This study focuses on a 10-m2 plot within a granitic hillslope in Cevennes mountainous area in France, in order to study infiltration and subsurface hydrological processes during heavy rainfalls and flash floods. The monitoring device included water content at several depths (0-70cm for the shallow soil water; 0-10m for the deep water) during both intense artificial and natural rainfall events, chemical and physical tracers, time-lapse electrical resistivity tomography. During the most intense events, the infiltrated water was estimated to be some hundreds of millimeters, which largely exceeds the topsoil capacity (≤40 cm deep in most of the cases). The weathered/fractured rock area below the soil clearly has an active role in the water storage and sub-surface flow dynamics. Vertical flow was dominant in the first 0-10m, and lateral flow was effective at 8-10 m depth, at the top of the saturated area. The speed of the vertical flow was estimated between 1 and 10 m/h, whereas it was estimated between 0.1 and 1 m/h for the lateral flow. The interpretation of the experiments led to a local pattern of the 2D-hydrological processes and profile properties. It suggests that fast triggering of floods at the catchment scale cannot be explained by a mass transfer within the hillslope, but should be due to a pressure wave propagation through the bedrock fractures, which allows exfiltration of the water downstream the hillslope.

Snow is Earth’s most climatically sensitive land cover type. Air temperature and moisture availability are first-order controls on snowfall. Maximum snowfall occurs at temperatures very near 0°C, so even a slight increase in temperature will shift a snowy winter to one with midseason rainfall and melt events. Traditional snow metrics are not able to adequately capture the changing nature of snow cover. For example, April 1 snow water equivalent (SWE, the amount of water represented by the snowpack) is used as a streamflow predictor. Still, it cannot express the effects of midwinter melt events, now expected in warming snow climates. The multiple impacts of a changing snowpack require a suite of climate indicators derived from readily measured or modelled data that serve as proxies for relevant snow-related and climate-driven processes. Such indicators need to be simple enough to “tell the story” of snowpack changes over space and time, but not overly simplistic as to be conflated with other variables or, conversely, overly complicated in their interpretation. This paper describes a targeted set of spatially explicit, multi-temporal snow metrics for multiple sectors, stakeholders, and scientists. They include metrics based on satellite data from NASA’s Moderate Resolution Imaging Spectroradiometer, meteorological observations and snow data from ground-based stations, and climate model output. We describe and provide examples for Snow Cover Frequency (SCF), Snow Disappearance Date (SDD), snowstorm temperature (ST), At-Risk Snow (ARS), and Frequency of a Warm Winter (FWW).

Catchment-scale response functions, such as transit time distribution (TTD) and evapotranspiration time distribution (ETTD), are considered fundamental descriptors of a catchment’s hydrologic and ecohydrologic responses to spatially and temporally varying precipitation inputs. Yet, estimating these functions is challenging, especially in headwater catchments where data collection is complicated by rugged terrain, or in semi-arid or sub-humid areas where precipitation is infrequent. Hence, we developed practical approaches for estimating both TTD and ETTD from commonly available tracer flux data in hydrologic inflows and outflows without requiring continuous observations. Using the weighted wavelet spectral analysis method of Kirchner and Neal [2013] for δ18O in precipitation and stream water, we specifically calculated TTDs that contribute to streamflow via spatially and temporally variable flow paths in a sub-humid mountain headwater catchment in Arizona, USA. Our results indicate that composite TTDs most accurately represented this system for periods up to approximately one month and that a Gamma TTD was most appropriate thereafter. The TTD results also suggested that some contribution of subsurface water was beyond the applicable tracer range. For ETTD and using δ18O as a tracer in precipitation and xylem waters, a Gamma ETTD type best matched the observations, and stable water isotopes were capable tracers for the majority of vegetation source waters. This study contributes to a better understanding of a fundamental question in mountain catchment hydrology; namely, how tracer input fluxes are modulated by spatially and temporally varying subsurface flow paths that support evapotranspiration and streamflow at multiple time scales.

