The main objective of the study is to evaluate the roles of climate change and human factors on runoff, baseflow, and hydrological drought characteristics at a watershed scale. The novelty of the study is to assess separately the cascading, indirect, accumulative effects of climate change and human factors on hydrological drought, i.e. runoff and baseflow. This involved analyzing change points to divide the available hydrometeorological data into a baseline and a perturbed period. We applied two hydrological models, SWAT and HBV-light, and two nonparametric climate elasticity of runoff to identify the contribution of climate change and human factors in influencing runoff and baseflow processes. The hydrological models were used to simulate naturalized runoff and baseflow during the perturbed period. The temporal variation in the characteristics of the baseflow regime is expressed as baseflow index. Drought indices, standardized runoff index and standardized baseflow index were used as hydrological drought indicators. A significant change in runoff reduction in the Kamienna watershed began in 1982, suggesting that human factors play a dominant role in influencing runoff. In addition, we found that an increase in baseflow and a decrease in hydrological drought events in the 2010s are a positive long-term effect of human factors such as construction of dams in the watershed. Finally, analyses of changes in land cover dynamics in the watershed over the past four decades using satellite imagery are used to confirm the presence of human interventions.

The assessment of climate change impacts on water resources and flood risk is typically underpinned by hydrological models calibrated and selected based on observed streamflow records. Yet, changes in climate are rarely accounted for when selecting hydrological models, which compromises their ability to robustly represent future changes in catchment hydrology. In this paper, we test a simple framework for selecting an ensemble of calibrated hydrological model structures in catchments where changing climatic conditions have been observed. We start by considering 78 model structures produced using the FUSE modular modelling framework and rely on a Pareto scheme to select model structures maximizing model efficiency in both wet and dry periods. The application of this approach in three case study basins in Peru enables the identification of structures with good robustness, but also good performance according to hydrological signatures not used for model selection. We also highlight that some model structures that perform well according to traditional efficiency metrics have low performance in contrasting climates or suspicious internal states and fluxes. Importantly, the model selection approach followed here helps to reduce the spread in precipitation elasticities and temperature sensitivities, providing a clearer picture of future hydrological changes. Overall, this work demonstrates the potential of using contrasting climatic conditions in a multi-objective framework to produce robust and credible simulations, and to constrain structural uncertainties in hydrological projections.

Suspended sediment delivery and deposition in proglacial lakes is generally sensitive to a wide range of hydrometeorologic and geomorphic controls. High discharge conditions are of particular importance in many glaciolacustrine records, with individual floods potentially recorded as distinctive turbidites. We used an extensive network of surface sediment cores and hydroclimatic monitoring data to analyze recent flood turbidites and associated sediment transfer controls over instrumental periods at Eklutna Lake, western Chugach Mountains, Alaska. Close to a decade of fluvial data from primary catchment tributaries show a dominating influence of discharge on sediment delivery, with various interconnections with other related hydroclimatic controls. Multivariate fluvial models highlight and help quantify some complexities in sediment transfer, including intra-annual variations, meteorological controls, and the influence of subcatchment glacierization. Sediments deposited in Eklutna Lake during the last half century are discontinuously varved and contain multiple distinctive turbidites. Over a 30-year period of stratigraphic calibration, we correlate the four thickest flood turbidites (1989, 1995, 2006, 2012) to specific regional storms. The studied turbidites correlate with late-summer and early-autumn rainstorms with a magnitude of relatively instantaneous sedimentation 3 to 15 times greater than annual background accumulation. Our network of sediment core data captured the broad extent and sediment variability among the study turbidites and background sediment yield. Within-lake spatial modelling of deposition quantifies variable rates of downlake thinning and sediment focusing effects, and highlights especially large differences between the thickest flood turbidites and background sedimentation. This we primarily relate to strongly contrasting dispersion processes controlled by inflow current strength and turbidity. Sediment delivery is of interest for this catchment because of reservoir and water supply operations. Furthermore, although smaller floods may not be consistently represented, the lake likely contains a valuable proxy record of regional flooding proximal to major population centers of south-central Alaska including Anchorage.

