ABSTRACT Contaminants found in treated wastewater discharged to streams, including pharmaceuticals and per‐ and polyfluoroalkyl substances (PFAS), are of global concern due to their deleterious effects on aquatic ecosystems and potential impacts to human health. Hyporheic zones have strong potential for contaminant attenuation. Assessing this potential requires collection of physical and biogeochemical data within the hyporheic zone. This study tested the applicability of a novel drive‐point multilevel system (DP‐MLS) for quantifying head profiles and characterizing contaminant concentrations in the hyporheic zone of a temperate region effluent dominated stream (EDS). DP‐MLS, each with 4 ports, were installed in the stream bed at two sites, DS‐1 and DS‐2, 0.2 and 4.7 km downstream of the effluent outfall, respectively. Head profiles were measured and groundwater collected for analysis of pharmaceuticals and PFAS temporally over two years. The DP‐MLS withstood rapid changes in stage, ice formation, and floating debris. Vertical hydraulic gradients (VHG) were generally upward but varied in magnitude indicating heterogeneity in hydraulic conductivity and variability in flow conditions. Upward VHG were also about 2X larger at DS‐1 than at DS‐2. Contaminant concentration profiles consistently showed penetration of pharmaceuticals and PFAS to 1 m below the bed at DS‐2 while there was less penetration, lower groundwater concentrations, and more temporal variability in concentrations at DS‐1. Integration of the physical and chemical data suggests weaker upwelling conditions at DS‐2 are more easily reversed during periods of high stream stage, which could facilitate migration of wastewater contaminants into the bed. However, further studies incorporating other transport processes and reach scale dynamics are required to fully characterize these exchanges. Overall, this study demonstrates the efficacy of these novel DP‐MLSs for characterization of the hyporheic zone and provides new insights into the occurrence, composition, and persistence of wastewater derived contaminants in the hyporheic zone of a well‐studied EDS.

ABSTRACT High mountain areas are experiencing rapid hydrological shifts, due to a decline of meltwater sources and a concomitant increase of solute concentrations in freshwater ecosystems. While research on water quality is increasing on river systems influenced by glaciers and rock glaciers, very limited knowledge exists on those influenced by other landforms such as moraines and talus slopes. During late summer 2021, we investigated the chemistry and δ 18 O in the water of 80 high‐elevation springs in 6 massifs of the Central‐Eastern European Alps. These springs were sourced either by glaciers, young moraines, rock glaciers, talus slopes, or reference slopes mantled with soil. End‐member mixing models revealed a large fraction of (snow or glacier) meltwater at all springs (> 60%), even though rainwater had a higher contribution (up to 40%) at lower elevations. Lithology and the likelihood of ice influence, an indicator that we built using the spring water temperature and the Alpine permafrost map, were the most important predictors of water chemistry in regression tree analyses. Springs draining catchments dominated by paragneisses and/or micaschists had high concentrations of sulphate, nickel, manganese, and other metals. In these geochemical hotspots, all glacier springs, 70% of rock glacier and young moraine springs, 60% of talus slope springs, and 25% of reference slope springs had poor water quality (European Union standards for drinking water). In areas with other lithologies, only 13% of springs exhibited poor water quality. For the springs on paragneisses and/or micaschists, the permafrost presence and the motion of different landform types may enhance nickel concentrations due to acid rock drainage. Finally, we discuss the importance of our findings for water management and freshwater ecosystems.

ABSTRACT Understanding the complex interactions between different types of droughts is essential for effective drought prevention and disaster management. This study employed the extended convergent cross mapping (CCM) algorithm to analyse the interactions between meteorological drought (standardised precipitation index), soil drought (standardised soil moisture index), vegetation drought (standardised normalised difference vegetation index), and groundwater drought (standardised groundwater index) in the middle reach of the Heihe River Basin in China at temporal scales of 1, 3, 6, and 12 months. A new method was proposed to assess the convergence of the CCM algorithm. The results showed that the temporal scale affected the intensity and the time lag of unidirectional or bidirectional interactions between different drought types. The intensity of the interaction between meteorological and other droughts increased, and the time lag shortened as the temporal scale increased. Meteorological drought had the longest lag time and the weakest effect on vegetation drought, indicating that precipitation was not the only factor affecting vegetation drought in this region. Groundwater drought influenced soil and vegetation droughts across all temporal scales, highlighting its crucial role in drought propagation in this region and underscoring the need for consideration.