Soil moisture is an important driver of growth in boreal Alaska, but estimating soil hydraulic parameters can be challenging in this data-sparse region. To better identify soil hydraulic parameters and quantify energy and water balance and soil moisture dynamics, we applied the physically-based, one-dimensional ecohydrologic Simultaneous Heat and Water (SHAW) model, loosely coupled with the Geophysical Institute of Permafrost Laboratory (GIPL) model, to an upland deciduous forest stand in interior Alaska over a 13-year period. Using a Generalized Likelihood Uncertainty Estimation (GLUE) parameterization, SHAW reproduced interannual and vertical spatial variability of soil moisture during a five-year validation period quite well, with root mean squared error (RMSE) of volumetric water content at 0.5 m as low as 0.020. Many parameter sets reproduced reasonable soil moisture dynamics, suggesting considerable equifinality. Model performance generally declined in the eight-year validation period, indicating some overfitting and demonstrating the importance of interannual variability in model evaluation. We compared the performance of parameter sets selected based on traditional performance measures (RMSE) that minimize error in soil moisture simulation, with those that were designed to minimize the dependence of model performance on interannual climate variability. The latter case moderately decreases traditional model performance but is likely more suitable for climate change applications, for which it is important that model error is independent from climate variability. These findings illustrate (1) that the SHAW model, coupled with GIPL, can adequately simulate soil moisture dynamics in this boreal deciduous region, (2) the importance of interannual variability in model parameterization, and (3) a novel objective function for parameter selection to improve applicability in non-stationary climates.

Patterns of δ18O and δ2H in Earth’s precipitation provide essential scientific data for use in hydrological, climatological, ecological and forensic research. Insufficient global spatial data coverage promulgated the use of gridded datasets employing geostatistical techniques (isoscapes) for spatiotemporally coherent isotope predictions. Cluster-based isoscape regionalization combines the advantages of local or regional prediction calibrations into a global framework. Here we present a revision of a Regionalized Cluster-Based Water Isotope Prediction model (RCWIP2) incorporating new isotope data having extensive spatial coverage and a wider array of predictor variables combined with high-resolution gridded climatic data. We introduced coupling of δ18O and δ2H (e.g. d-excess constrained) in the model predictions to prevent runaway isoscapes when each isotope is modelled separately. We validated RCWIP2 isoscape performance by cross-checking observed versus modelled d-excess values. We improved model error quantification by adopting full uncertainty propagation in all calculations. RCWIP2 improved the RMSE over previous isoscape models by ca. 0.6 ‰ for δ18O and 5 ‰ for δ2H with an uncertainty <1.0 ‰ for δ18O and <8 ‰ for δ2H for most regions of the world. The improved RCWIP2 isoscape grids and maps (season, monthly, annual, regional) are available for download at https://isotopehydrologynetwork.iaea.org.

Accurately quantifying the evaporation loss of surface water is essential for regional water resources management, especially in arid and semi-arid areas where water resources are already scarce. The long-term monitoring of stable isotopes (δ18O and δ2H) in water can provide a sensitive indicator of water loss by evaporation. In this study, we obtained surface water samples of Shiyang River Basin from April to October between 2017 and 2019. The spatial and temporal characteristics of stable isotopes in surface water show the trend of enrichment in summer, depletion in spring, enrichment in deserts and depletion in mountains. The Surface Water Line (SWL) has been defined by the lines: δ2H=7.61δ18O+14.58 for mountainous area, δ2H=4.19δ18O-17.85 for oasis area, δ2H=4.08δ18O-18.92 for desert area. The slope of SWL shows a gradual decrease from mountain to desert, indicating that the evaporation of surface water is gradually increasing. The evaporation loss of stable isotopes in surface water is 24.82% for mountainous area, 32.19% for oasis area, and 70.98% for desert area, respectively. Temperature and air humidity are the main meteorological factors affecting the evaporation loss, and the construction of reservoirs and farmland irrigation are the main man-made factors affecting the evaporation loss.

Field measurements were conducted to study the influence of aquatic vegetation on flow structures in floodplains with the hydrodynamic conditions dominated by combined current and wind-driven wave. Wave and turbulent flow velocity components were decomposed from the time series of instantaneous velocity and analyzed separately. With the ratio of wave excursion to stem spacing less than 0.5, the interaction between wave and vegetation was weak in present study, leading to the vertical distributions of time-averaged velocity (Uhoriz) and turbulent kinetic energy (TKE) with the presence of vegetation similar with the vegetated flow structures under pure current conditions. For emergent vegetations, Uhoriz and TKE distributed uniformly through the entire water column or increased slightly from bed to water surface. Similar distributions were present in the lower part of submerged vegetations. Within the upper part of submerged vegetations, Uhoriz and TKE increased rapidly toward water surface and TKE reached its maximum near the top of vegetation. With small Ew/S the wave orbital velocity (Uw) within vegetation was not attenuated when compared with the Uw above vegetation, and Uw through the entire water column can be predicted by the linear wave theory. However, wind-driven waves made the turbulence generated near the top of canopy penetrate a deeper depth into vegetation than predictions under pure current conditions.

