Mountainous Headwater Catchments Research Articles

Influence of Subsurface Critical Zone Structure on Hydrological Partitioning in Mountainous Headwater Catchments

AbstractHeadwater catchments play a vital role in regional water supply and ecohydrology, and a quantitative understanding of the hydrological partitioning in these catchments is critically needed, particularly under a changing climate. Recent studies have highlighted the importance of subsurface critical zone (CZ) structure in modulating the partitioning of precipitation in mountainous catchments; however, few existing studies have explicitly taken into account the 3D subsurface CZ structure. In this study, we designed realistic synthetic catchment models based on seismic velocity‐estimated 3D subsurface CZ structures. Integrated hydrologic modeling is then used to study the effects of the shape of the weathered bedrock and the associated storage capacity on various hydrologic fluxes and storages in mountainous headwater catchments. Numerical results show that the weathered bedrock affects not only the magnitude but also the peak time of both streamflow and subsurface dynamic storage.

Hydrological balance and runoff from a montane peat bog traced by water temperature

ABSTRACT Peatlands are hydrologically significant landscape units in mountain headwater catchments in Central Europe. In this study, the contribution of a peat bog to runoff and its annual water balance were analysed and compared with those of a forested mineral soil hillslope. A mixing model method using the water temperature as a tracer was applied, and the resulting ratio of peat bog water in streams was incorporated into a hydrological model. The mean peat bog contribution during baseflow ranged from 33.4% during the cold season to 48.9% during the warm season, while during runoff events, the contribution reached over 83%. The simulated water balance results showed a higher actual evapotranspiration on the mineral hillslope, which suggests a higher total runoff from the peat bog (1523 versus 1327 mm/year on the mineral hillslope). The results of this study enable more reliable modelling assessments of peatland reactions to climate change.

Geophysics‐Informed Hydrologic Modeling of a Mountain Headwater Catchment for Studying Hydrological Partitioning in the Critical Zone

AbstractHydrologic modeling has been a useful approach for analyzing water partitioning in catchment systems. It will play an essential role in studying the responses of watersheds under projected climate changes. Numerous studies have shown it is critical to include subsurface heterogeneity in the hydrologic modeling to correctly simulate various water fluxes and processes in the hydrologic system. In this study, we test the idea of incorporating geophysics‐obtained subsurface critical zone (CZ) structures in the hydrologic modeling of a mountainous headwater catchment. The CZ structure is extracted from a three‐dimensional seismic velocity model developed from a series of two‐dimensional velocity sections inverted from seismic travel time measurements. Comparing different subsurface models shows that geophysics‐informed hydrologic modeling better fits the field observations, including streamflow discharge and soil moisture measurements. The results also show that this new hydrologic modeling approach could quantify many key hydrologic fluxes in the catchment, including streamflow, deep infiltration, and subsurface water storage. Estimations of these fluxes from numerical simulations generally have low uncertainties and are consistent with estimations from other methods. In particular, it is straightforward to calculate many hydraulic fluxes or states that may not be measured directly in the field or separated from field observations. Examples include quickflow/subsurface lateral flow, soil/rock moisture, and deep infiltration. Thus, this study provides a useful approach for studying the hydraulic fluxes and processes in the deep subsurface (e.g., weathered bedrock), which needs to be better represented in many earth system models.

Deforestation alters dissolved organic carbon and sulfate dynamics in a mountainous headwater catchment—A wavelet analysis