ABSTRACT Climate change profoundly influences local microclimates, and plant transpiration processes are directly regulated by these microclimatic conditions. As a key topographic factor, mountain slope aspects shape local microclimatic characteristics through the redistribution of hydrothermal resources. Although plant growth and physiological processes are more directly influenced by local microclimates than by macroclimates, the differentiated responses of plant transpiration dynamics to environmental factors across different slope aspects and their underlying driving mechanisms remain unclear. This study investigates Zanthoxylum bungeanum forests on sunny and shady slopes at the same altitude in plateau gorges within a subtropical monsoon climate zone. Through in situ monitoring of sap flow (SF), local meteorological factors, soil water content (SWC) and stable isotope composition, we explored the effects of differences in microclimate and SWC on plant transpiration and water use strategies. The results showed that: (1) Water use patterns exhibited significant seasonal and slope‐related differences. During the rainy season, plants on both slopes primarily relied on shallow soil water. In the dry season, plants on the shady slope consistently utilised deep soil water (30–60 cm), whereas plants on the sunny slope rapidly shifted to using shallow and intermediate water sources following light rainfall, facing a higher risk of water source depletion. (2) During both dry and rainy seasons, SF on both slopes was primarily driven by vapour pressure deficit (VPD) and air temperature (Ta), which exerted threshold control effects on SF. These thresholds varied with slope aspect and season. Specifically, the VPD threshold during the rainy season was sunny slope (2.7 kPa) < shady slope (3.2 kPa); during the dry season, it was sunny slope (31.9°C) > shady slope (30.1°C). The Ta threshold during the rainy season was sunny slope (34.3°C) < shady slope (38.4°C); during the dry season, it was sunny slope (31.9°C) > shady slope (30.1°C). Furthermore, the stomatal conductance of plants on the shady slope was more sensitive to changes in VPD and Ta than that of plants on the sunny slope. Therefore, under drought conditions, plants on the shady slope exhibited a more robust water use strategy, with greater stability in transpiration and stomatal regulation, significantly reducing the risk of hydraulic dysfunction and demonstrating stronger drought adaptability compared to plants on the sunny slope.

ABSTRACT The Mediterranean region is highly vulnerable to climate change (CC), with expected decreases in precipitation and rising temperatures leading to increased evapotranspiration and reduced water availability. The Tagus River Basin (TRB), shared by Spain and Portugal, is the Iberian Peninsula's third largest and most populated basin. Its upper section (UTRB) is especially critical, as it hosts 7.5 million people and supplies the largest inter‐basin water transfer in Spain. CC scenarios considered for the TRB management plan were assessed using a conceptual model—not able to reproduce its complex hydrological behaviour—for all Spain, jeopardising the reliability of the estimated impacts and requiring advanced methodologies to further evaluate future CC scenarios and to provide robust information for water resource management. In this study, we used a detailed and reliable SWAT+ model of the UTRB—calibrated considering the simulation of hydrological processes—together with up‐to‐date downscaled and bias‐corrected projections, to simulate and analyse four CC scenarios using 11 global climate models. Scenarios were evaluated at different spatiotemporal scales and considering their impact on numerous hydrological processes. Our results indicate that uncertainty related to climate scenarios and models is noticeably larger for the end of the 21st century, as the magnitude of the predicted impacts, which include a large reduction of water availability for all processes, especially under the most pessimistic scenarios. We found greater reductions in the UTRB than those identified in the Spanish administration's national‐scale study, which suggests that climate‐change impacts may be underestimated and that further study of CC effects on Spanish water resources is needed. The comprehensive evaluation of CC scenarios, and especially the use of a detailed hydrological model, provides a reliable estimation of the magnitude and uncertainty of CC impacts in this relevant basin, while also offering key methodological guidance for replicating this robust assessment in other basins.

ABSTRACT Himalayan river basins are highly vulnerable to global warming particularly in glacier‐ and snow‐fed regions like the upstream Teesta River Basin. This study investigates the spatio‐temporal impacts of future climate scenarios on hydro‐sedimentological processes using the SWAT model coupled with CMIP6 projections under four emission pathways (SSP1‐2.6, SSP2‐4.5, SSP3‐7.0 and SSP5‐8.5). Temporal trends and scales of key hydro‐sedimentological variables were assessed using the Mann‐Kendall test and Sen's slope while spatial variability was analysed using model outputs to comprehend how climate signals manifest across different parts of the basin. Both precipitation and temperatures are projected to rise significantly with more intense changes under high‐emission scenarios by the late 21st century. Minimum temperatures are expected to increase more swiftly than maximum depicting a strong warming trend. Southeastern and western regions, already wetter, may see further increases in water availability through enhanced precipitation, lateral flow and groundwater recharge. Valley reaches with stronger surface runoff generate higher sediment loads, indicating that runoff plays a central role in sediment transport. Across scenarios, streamflow, water yield and sediment yield increase by about 0%–29%, 4%–40% and 10%–91%, with the largest changes under SSP3‐7.0 and SSP5‐8.5. Lower‐emission pathways show smaller early changes, but impacts grow over time. An expansion of moderate to high rainfall areas may raise flood and erosion risk, creating added pressure for water and sediment management. The novelty of this study lies in its integrated approach by combining recent climate projections with high‐resolution modelling and spatial–temporal analysis in the data‐limited Himalayan region. The study also provides a framework that can be applied to other cryospheric basins around the world. This makes it highly relevant for global assessments of climate‐driven risks to water and sediment.