Soil moisture signatures provide a promising solution to overcome the difficulty of evaluating soil moisture dynamics in hydrologic models. Soil moisture signatures are metrics that quantify the dynamic aspects of soil moisture timeseries and enable process-based model evaluations. To date, soil moisture signatures have been tested only under limited land-use types. In this study, we explore soil moisture signatures’ ability to discriminate different dynamics among contrasting land-uses. We applied a set of nine soil moisture signatures to datasets from six in-situ soil moisture networks worldwide. The dataset covered a range of land-use types, including forested and deforested areas, shallow groundwater areas, wetlands, urban areas, grazed areas, and cropland areas. Our set of signatures characterized soil moisture dynamics at three temporal scales: event, season, and a complete timeseries. Statistical assessment of extracted signatures showed that (1) event-based signatures can distinguish different dynamics for all the land-uses, (2) season-based signatures can distinguish different dynamics for some types of land-uses (deforested vs. forested, urban vs. greenspace, and cropped vs. grazed vs. grassland contrasts), (3) timeseries-based signatures can distinguish different dynamics for some types of land-uses (deforested vs. forested, urban vs. greenspace, shallow vs. deep groundwater, wetland vs. non-wetland, and cropped vs. grazed vs. grassland contrasts). Further, we compared signature-based process interpretations against literature knowledge; event-based and timeseries-based signatures generally matched well with previous process understandings from literature, but season-based signatures did not. This study will be a useful guideline for understanding how catchment-scale soil moisture dynamics in various land-uses can be described using a standardized set of hydrologically relevant metrics.

Quantifying and understanding catchment sediment yields is crucial both from a scientific and environmental management perspective. To deepen the understanding of land use impacts and climate change on sediment load, we explore mechanisms of the suspended sediment yield formation in the Northern Caucasus during the Anthropocene. We examine how sediment flux of various river basins with different land-use/landcover and glacier cover changes during the 1925-2018 period. Our analysis is based on observed mean annual suspended sediment discharges (SSD, kg·s−1) and annual fluxes (SSL, t·yr−1) from 33 Roshydromet gauging stations (Russia). SSL series have been analyzed to detect statistically significant changes during the 1925-2018 period. The occurrence of abrupt change points in SSD was investigated using cumulative sum (CUSUM) charts. We found that SSL has decreased by −1.81% per year on average at most gauges. However, the decline was not linear. Several transition years are expected in the region: increasing trends from the 1950s and decreasing trends from 1988-1994. Correlation analyses showed that variation in SSL trend values is mainly explained by gauging station altitude, differences in land use (i.e., the fraction of cropland), and catchment area. Nonetheless, more accurate quantifications of SSL trend values and more refined characterizations of the catchments regarding (historical) land use, soil types/lithology, weather conditions, and topography may reveal other tendencies.

This study characterizes the dynamics of exchange fluxes between Brazos River Alluvium Aquifer and the Brazos River, TX, USA. Seven alternative conceptual models for the connection between the river and the aquifer were simulated in HYDRUS 2D using small-scale, high-resolution transects across the river. These models assumed varying aquifer lithology and river incision depths and considered processes such as riverbed clogging and seepage face flows. The simulations were forced by observed river stage values and tested against observed hydraulic heads in two nearby monitoring wells. The nearly 1.5 years of sub-hourly measurements spanned both flood and drought periods. The best-fit conceptual model supported a hypothesized hydraulic disconnection between the subsurface near the river and the wider alluvial aquifer. In contrast to the assumptions of previous studies, these data were more consistent with the presence of an abandoned paleochannel rather than riverbed clogging or other low-permeability zones. The implications for groundwater-surface water exchanges, and their modelling, are profound. Across the range of models, the difference in average baseflow predicted was nearly 13 m3/d/m, equivalent to seven times the firm water rights allocated for river users.