Deforestation has a wide range of effects on hydrological and geochemical processes. Dissolved organic carbon (DOC) dynamics, a sensitive environmental change indicator, is expected to be affected by deforestation, with changes in atmospheric sulfur (S) deposition compounding this. However, how precisely anthropogenic disturbance (deforestation) under a declining atmospheric S input scenario affects the underlying spatiotemporal dynamics and relationships of river DOC and sulfate with hydro-climatological variables e.g., stream water temperature, runoff, pH, total dissolved iron (Fetot), and calcium (Ca2+) remains unclear. We, therefore, examined this issue within the TERENO Wüstebach catchment (Eifel, Germany), where partial deforestation had taken place in 2013. Wavelet transform coherence (WTC) analysis was applied based on a 10-year time series (2010–2020) from three sampling stations, whose (sub) catchment areas have different proportions of deforested area (W10: 31%, W14: 25%, W17: 3%). We found that water temperature and DOC, sulfate, and Fetot concentrations showed distinct seasonal patterns, with DOC averaging concentrations ranging from 2.23 (W17) to 4.56 (W10) mg L–1 and sulfate concentration ranging from 8.04 (W10) to 10.58 (W17) mg L–1. After clear-cut, DOC significantly increased by 59, 58% in the mainstream (W10, W14), but only 26% in the reference stream. WTC results indicated that DOC was negatively correlated with runoff and sulfate, but positively correlated with temperature, Ca2+, and Fetot. The negative correlation between DOC with runoff and sulfate was apparent over the whole examined 10-year period in W17 but did end in W10 and W14 after the deforestation. Sulfate (SO4) was highly correlated with stream water temperature, runoff, and Fetot in W10 and W14 and with a longer lag time than W17. Additionally, pH was stronger correlated (higher R2) with sulfate and DOC in W17 than in W10 and W14. In conclusion, WTC analysis indicates that within this low mountainous forest catchment deforestation levels over 25% (W10 and W14) affected the coupling of S and C cycling substantially more strongly than “natural” environmental changes as observed in W17.

Understanding the Hydrogeochemical Response of a Mountainous Watershed Using Integrated Surface‐Subsurface Flow and Reactive Transport Modeling

AbstractClimate change and other disturbances significantly impact hydrogeochemical exports from mountainous headwater catchments such as the Upper Colorado River Basin. Developing a mechanistic understanding of how the physical and chemical processes interact in time and space in an integrated manner is key to quantifying the future impacts of such disturbances. The hydrogeochemical response of a mountainous catchment in the 2010–2019 period is evaluated quantitatively using a high‐resolution model that simulates integrated hydrology, and transport and reactions for selected solutes and minerals. The model assumes that pyrite is present only at depth while calcite is distributed uniformly, and captures the observed C‐Q reasonably well. Distinct C‐Q dynamics are observed in an average (WY16), a wet (WY17), and a dry (WY18) water year. The model also quantifies the water fraction from surface, shallow and deep groundwater compartments using tracers, and suggests greater groundwater contributions to peak stream discharge in the dry WY18. Results demonstrate that calcium concentrations do not change significantly from year to year, while sulfate shows significant temporal variability. Pyrite dissolution is affected by the changing hydrological drivers where it is enhanced in the dry WY18; calcite dissolution supplements calcium dilution under high flow conditions. The model simulates the reaction hotspots controlled by hydrological conditions, and the spatially‐resolved results show that higher soil saturation and less snowpack occur earlier on the south‐facing side than on the north‐facing side. This is a first‐of‐its‐kind demonstration of a model that integrates hydrologic processes, including evapotranspiration, and reactive transport to enable a predictive understanding of hydrogeochemical exports.

Tandem Use of Multiple Tracers and Metrics to Identify Dynamic and Slow Hydrological Flowpaths

Current understanding of the dynamic and slow flow paths that support streamflow in mountain headwater catchments is inhibited by the lack of long-term hydrogeochemical data and the frequent use of short residence time age tracers. To address this, the current study combined the traditional mean transit time and the state-of-the-art fraction of young water (Fyw) metrics with stable water isotopes and tritium tracers to characterize the dynamic and slow flow paths at Marshall Gulch, a sub-humid headwater catchment in the Santa Catalina Mountains, Arizona, USA. The results show that Fyw varied significantly with period when using sinusoidal curve fitting methods (e.g., iteratively re-weighted least squares or IRLS), but not when using the transit time distribution (TTD)-based method. Therefore, Fyw estimates from TTD-based methods may be particularly useful for intercomparison of dynamic flow behavior between catchments. However, the utility of 3H to determine Fyw in deeper groundwater was limited due to both data quality and inconsistent seasonal cyclicity of the precipitation 3H time series data. Although a Gamma-type TTD was appropriate to characterize deep groundwater, there were large uncertainties in the estimated Gamma TTD shape parameter arising from the short record length of 3H in deep groundwater. This work demonstrates how co-application of multiple metrics and tracers can yield a more complete understanding of the dynamic and slow flow paths and observable deep groundwater storage volumes that contribute to streamflow in mountain headwater catchments.