Delays in forest recovery from terrestrial acidification combined with climate change is leading Acadian Forest ecosystems into new territory. Kejimkujik Calibrated Catchments (KCC) Study Program was established in an around Kejimkujik National Park and Historic Site (KNPHS) in Southwest Nova Scotia (SWNS) in the late 1970s to increase our understanding of the impacts of acid precipitation on relatively pristine ecosystems. KCC now have one of the longest continuously monitored water chemistry records in North America, with data collection beginning in 1980. Its infrastructure includes three gauged streams, twelve forest inventory plots, an atmospheric deposition monitoring station, and three streams with continuous water quality monitoring and regular lab analysis of stream chemistry, and recent LiDAR coverage. The KCC fits into a wider network of monitored lakes. Data collected at the KCC form a key datapoint in comparisons of catchment response to terrestrial acidification in the context of a warming climate, due to their high and increasing DOC levels, highly dilute waters, lowland topography and extensive wetlands. KCC are also emerging as an important source of information for species at risk protection as SWNS was declared one of the 11 national priority places for biodiversity protection.

Soil and nutrient loss play a vital role in eutrophication of water bodies. Several simulated rainfall experiments have been conducted to investigate the effects of a single controlling factor on soil and nutrient loss. However, the role of precipitation and vegetation coverage in quantifying soil and nutrient loss is still unclear. We monitored runoff, soil loss, and soil nutrient loss under natural rainfall conditions from 2004 to 2015 for 50-100 m2 runoff plots around Beijing. Soil erosion was significantly reduced when vegetation coverage reached 20 and 60%. At levels below 30%, nutrient loss did not differ among different vegetation cover levels. Minimum soil N and P losses were observed at cover levels above 60%. Irrespective of the management measure, soil nutrient losses were higher at high-intensity rainfall (Imax30>15 mm/h) events compared to low-intensity events (p < 0.05). We applied structural equation modelling (SEM) to systematically analyze the relative effects of rainfall characteristics and environmental factors on runoff, soil loss, and soil nutrient loss. At high-intensity rainfall events, neither vegetation cover nor antecedent soil moisture content (ASMC) affected runoff and soil loss. After log-transformation, soil nutrient loss was significantly linearly correlated with runoff and soil loss (p < 0.01). In addition, we identified the direct and indirect relationships among the influencing factors of soil nutrient loss on runoff plots and constructed a structural diagram of these relationships. The factors positively impacting soil nutrient loss were runoff (44-48%), maximum rainfall intensity over a 30-min period (18-29%), rainfall depth (20-27%), and soil loss (10-14%). Studying the effects of rainfall and vegetation coverage factors on runoff, soil loss, and nutrient loss can improve our understanding of the underlying mechanism of slope non-point source pollution.

Aggregate disintegration is a critical process in soil splash erosion. However, the effect of soil organic carbon (SOC) and its fractions on soil aggregates disintegration is still not clear. In this study, five soils with similar physical and chemical properties and different contents of SOC have been used. The effects of slaking and mechanical striking on splash erosion were distinguished by using deionized water and 95% ethanol as raindrops. The simulated rainfall experiments were carried out in four heights (0.5, 1.0, 1.5, and 2.0 m). The result indicated that the soil aggregate stability increased with the increases of SOC and light fraction organic carbon (LFOC). The relative slaking and the mechanical striking index increased with the decreases of SOC and LFOC. The reduction of macroaggregates in eroded soil gradually decreased with the increase of SOC and LFOC, especially in alcohol test. The amount of macroaggregates (>0.25mm) in deionized water tests were significantly less than that in alcohol tests under the same rainfall heights. The contribution of slaking to splash erosion increased with the decrease of heavy fractions organic carbon (HFOC). The contribution of mechanical striking was dominant when the rainfall kinetic energy increased to a range of threshold between 9 J m-2 mm-1 and 12 m-2 mm-1. This study could provide the scientific basis for deeply understanding the mechanism of soil aggregates disintegration and splash erosion.