Tropical montane cloud forests (TMCF) receive additional (‘occult’) inputs of water from fog and wind-driven rain. Together with the concomitant reduction in evaporative losses, this typically leads to high soil moisture levels (often approaching saturation) that are likely to promote rapid subsurface flow via macropores. Although TMCF make up an estimated 6.6% of all remaining montane tropical forest and occur mostly in steep headwater areas that are protected in the expectation of reduced downstream flooding, TMCF hillslope hydrological functioning has rarely been studied. To better understand the hydrology of a TMCF on heterogeneously layered volcanic ash soils under very high net rainfall conditions (up to 6,740 mm yr-1), we examined temporal and spatial soil water dynamics and their contribution to subsurface runoff and stormflow for a year (1 July 2003 – 30 June 2004) in a small headwater catchment on the Atlantic (windward) slope near Monteverde, NW Costa Rica. Particular attention was paid to partitioning of water fluxes into lateral subsurface flow and vertical percolation. The presence of a gravelly volcanic layer at shallow depth of very high hydraulic conductivity intercalated between two layers of lower conductivity, controlled both surface infiltration and delayed vertical water movement deeper into the soil profile. Soil water fluxes during rainfall were dominated by rapid lateral flow in the gravelly layer, particularly at high soil water contents. In turn, this lateral subsurface flow controlled the magnitude and timing of stormflow from the catchment. Stormflow amount increased rapidly once topsoil moisture exceeded a threshold value of ~0.58 cm3 cm-3. Responses were not affected appreciably by rainfall intensity because soil hydraulic conductivities across the profile largely exceeded prevailing rainfall intensities.

The δ2H and δ18O composition of 77 precipitation samples collected between January 2014 and April 2019 from two sites across the Guadalquivir Basin, SW Spain, were analyzed. Sampling was performed within intervals of at least 14 days basis if rain occurred but frequently intervals were longer according to the rainfall incidence. Precipitation weighted averages and local meteoric water lines are presented for use in hydrological applications. δ2H and δ18O values show a remarkably high variability attributed to the influence of Atlantic and Mediterranean vapor sources. Precipitation weighted average d-excess values of 12.5-13 ‰ confirm the Western Mediterranean influence. Temperature and amount effects were found to be weak but significant influence of secondary evaporation for single rainfall events during summer was identified by reduced d-excess values and enriched isotopic signatures plotting close or below the Global Meteoric Water line (GMWL). Isotopic signatures of both sites are very similar in general and any temperature related urban effects of Seville city compared to the rural site Doñana could not be identified with the present data

There is still limited understanding of how waters mix, where waters come from and for how long they reside in tropical catchments. In this study, we used a tracer-aided model (TAM) and a gamma convolution integral model (GM) to assess runoff generation, mixing processes, water ages and transit times (TT) in the pristine humid tropical rainforest Quebrada Grande catchment in central Costa Rica. Models are based on a four-year data record (2016 to 2019) of continuous hydrometric and stable isotope observations. Both models agreed on a young water component of fewer than 95 days in age for 75% of the study period. The streamflow water ages ranged from around two months for wetter years (2017) and up to 9.5 months for drier (2019) years with a better agreement between the GM estimated TTs and TAM water ages for younger waters. Such short TTs and water ages result from high annual rainfall volumes even during drier years with 4,300 mm of annual precipitation (2019) indicating consistent quick near-surface runoff generation with limited mixing of waters and a supra-regional groundwater flow of likely unmeasured older waters. The TAM in addition to the GM allowed simulating streamflow (KGE > 0.78), suggesting an average groundwater contribution of less than 40% to streamflow. The model parameter uncertainty was constrained in calibration using stable water isotopes (δ2H), justifying the higher TAM model parameterization. We conclude that the multi-model analysis provided consistent water age estimates of a young water dominated catchment. This study represents an outlier compared to the globally predominant old water paradox, exhibiting a tropical rainforest catchment with higher new water fractions than older water.