On the performance of conceptual and physically based modelling approach to simulate a headwater catchment in Brazil

Conceptual large-scale distributed hydrological models (e.g. MHD-INPE) were developed to not only be used with limited available data but also to match the scale of atmospheric models. However, it is unknown if it can be representative for small watersheds in rural mountainous regions, which are crucial not only for agriculture, but also for water supply for several uses. Therefore, the objective of this study is to assess MHD-INPE performance in a headwater catchment with complex terrain alongside to a distributed physically based model (DHSVM). The Lavrinha Watershed (LW) has a drainage area of 6.7 km2, being 63% of the catchment occupied by native vegetation (Atlantic Rainforest), while the remaining area is mainly occupied by pasture for livestock farming. MHD-INPE and DHSVM were applied using a soil moisture zone map derived from the height above the nearest drainage (HAND) algorithm. The runoff simulated by the MHD-INPE better fits the observed data, with a validation Nash-Sutcliffe efficiency (NSE) of 0.70 for the daily scale, compared to a 0.55 in DHSVM. In the evapotranspiration simulation, both models showed similar trends, being 49% of the precipitation in the MHD-INPE and 46% in the DHSVM, while the observed value was 49%. For the baseflow, the MHD-INPE fitted better to the observed streamflow, whereas the DHSVM underestimated it during the dry season. Thus MHD-INPE was able to accurately simulate the streamflow in a mountainous headwater catchment in southeast Brazil, despite its large spatial scale.

How Important Are Those Fracture Zones? Scale Dependent Characteristics Revealed Through Field Studies and an Integrated Hydrological Model of a Mountain Headwater Catchment

Modeling groundwater flow in bedrock can be particularly challenging due to heterogeneities associated with fracture zones. However, fracture zones can be difficult to map, particularly in forested areas where tree cover obscures land surface features. This study presents the evidence of fracture zones in a small, snowmelt-dominated mountain headwater catchment and explores the significance of these fracture zones on groundwater flow in the catchment. A newly acquired bare earth image acquired using LiDAR identifies a previously undetected linear erosion zone that passes near a deep bedrock well at low elevation in the catchment. Borehole geophysical logs indicate more intense fracturing in this well compared to two wells at higher elevation. The well also exhibited a linear flow response during a pumping test, which is interpreted to reflect the influence of a nearby vertical fracture zone. The major ion chemistry and stable isotope composition reveal a slightly different chemical composition and a more depleted isotopic signature for this well compared to other groundwaters and surface waters sampled throughout the catchment. With this evidence of fracturing at the well scale, an integrated land surface – subsurface hydrologic model is used to explore four different model structures at the catchment scale. The model is refined in steps, beginning with a single homogeneous bedrock layer, and progressively adding 1) a network of large-scale fracture zones within the bedrock, 2) a weathered bedrock zone, and 3) an updated LiDAR-derived digital elevation model, to gain insight into how increasing subsurface geological complexity and land surface topography influence model fit to observed data and the various water balance components. Ultimately, all of the models are considered plausible, with similar overall fit to observed data (snow, streamflow, pressure heads in piezometers, and groundwater levels) and water balance results. However, the models with fracture zones and a weathered zone had better fits for the low elevation well. These models contributed slightly more baseflow (~14% of streamflow) compared to models without a weathered zone (~1%). Thus, in the watershed scale model, including a weathered bedrock zone appears to more strongly influence the hydrology than only including fracture zones.