ABSTRACT Urban flooding is becoming increasingly prevalent worldwide due to climate change and rapid urbanisation, posing significant challenges for stormwater regulation, surface runoff management and drainage system effectiveness. Yet, despite the trend of recommending that nature‐based solutions (NBS) be deployed in flood mitigation efforts, information about the use of spatially resolved indicators of urban hydrological dysfunction is seldom used to inform their implementation. The paper makes contributions to the frontier of urban flood hydrology by proposing the concept of social sensing as an auxiliary observation layer that can be used to attribute processes related to flooding, such as surface runoff accumulation, drainage inefficiency and repetitive flooding. Based on the use of social media data in Pakistan, we combine sentiment analysis, topic modelling and spatial analysis to develop proxy variables for localised flood dynamics. Findings indicate a high degree of spatiotemporal alignment between clusters of negative sentiment and reported flood‐prone areas, suggesting that runoff pathways are highly concentrated and drainage is poor. Discourse analysis also links NBS‐related stories to key hydrological processes, such as runoff attenuation, increased infiltration and the moderation of flood peaks. The paper integrates social sensing and hydrological interpretation, forming a process‐based decision‐support system that can prioritise NBS interventions in urban flood‐prone environments. The results indicate that conventional hydrological data can be enhanced with crowd‐derived observations to capture the population‐level response to flood dynamics, thereby increasing situational awareness of urban runoff dynamics and drainage collapse. This social‐hydrological integration offers a versatile path to improving flexible flood management in data‐scarce, rapidly urbanising areas.

ABSTRACT Saturated hydraulic conductivity (K) is often used to characterise the hydrological dynamics of peatlands. There are several methods to determine hydraulic conductivity, both in the field and in the laboratory. In the field, the piezometer method is one of the most widely used, while in the laboratory, the modified cube method (MCM) is often used in peaty soils due to its practicality. It also allows the estimation of anisotropy as K can be measured vertically or horizontally. Patagonia hosts the largest concentration of peatlands in South America, which are recognised for their pristine conservation status. However, there are few studies that address the hydrological dynamics of peatlands in the region. In this work, we aimed to investigate the relationships of K with other properties in southern peatlands and relate it to peat type classification. We also evaluate the performance of a modified MCM method, using a variable falling head; finally, we compare the laboratory and field methods used. The study was carried out in a pristine raised bog in southern Tierra del Fuego (Argentina). K was a function of depth, bulk density, humification index, and root density. Peat type classification adequately summarised these ecohydrological properties. Anisotropy was observed, but no preferential flow direction was evident. Both methods were consistent, showing that high and low permeability sites were well represented both in the field and in the laboratory. The alternative MCM method using the falling head mode obtained low experimental error and high correlation with the traditional MCM method. This method proved to be an interesting alternative as its application is simpler, reliable and recommended especially for low permeability samples.

The data that support the findings of this Special Issue are available from the corresponding author of each paper upon request. However, the data are not publicly available due to privacy restrictions.

ABSTRACT There is a gap in hydrological and water data in New Mexico, a state known for its arid to semi‐arid climate. The lack of access to these data presents a challenge to effective water management. The Dynamic Statewide Water Budget (DSWB) model represents an effort to apply water balance and system dynamics methodologies to estimate and quantify water budget components and to provide an accessible online tool for hydrological and water data in New Mexico. This paper highlights the model and presents the analysis and evaluation of the dynamic behaviour of selected hydrological variables in relation to climate change and drought, using data obtained from the model. The online DSWB interactive tool was also reviewed and evaluated for its role in providing hydrological data characterisation and demonstrating modelled results that compare multiple variables and scenarios across different spatial scales. The model performance was evaluated against measured and published data using several statistical indicators, including the correlation coefficient (0.95), root mean square error (8.24 kAF/year), Nash–Sutcliffe efficiency (0.84), performance index (0.96), coefficient of residual mass (0.04), index of agreement (0.92), mean absolute error (6.82 kAF/year), and mean absolute relative error (14.39%). All these metrics indicate satisfactory model performance. The findings indicate a decline in runoff from snowmelt and significant variability in surface water inflow due to climate change. They also demonstrate an interrelated relationship between drought conditions, surface water inflow, and groundwater storage, highlighting the interconnected dynamics among them. Drought further affects evaporation rates from reservoirs, while declining surface water availability leads to increased groundwater pumping to meet irrigation demand, emphasising the reliance on groundwater during drought periods. Groundwater storage has also been steadily decreasing, especially in regions that depend on fossil aquifers receiving little to no recharge. Overall, the model results indicate acceptable levels of accuracy and reliability, making it a useful tool for water management by providing hydrological estimates and predictions, as well as identifying key trends in water conditions. The DSWB model reflects a serious effort and promising attempt to fill existing knowledge gaps related to the water budget and provides a practical framework that can be utilised to improve water resource management in New Mexico. This model may also pave the way for developing additional water budget models based on the same methodology in other arid and semi‐arid regions around the world with hydrological data limitations.