Stream thermal regimes are critical to the stability of freshwater habitats. There is growing concern that climate change will result in stream warming due to rising air temperatures, decreased shading in forested areas due to wildfires, and changes in streamflow. Groundwater plays an important role in controlling stream temperatures in mountain headwaters, where it makes up a considerable portion of discharge. This study investigated the controls on the thermal regime of a headwater stream, and the surrounding groundwater processes, in a catchment on the eastern slopes of the Canadian Rocky Mountains. Groundwater discharge to the headwater spring is partially sourced by a seasonal lake. Spring, stream, and lake temperature, water level, discharge and chemistry data were used to build a conceptual model of the system. Meteorological data was used to set up a stream temperature model. A tracer test was carried out to estimate hyporheic exchange along the study reach. This study presents a unique example of an indirectly lake-headed stream i.e., where the interaction of groundwater and lake water, and the hydraulic gradient determine the resulting stream temperature. Energy balance of the stream is mainly controlled by radiation. Sensible and latent heat fluxes play a secondary role, but their effects generally cancel out. Hyporheic exchange is present but plays only a minor role in the energy balance. During snowfall events, the latent heat associated with melting of direct snowfall onto the water surface was responsible for rapid stream cooling. An increase in advective inputs from groundwater and hillslope pathways did not result in observed cooling of stream water during rainfall events. The results from this study will assist water resource and fisheries managers in adapting to stream temperature changes under a warming climate.

Quantifying species-specific tree transpiration across watershed zones is important for estimating watershed evapotranspiration (ET) and predicting drought effects on vegetation. The objectives of this study are to 1) assess sap flux density (Js) and tree-level transpiration (Ts) across three contrasting zones (riparian buffer, mid-hillslope, and upland-hillslope), 2) determine how species-specific Js responds to vapor pressure deficit (VPD), and 3) compare watershed-level transpiration (Tw) derived from each zone. We measured Js and Ts in eight tree species in the three zones in a 12-ha forested watershed. In the dry year of 2015, loblolly pine (Pinus taeda), Virginia pine (Pinus virginiana), and sweetgum (Liquidambar styraciflua) Js rates were significantly higher in the buffer when compared to the other two zones. In contrast, Js in tulip poplar (Liriodendron tulipifera) and red maple (Acer rubrum) were significantly lower in the buffer than in the mid-hillslope. Daily Ts varied by zone and ranged from 10 to 93 liters in the dry year and 9 to 122 liters in the wet year. Js responded nonlinearly to VPD in all trees and zones. Annual Tw based on scaled-Js data was 447 mm, 377 mm, and 340 mm for the buffer, mid-hillslope, and upland-hillslope, respectively. We conclude that large spatial variability in Js and scaled Tw were driven by differences in soil moisture at each zone and forest composition. Consequently, spatial heterogeneity of vegetation and soil moisture must be considered when accurately quantifying watershed level ET.

Complex networks of both natural and engineered flow paths control the hydrology of streams in major cities through spatio-temporal variations in connection and disconnection of water sources. We used spatially extensive and temporally intensive sampling of water stable isotopes to disentangle the hydrological sources of the heavily urbanized Panke catchment (≈ 220 km²) in the north of Berlin, Germany. The isotopic data enabled us to partition stream water sources across the catchment using a Bayesian mixing analysis. The upper part of the catchment streamflow here is dominated by groundwater from gravel aquifers underlying surrounding agricultural land. In dry summer periods, streamflow becomes intermittent; possibly as a result of local groundwater abstractions. Urban storm drainage is also an important part of runoff generation, dominating the responses to precipitation events. Although this dramatically changes the isotopic composition of the stream, it only accounts for 10-15% of annual streamflow. Moving downstream, subtle changes in sources and isotope signatures occur as catchment characteristic vary and the stream is affected by different tributary inflows. However, effluent from a wastewater treatment plant (WWTP) serving 700,000 people dominates the stream in the lower catchment where urbanisation effects are more dramatic. The associated increase in sealed surfaces downstream also reduces the relative contribution of groundwater to streamflow. The volume and isotopic composition of storm runoff is again dominated by urban drainage. As a result, only about 10% of annual runoff in the lower catchment comes from urban storm drains. The study shows the potential of stable water isotopes as inexpensive tracers in urban catchments that can provide a more integrated understanding of the complex hydrology of major cities. This offers an important evidence base for guiding the plans to develop and re-develop urban catchments to protect, restore and enhance the ecological and amenity value of these important resources.