An improved practical approach for estimating catchment‐scale response functions through wavelet analysis

AbstractCatchment‐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 calculated TTDs that contribute to streamflow via spatially and temporally variable flow paths in a sub‐humid mountain headwater catchment in Arizona, United States. Our results indicate that composite TTDs (a combination of Piston Flow and Gamma TTDs) most accurately represented this system for periods up to approximately 1 month, and that a Gamma TTD was most appropriate thereafter during both winter and summer seasons and for the overall time‐weighted TTD; a Gamma TTD type was applicable for all periods during the dry season. The TTD results also suggested that old waters, i.e., beyond the applicable tracer range, represented approximately 3% of subsurface contributions to streamflow. For ETTD and using δ18O as a tracer in precipitation and xylem waters, a Gamma ETTD type best matched the observations for all seasons and for the overall time‐weighted pattern, and stable water isotopes were effective tracers for the majority of vegetation source waters. This study addresses a fundamental question in mountain catchment hydrology; namely, how do the spatially and temporally varying subsurface flow paths that support catchment evapotranspiration and streamflow modulate water quantity and quality over space and time.

Modeling the Impact of Riparian Hollows on River Corridor Nitrogen Exports

Recent studies in snowmelt-dominated catchments have documented changes in nitrogen (N) retention over time, such as declines in watershed exports of N, though there is a limited understanding of the controlling processes driving these trends. Working in the mountainous headwater East River Colorado watershed, we explored the effects of riparian hollows as N-cycling hotspots and as important small-scale controls on observed watershed trends. Using a modeling-based approach informed by remote sensing and in situ observations, we simulated the N-retention capacity of riparian hollows with seasonal and yearly hydrobiogeochemical perturbations imposed as drivers. We then implemented a scaling approach to quantify the relative contribution of riparian hollows to the total river corridor N budget. We found that riparian hollows primarily serve as N sinks, with N-transformation rates significantly limited by periods of enhanced groundwater upwelling and promoted at the onset of rainfall events. Given these observed hydrologic controls, we expect that the nitrate (NO3-) sink capacity of riparian hollows will increase in magnitude with future climatic perturbations, specifically the shift to more frequent rainfall events and fewer snowmelt events, as projected for many mountainous headwater catchments. Our current estimates suggest that while riparian hollows provision ~5–20% of NO3- to the river network, they functionally act as inhibitors to upland NO3- reaching the stream. Our work linking transient hydrological conditions to numerical biogeochemical simulations is an important step in assessing N-retaining features relative to the watershed N budget and better understanding the role of small-scale features within watersheds.

Seasonal connections between meteoric water and streamflow generation along a mountain headwater stream

AbstractIn snowmelt‐driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ18O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high‐relief mountainous catchments. We characterized δ18O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ18O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow‐derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ18O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ18O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late‐winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid‐summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions.

Tracing total and dissolved material in a western Canadian basin using quality control samples to guide the selection of fingerprinting parameters for modelling

The source dynamics of total and dissolved material in riverine systems are being affected by anthropogenic activities resulting in the degradation of waterways worldwide. Identifying the main sources of total and dissolved material is thus central to the management of increasingly scarce water resources. Here, we utilize data generated from water quality monitoring programs to investigate the sources of total and dissolved material in a large, semi-arid basin in western Canada. Our research focuses on the confluence of two major tributaries in the South Saskatchewan River Basin (SSRB) in the Province of Alberta: the Bow River (25,611 km2) and the Oldman River (28,270 km2). A tributary tracing technique coupled with a Deconvolutional-MixSIAR (D-MIXSIAR) modelling approach is used to estimate the potential source contributions of total and dissolved material from major tributary sites to target node sites on the main stem of the Bow River and Oldman River in addition to target nodes downstream of their confluence. In total, 812 samples were taken from 29 sites across the SSRB. A novel approach to selecting fingerprints for modelling is presented based on the analyses of additional quality control samples (146 duplicate and 172 blank samples). Overall, the Rocky Mountain headwater catchments were found to dominate the supply of material modelled using total recoverable (68%) and dissolved (76%) metals. There were seasonal fluctuations in source dynamics evident where the Bow River dominated the supply of total (69%) and dissolved (57%) material during the ice-covered season (November-March), and the Oldman River dominated the supply of total (73%) and dissolved (59%) material during the open water season (April-October). On the one hand, these seasonal dynamics are potentially the result of the extensive regulation of flow, particularly along the Bow River. On the other hand, the intensification of agriculture in the prairie/plain catchments may also facilitate the excess supply of total relative to dissolved material. For example, the Little Bow River, with ~70% agricultural land cover, contributed ~14 times more total material than anticipated based on discharge and 1.6 times more than anticipated based on unit area during the open water season. Overall, this research has improved our understanding of the source dynamics of total and dissolved material in the SSRB, providing the foundation for focussed studies targeting the main sources of total and dissolved material in this large, semi-arid basin in western Canada. In addition, our research highlights the potential of using existent data generated from water quality monitoring programs along with quality control best practices to help improve our understanding of the source dynamics of total and dissolved material in waterways around the world.