Our understanding of the mechanisms routing precipitation inputs to evapotranspiration and streamflow in catchments is still very fragmented, particularly in the case of saturated flows. Here we explore five mechanisms by which plants and streams compete with each other for water, based on multiple scales of observations in a flat semiarid sedimentary catchment of central Argentina subject to abrupt hydrological transformations. Since the 80s, the “El Morro” catchment (1334 km2, -33.64°, -65.36°) experienced a fast expansion of crops over native forests and grasslands, rapid water table level rises (~0.3 m y-1), spontaneous expansion of wetlands and permanent streams by groundwater sapping. Based on episodic and continuous groundwater level, stream flow, and remote sensing data we show that plants not only take away water from streams by drying the unsaturated zone (mechanism 1), but by tapping the saturated zone in the expanding waterlogged environments (mechanism 2) and in the upland environments that remain uncultivated and display increasing tree cover (mechanism 3). Conversely, streams take away water from plants through pulsed bed-deepening and water table depression (mechanism 4), and riparian and wetland zones burying with fresh sediments (mechanism 5). While earlier work established widespread support for mechanisms 1 preventing stream formation, diurnal and seasonal fluctuations of water table levels and base streamflow records in this study proved the importance of mechanisms 2 and 3 under the current high-water table conditions. These data together with remotely-sensed greenness showed a growing but localized relevance of mechanism 4 and 5 as the stream network developed. The distinction of recharge- vs. topography-controlled groundwater systems is useful to organize the interplay of these concurrent mechanisms. Findings point to the unsaturated-saturated contact zone as a crucial and dynamic hub for water partition and for ecological, geomorphological, and hydrological knowledge integration.

Recent decades have seen a change in the runoff characteristics of the Suntar River in Eastern Siberia. This study attempts to attribute these changed hydrological conditions through parameterizing a hydrological model based on historical short-term observations conducted in 1957-1959 at the Suntar-Khayata research station. The Hydrograph model is applied as it has the advantage of using observed physical properties of landscapes as its parameters. The developed parametrization of the goltsy landscape (rocky-talus) is verified by comparison of the results of simulations of variable states of snow and frozen ground with observations carried out in 1957-1959. Continuous simulations of streamflow on a daily time step are conducted for the period 1957-2012 in the Suntar River (area 7680 km2, altitude 828-2794 m) with mean and median values of Nash-Sutcliff criteria reaching 0.58 and 0.67 respectively. The results of simulations have shown that the largest component of runoff (about 70%) is produced in the high-altitude area which comprises only 44% of the Suntar River basin area. The simulated streamflow reproduces the patterns of recently observed changes, including the increase in low flows, suggesting that the increase in the proportion of liquid precipitation in autumn due to air temperature rise is an important factor in driving streamflow changes in the region. The data presented are unique for the vast mountainous parts of North-Eastern Eurasia which play an important role in global climate system. The results indicate that parameterizing a hydrological model based on observations rather than blind calibration allows the model to be used in studying the response of river basins to climate change with greater confidence.

The Jordan River Basin, and its seven sub-catchments of the Central Wasatch Mountains immediately east of Salt Lake City, UT, are home to an array of research infrastructrure that collectively form the Wasatch Environmental Observatory (WEO). Each sub-catchment is comprised of a wildland to urban land use gradient that spans an elevation range of over 2000 m in a linear distance of ~25km. Geology varies across the sub-catchments, ranging from granitic, intrusive to mixed sedimentary rocks in uplands that drain to the alluvial or colluvial sediments of the former Lake Bonneville. Vegetation varies by elevation, aspect, distance to stream channels, and land use.  The sharp elevation gradient results in a range of precipitation from 700 to 1200 mm/yr (roughly 2/3 as snow) and mean annual temperature from 3.5 o to 6.8o C. Spring snowmelt dominates annual discharge. Although climate is relatively similar across the catchments, annual water yield varies spatially by more than a factor of 3, ranging from 0.18 to 0.63. With historical strengths in ecohydrology, water supply, and social-ecological research, current infrastructure supports both basic and applied research in meteorology, climate, atmospheric chemistry, hydrology, ecology, biogeochemistry, resource management, sustainable systems, and urban redesign. Climate and discharge data span over a century for the seven sub-catchments of the larger basin. These data sets, combined with multiple decades of hydrochemistry, isotopes, ecological data sets, social survey data sets, and high-resolution LiDAR topography and vegetation structure, provide a baseline for long-term data collected by NEON, public agencies, and individual research projects. The combination of long-term data with active state of the art observing facilities allows WEO to serve as a unique natural laboratory for addressing research questions facing rapidly growing, seasonally snow-covered, semi-arid regions worldwide and an excellent facility for providing student education and research training.