Permafrost thaw has been observed in recent decades in the Northern Hemisphere and is expected to accelerate with continued global warming. Predicting the future of permafrost requires proper representation of the interrelated surface/subsurface thermal and hydrologic regimes. Land surface models (LSMs) are well suited for such predictions, as they couple heat and water interactions across soil-vegetation-atmosphere interfaces and can be applied over large scales. LSMs, however, are challenged by the long-term thermal and hydraulic memories of permafrost and the paucity of historical records to represent permafrost dynamics under transient climate conditions. In this study, we address the challenge of model initialization by characterizing the impact of initial climate conditions and initial soil frozen and liquid water contents on the simulation length required to reach equilibrium. Further, we quantify how the uncertainty in model initialization propagates to simulated permafrost dynamics. Modelling experiments are conducted with the Modélisation Environmentale Communautaire – Surface and Hydrology (MESH) framework and its embedded Canadian Land Surface Scheme (CLASS). The study area is in the Liard River basin in the Northwest Territories of Canada with sporadic and discontinuous regions. Results show that uncertainty in model initialization controls various attributes of simulated permafrost, especially the active layer thickness, which could change by 0.5-1.5m depending on the initial condition chosen. The least number of spin-up cycles is achieved with near field capacity condition, but the number of cycles varies depending on the spin-up year climate. We advise an extended spin-up of 200-1000 cycles to ensure proper model initialization under different climatic conditions and initial soil moisture contents.

A knowledge of the exchanges of energy and water over the terrestrial surface is the first step to understand the ecohydrological mechanisms, particularly in water-limited ecosystems in the dryland environments. However, patterns of energy exchange and evapotranspiration (ET) are not well understood in the oasis-desert ecotone, which plays an important role in protecting oasis against the threat of desertification in northwestern China’s arid regions. Here the continuous measurements of surface energy fluxes were made using eddy covariance in conjunction with auxiliary measurements for two years (2014-2015) at a shrubland within an oasis-desert ecotone in the arid regions, northwestern China. Statistical analysis on 30-min time scale indicates that about 50% of daytime net radiation (Rn) over the shrubland is dissipated as H on average, which peaks in spring; one third Rn is consumed by soil heat flux (G). Only 9% of Rn was consumed for latent heat flux (λE), which peaks in summer (21% in 2014 and 16% in 2015), corresponding to the season with highest rainfall among all seasons. Daily mean ET is about 1 mm·d−1 during growing season of the shrub species. The rapid and transient increase in ET occurs following a rainfall event. A switch in surface soil moisture from 0.04 to 0.11 m3·m−3 causes an increase in Rn by about 11% and λE by 151% at the shrubland, respectively. Accumulated annual ET were 195 and 181 mm in 2014 and 2015, respectively, exceeding the corresponding P by about 87 and 77 mm, indicating that groundwater may be another important source of water for ET over the shrubland aside from P. These results provide valuable insight into the mechanisms of sustaining energy and water balance at the ecotone, and then produce some management guidelines for allocating water resources and protecting vegetation.

Surface water storage—including wetlands and other small waterbodies—has largely been disregarded in traditional hydrological models. In this paper, the grid resampling method is adopted to study the influence of the digital elevation model (DEM) grid resolution on depression storage (DS) considering different rainfall return periods. It is observed that the DEM grid size highly affects DS, and the higher the grid resolution is, the larger the DS value. However, when the grid resolution reaches a certain value, the maximum DS value decreases. This suggests that a critical grid resolution value exists at which the water storage capacity of depressions is maximized, namely, 20 m in this work. This phenomenon is further verified in two test cases with and without the infiltration process, i.e., calculations of the local area and without infiltration area, respectively. This research may facilitate the accurate computation of the DS process, which is greatly affected by the grid resolution, thereby improving the reliability of hydrological models.