Sr and U isotopes reveal the influence of lithologic structure on groundwater contributions along a mountain headwater catchment (Hyalite Canyon, MT)

Changing precipitation patterns are projected to reduce snowpack storage and late summer stream flows in mountain headwaters of the western United States. Ecosystems, agriculture, and municipalities depend on late summer flow in streams supplying water to the mountain front. Therefore, improved understanding of groundwater storage contributions to mountain streams is increasingly important. In this work, we use 87Sr/86Sr ratios and 234U/238U activity ratios ([234U/238U]) as indicators of water-rock interaction contributing to runoff, groundwater, and surface-subsurface hydrologic exchanges along Hyalite Creek, a mountain headwater tributary within the upper Missouri River basin. Main stem and tributary flow was sampled longitudinally from 2016 to 2018, focusing on presumed baseflow conditions in February and August. Changing stream chemistry of the main stem is associated with groundwater inflows from local springs discharging from Madison Group limestones, and inputs of water from the Archean gneiss exposed along the mountain front. We use mixing models to estimate that locally in the stream reach traversing a spring outlet, distinct inflows from the Madison aquifer contribute ~4% of streamflow during baseflow conditions. Contributions to streamflow by local inflows from Archean gneiss varied seasonally, with increases in 87Sr/86Sr values in Hyalite Creek that suggest contributions of ~2% in August to ~8% in February; at the same time, decreasing [234U/238U] values indicate an otherwise undetected additional endmember, and 222Rn assays show that total inflows are likely more extensive. Our results reveal distinct groundwater contributions to streamflow from specific rock units within this mountain headwater system, and elucidate pathways of mountain stream flow generation in lithologically diverse watersheds.

Differential C-Q Analysis: A New Approach to Inferring Lateral Transport and Hydrologic Transients Within Multiple Reaches of a Mountainous Headwater Catchment

Concentration-discharge (C-Q) relationships have been widely used as “hydrochemical tracers” to determine the variability in riverine solute exports across event, seasonal, annual, and decadal time scales. However, these C-Q relationships are limited to investigating solute transport dynamics at individual sampling stations, such that they create an incomplete understanding of the solute behavior upstream or downstream of the sampling station. Therefore, the objective of this study is to develop, apply and assess a differential C-Q approach that can characterize spatial variability in solute behavior across stations, as well as investigate their controls, by following a different spatial scheme and organizing the river into multiple sections. The differential C-Q approach captures the difference in concentration in a river segment over the difference in discharge, thereby accounting for gains, losses or fractional solute turnover between sampling stations. Using water quality data collected over four water years (2014-18) in a mountainous headwater catchment of the East River, Colorado, this study compares traditional and differential C-Q relationships in predicting solute behavior between three sampling stations distributed throughout the river. Results from the differential C-Q analysis demonstrate significant differences in solute behavior within upstream versus downstream reaches of the East River watershed. In particular, the meandering downstream section is marked by significant gains in both groundwater and solute concentrations as opposed to the dilution and the declining trends observed in the high-relief, steep terrain upstream reach. Shale mineralogy was determined to have a major influence on in-stream concentrations pertaining to Ca, DIC, DOC, Mg, Mo, NO3 and SO4. The analyses further revealed that total P concentration in the downstream reach exceeded the U.S. Environmental Protection Agency’s desired goal for control of eutrophication (110 ppb). Overall, differential C-Q metrics yield a better understanding of the lateral storage and interactions within catchments than traditional analyses, and holds potential for aiding water quality managers in the identification of critical stream reaches that assimilate harmful chemicals.

