwater recharge in and around Polder A is primarily induced by floodwater, whilst Polder 10 also reflects the influence of local rainfall-driven recharge. The flood regimes of the two polders showed marked inter-annual variation, largely dependent on flows from the upper Oder catchment. Understanding these patterns and processes of inundation is important for both managing flows and sustaining valuable wetland habitats within the National Park. Given projected climate change in eastern Europe and possible management alterations to the flow regime of the Oder, the potential implications for these habitats needs urgent attention.

Mountain meadows are ecologically important, but often degraded, groundwater dependent ecosystems that retain and store water in upland forested landscapes. They tend to occur in low-gradient, broad valleys where water naturally slows and sediment accumulates, making them efficient locations for restoration. Over a century and a half of land use has degraded many meadows in the Sierra Nevada, reducing their hydrological and ecological functionality. Process-based restoration is a potentially economical and scalable restoration approach for numerous, small, remote, degraded mountain meadows. The approach uses onsite materials and leverages fluvial processes to achieve restoration objectives including increases in wetted area, groundwater elevations, sediment capture, and development of multithreaded channels. These changes in hydrological functionality can lead to improved ecological function over time. This study compares pre- and post-restoration surface and groundwater conditions in a degraded riparian meadow in the Sierra Nevada, California U.S.A. to understand changes in meadow hydrogeomorphic function following process-based restoration. Restoration included the installation of 35 postless beaver dam analog structures in ~1 km of incised meadow channel. Stage-discharge data at the inlet and outlet of the project area were paired with groundwater data collected from 15 wells distributed across the meadow in a power law model to estimate increased water storage of 3700 m 3 (~3 acre-ft) due to restoration. After the wet winter of 2023, we estimated that pools behind structures filled to over half their volume with fine sediment. We also applied hydrodynamic modeling to evaluate fluvial changes at high flows and found that restoration increased flow complexity and wetted surface area. These short-term responses highlight the potential speed and effectiveness of low-tech, process-based restoration in achieving desired restoration outcomes.

Changes in evapotranspiration and its response to control variables are crucial for understanding water balance and climate change in high-altitude areas. Environmental changes will inevitably disturb regional water cycles and water balance, especially in the high-altitude alpine regions of the Qilian Mountains. To better understand the variation of evapotranspiration at different altitudes in the high-altitude region of the Qilian Mountains and the applicability of the model and its response to environmental factors, we measured the variation of actual evapotranspiration at three altitude gradients using meteorological stations and automatic observation of continuous data with a weighing-type micro-lysimeter at three altitude gradients of 3797 m, 4250 m, and 4303 m in the Shaliu River basin of the Qilian Mountains during the growing season from June 2020 to October 2022 in our research. Using ten models to calculate the variation of reference evapotranspiration, and fitting them to the actual evapotranspiration, we selected the most suitable model. The results showed that the cumulative total evapotranspiration during the growing season in our study period was 1974.556 mm, 2203.066 mm, and 2201.393 mm, respectively, with intra-annual fluctuations consistent across the three elevation gradients. The value of evapotranspiration in August showed the highest at the monthly scale of 4.809 mm·day -1 and a bimodal variation at the daily scale with peaks at 10:00 and 15:00. The model of Dalton simulations showed the best results with the lowest analysis of residuals (RA), root mean square error (RMSE), and percentage error (PE), which had values of 3.291 mm·day -1, 3.994 mm·day -1, and 0.692%, and the values of R 2 between simulated and measured values of 0.622, 0.609, and 0.420. Water balance results showed that a portion of evapotranspiration in the study area originated from deep soil moisture. Partial Least Squares Regression (PLSR) analysis and enhanced regression tree model results indicated that precipitation was the most important variable, with Variable Importance in Projection (VIP) scores of 2.079 and a relative contribution to evapotranspiration of 52.6%. Overall, moisture conditions and precipitation were important factors limiting evapotranspiration variation in our research area. Our findings have implications for future climate change conditions. This conclusion is important for future water budget details in alpine mountains under climate change.

This paper demonstrates that the multivariate monitoring methods are capable to underpin the systematic investigation of the hysteretic behavior occurring during gradually-varied flows. For this purpose, we present simultaneous measurements of stage, index velocity, and free-surface slope acquired continuously with high-frequency sampling instruments deployed at several river gauging sites exposed to a range of storm magnitudes. The experimental evidence reveals intrinsic features of unsteady open-channel flow mechanics that are hinted by pertinent governing equations but rarely substantiated with in-situ measurements. The illustrations are intentionally made for fluvial waves propagating at sites located in lowland areas where the relationships among flow variables are most likely displaying hysteretic loops and phasing in the hydraulic variable progression. The set of presented measurements highlights that: a) the hysteretic behavior is apparent in both time-independent and time-dependent graphical representations of any two of the hydraulic variables; b) the severity of the hysteresis is commensurate with the geomorphic, hydraulic, and hydrological characteristics of the measurement site; and c) there is a pressing need for changing the flow paradigms currently used in tracking flow variables during gradually-varied flows. Also discussed are research needs associated with flow hysteresis for advancing the understanding of the mechanisms underlying the movement and storage of water in the lowland river environments as well as for increasing the accuracy of streamflow monitoring, modeling, and forecasting.

This research analyses the snow depth distribution in canopy gaps across two plots in Central Pyrenees, to improve understanding of snow–forest and topography interactions. Snow depth maps, forest structure–canopy gap (FSCG) characteristics and topographic variables were generated by applying Structure from Motion algorithms (SfM) to images acquired from Unmanned Aerial Vehicles (UAVs). Six flights were conducted under different snowpack conditions in 2021, 2022 and 2023. Firstly, the snow depth database was analyzed in terms of the ratio between the radius of the canopy gap and the maximum height of the surrounding trees ( r/ h), in order to classify the gaps as small-size, medium-size, large-size or open areas at both sites independently. Then the Kendall correlation coefficients between the snow depth, FSCG and topographic variables were computed, and a Random Forest (RF) model for each survey day was implemented, to determine the influence of these variables for explaining snow depth patterns. The results demonstrate the high reliability of the UAV SfM photogrammetry approach for measuring snowpack dynamics at fine scale in canopy gaps and open areas. At site 1, the larger the r/ h observed, the greater was the snow depth obtained. This pattern was not evident at site 2, which presented high variability related to the survey dates and categories, highlighting the relevance of topography for determining optimum snow accumulation in forested areas. Slope systematically exhibited a negative and significant correlation with snow depth, and was consistently the highest-ranked variable for explaining snow distribution at both sites according to the RF models. Distance to the Canopy Edge also presented high influence, especially at site 1. The findings suggest differences in the main drivers throughout each site and survey of the topographic and FSCG variables are needed to understand snow depth distribution over heterogeneous mountain forest domains.