Catchments in the Luquillo Mountains of Puerto Rico are warm, wet, and tropical with steep elevational relief creating gradients in temperature and rainfall. Long-term objectives of research at the site are to understand how changing climate and disturbance regimes alter hydrological and biogeochemical processes in the montane tropics and to provide information critical for managing and conserving tropical forest ecosystems globally. Measurements of hydrology and meteorology span decades, and currently include temperature, humidity, precipitation, cloud base, throughfall, groundwater table elevation, and stream discharge. The chemistry of rain, throughfall, and streams is measured weekly, and lysimeters and wells are sampled monthly to quarterly. Multiple data sets document the effects of major hurricanes including Hugo (1989), Georges (1998), and Maria (2017) on vegetation, biota, and catchment biogeochemistry and provide some of the longest available records of biogeochemical fluxes in tropical forests. Here we present an overview of the findings and the data sets that have been generated from the Luquillo Mountains, highlighting their importance for understanding montane tropical watersheds in the context of disturbance and global environmental change

KEYWORDSArctic, benthic, catchment, climate change, discharge, long-term monitoring, nutrient enrichment, nutrients

Two experimental basins – the Cariri basins – were installed in a typically semiarid region in the State of Paraíba, Brazil, for obtaining reliable estimates of runoff and soil erosion in different scales to evaluate the influence of the human activities and other factors over the processes of runoff and erosion. In the first basin, located in the municipality of Sumé, the field studies were carried out at three different scales: four micro-basins with an area of around 0.5 ha; nine standard Wischmeier-type erosion plots of 100 m2 and seven sample plots of 1 m2. The experimental units had varied vegetal cover and management and, except the sample plots, were subjected to natural rainfall events only, and were monitored from 1982 to 1991. The total runoff and total sediment yield were determined for each of the events of precipitation. The installations of the second basin, in the near municipality of São João do Cariri, were planned for the continuation of the studies initiated at Sumé, and include erosion plots (100 m2), micro-basins, and sub-basins, which are being monitored for runoff and sediment production up to now. Among them, two nested micro-basins were monitored to detect any scale effect at the micro-basin level. Nearly 600 events of natural precipitation, that produced runoff in at least one of the experimental units, have been registered. This bulk of data was utilised to evaluate the influence of various factors, including cultivation practices. The data collected so far has been successfully used to calibrate hydrological models for plots and micro-basins. Parameters have been tested by means of cross validations among micro-basins and sub-basins. The data sets, in part, have been made available to researchers in Brazil and now the updated data files are being made available to all the researchers in hydro-sedimentology at: https://doi.org/10.5281/zenodo.4044690

Local community and research interest to better understand regional climate change impacts has led to the establishment of a long-term soil moisture and weather observation network in the Roaring Fork catchment of the Colorado River Headwaters. This catchment-wide suite of 10 stations collects frequent and continuous data on soil moisture, soil temperature, rain, air temperature, relative humidity, and (at some stations) snow across an elevational gradient from 1,800m to 3,680m in elevation. We demonstrate how this effort can support research on mountain hydrology with applications for resource management and climate change adaptation decision making. We also share perspectives on the value and opportunities a community science approach can bring to catchment studies moving forward. All data from this project are publicly available.

IntroductionThe Turkey Lakes Watershed (TLW) study (https://www.canada.ca/en/environment-climate-change/services/turkey-lakes-watershed-study.html) was established in 1979 and is one of the longest running watershed-based ecosystem studies in Canada (Foster, Beall & Kreutzweiser, 2005; Jeffries, Kelso & Morrison, 1988; Morrison, Cameron, Foster & Groot, 1999). The watershed drains 10.5 km2 of Eastern Temperate Mixed Forest (Baldwin et al., 2018) or Great Lakes – St. Lawrence forest region (Rowe, 1972) within the Boreal Shield Ecozone (Wiken, 1986), and is located approximately 60 km north of Sault Ste. Marie, Ontario (47°03’N, 84°25’W) (Figure 1). Researchers from several federal government departments (Natural Resources Canada (NRCAN), Environment and Climate Change Canada (ECCC) and Fisheries and Oceans Canada (DFO) established this research watershed to evaluate the impacts of acid rain on terrestrial and aquatic ecosystems (e.g., Foster, Hazlett, Nicolson & Morrison, 1989; Hazlett, Curry & Weldon, 2011; Jeffries, Semkin, Beall & Franklyn, 2002; Kelso 1988). Since its inception, many studies have taken a multi-disciplinary, whole-ecosystem approach to investigate the processes governing terrestrial and aquatic responses to natural and anthropogenic disturbances. This holistic approach has allowed research to expand from its original acidification focus to address a range of other ongoing and emerging environmental issues (e.g. habitat alteration, organic contaminants, forest management, climate change) and to involve numerous academic, government and industrial collaborators.