Using data from five long-term field sites measuring soil moisture, we show the limitations of using soil moisture observations alone to constrain modelled hydrological fluxes. We test a land surface model, MESH/CLASS, with two configurations: one where the soil hydraulic properties are determined using a pedotransfer function (the texture-based calibration) and one where they are assigned directly (the hydraulic properties-based calibration). The hydraulic properties-based calibration outperforms the texture-based calibration in terms of reproducing changes in soil moisture storage within a 1.6 m deep profile at each site, but both perform reasonably well, especially in the summer months. When the models are constrained using observations of changes in soil moisture, the predicted hydrological fluxes are subject to very large uncertainties associated with equifinality. The uncertainty is larger for the hydraulic properties-based calibration, even though the performance was better. We argue that since the pedotransfer functions constrain the model parameters in the texture-based calibrations in an unrealistic way, the texture-based calibration underestimates the uncertainty in the fluxes. We recommend that reproducing observed cumulative changes in soil moisture storage should be considered a necessary but insufficient criterion of model success. Additional sources of information are needed to reduce uncertainties, and these could include improved estimation of the soil hydraulic properties and direct observations of fluxes, particularly evapotranspiration.

Investigating the changes in streamflow regimes is useful for understanding the mechanisms associated with hydrological processes in different watersheds and for providing information to facilitate water resources management. In this study, we selected three watersheds, i.e., Sandu River, Hulu River, and Dali River on the Loess Plateau, to examine the changes in the streamflow regimes and to determine their responses to different soil and water conservation measures (terracing, afforestation, and damming). The daily runoff was collected continuously by three hydrological gauges close to the outlets of the three watersheds from 1965 to 2016. The eco-surplus, eco-deficit, and degree of hydrological change were assessed to detect hydrological alterations. The Budyko water balance equation was applied to estimate the potential impacts of climate change and human activities on the hydrological regime changes. Significant decreasing trends (P < 0.05) were detected in the annual streamflow in the Sandu and Dali River watersheds, but not in the Hulu River watershed where afforestation dominated. The annual eco-surplus levels were low and they decreased slightly at three stations, whereas the eco-deficit exhibited dramatic increasing trends in the Sandu and Dali River watersheds. In the Sandu River watershed (dominated by terraces), the runoff exhibited the most significant reduction and the eco-deficit was the highest among the three watersheds. The integral degrees of hydrological change were higher in the Sandu River watershed than the other two watersheds, thereby suggesting substantial variations in the magnitude, duration, frequency, timing, and rate of change in the daily streamflow. In the Dali River watershed (dominated by damming), the changes in the extreme flow were characterized by a decreasing number appearing in high flow. In these watersheds, human activities accounted for 74.1% and 91.78% of the runoff reductions, respectively. In the Hulu River watershed (dominated by afforestation), the annual runoff exhibited an insignificant decreasing trend but with a significant increase in the low flow duration. Rainfall changes accounted for 64.30% of the runoff reduction.

Mountain regions are an important regulator in the global water cycle through their disproportionate water contribution. Often referred to as the “Water Towers of the World”, mountains contribute 40 to 60% of the world’s annual surface flow. Shade is a common feature in mountains, where complex terrain cycles land surfaces in and out of shadows over daily and seasonal scales. This study investigated turbulent water and carbon dioxide fluxes over the snow-free period in a subalpine wetland in the Canadian Rocky Mountains, from June 7th to September 10th, 2018. Shading had a significant and substantial effect on water and carbon fluxes at our site. Each hourly increase of shade per day reduced evapotranspiration (ET) and gross primary production (GPP) by 0.42 mm and 0.77 gCm-2, equivalent to 17% and 15% per day, respectively, over the entire study period. However, during only peak growing season, when leaves were fully out and mature, shade caused by the local complex terrain, reduced ET and increased GPP, likely due to enhanced diffuse radiation. The overall result was increased water use efficiency at the site during periods of increased shading during the peak growing season. In addition to incoming solar radiation (Rg), temporal variability in ET was found to relate to temporal variability in soil temperature, moisture and vapour pressure deficit. Shade impacted the curvature and intercept of the nonlinear ET-Rg relationship at our site. In contrast, temporal variability in GPP at our site was dependent largely on Rg only. Our findings suggest that shaded subalpine wetlands can store large volumes of water for late season runoff and are productive through short growing seasons.