Satellite-derived foresummer drought sensitivity of plant productivity in Rocky Mountain headwater catchments: spatial heterogeneity and geological-geomorphological control

Long-term plot-scale studies have found water limitation to be a key factor driving ecosystem productivity in the Rocky Mountains. Specifically, the intensity of early summer (the ‘foresummer’ period from May to June) drought conditions appears to impose critical controls on peak ecosystem productivity. This study aims to (1) assess the importance of early snowmelt and foresummer drought in controlling peak plant productivity, based on the historical Landsat normalized-difference vegetation index (NDVI) and climate data; (2) map the spatial heterogeneity of foresummer drought sensitivity; and (3) identify the environmental controls (e.g. geomorphology, elevation, geology, plant types) on drought sensitivity. Our domain (15 × 15 km) includes four drainages within the East Water watershed near Gothic, Colorado, USA. We define foresummer drought sensitivity based on the regression slopes of the annual peak NDVI against the June Palmer Drought Severity Index between 1992 and 2010. Results show that foresummer drought sensitivity is spatially heterogeneous, and primarily dependent on the plant type and elevation. In support of the plot-based studies, we find that years with earlier snowmelt and drier foresummer conditions lead to lower peak NDVI; particularly in the low-elevation regions. Using random forest analysis, we identify additional key controls related to surface energy exchanges (i.e. potential net radiation), hydrological processes (i.e. microtopography and slope), and underlying geology. This remote-sensing-based approach for quantifying foresummer drought sensitivity can be used to identify the regions that are vulnerable or resilient to climate perturbations, as well as to inform future sampling, characterization, and modeling studies.

Vegetation dynamics of ephemeral and perennial streams in mountainous headwater catchments

Ephemeral and perennial streams of mountainous catchments in Sabaragamuwa Province of Sri Lanka and Hong Kong of China were studied for two years on vegetation dynamics. Each year, sampling was conducted during a period when ephemeral streams had low surface flows. Sampling was realized contiguously using belt transects. The standing crop biomass (hereafter biomass) of herbaceous vegetation in ephemeral channels was comparatively lower than perennials and so was the herb diversity. Herb diversity showed a peak from 1.5 to 4.5 m from the centerline/thalweg of ephemeral and perennial streams. Out of 24 herbs, only three were common for both. A peak herb biomass zone was observed in perennials in the same region where diversity peaked. In ephemerals, herb biomass increased laterally up to ∼1.5 m, and was constant thereafter. Seedling experiment results tallied with the field diversity observations of both stream types, and suggested that seed dispersion was the main reason for herb colonization. Furthermore, it showed sapling emergence to be significantly higher in perennials than ephemerals. Return period of annual maximum monthly rainfall was a strong indicator of age of trees in ephemeral streams, and elucidated the possibility of hindcasting past flow episodes. Electrical conductivity was significantly high in ephemeral streams among all the water quality parameters. The contents of the water nutrients were approximately the same in both stream types. While recommending further studies on eco-hydrology of ephemerals, we recognize ephemeral streams to be valuable references in climate change studies due to their responsiveness and representativeness in long term hydrological changes.

Spatial variations of hydrochemistry and stable isotopes in mountainous river water from the Central Asian headwaters of the Tajikistan Pamirs

Water resources in Central Asia from the mountainous headwater catchments is changing due to the shrinkage of glaciers in the Tian Shan and Pamir mountain systems. In order to predict future changes in water quality, it is crucial to understand what factors are governing the spatial variations of water chemistry and hydrological processes in mountainous headwater catchments. In this study, water chemistry including major ions and stable isotopes in the headwaters of major Tajikistan rivers was studied. Results showed that Tajikistan river water had an alkaline pH value (mean: 8.2) and total dissolved solids (mean: 368.5mg/L) were higher than the global average value. Ca2+, Na+, HCO3−, and SO42− in the rivers were the most abundant cations and anions, controlled by the rock weathering process and evaporation-crystallization processes. The hydrochemical facies of river water was dominated by Ca-HCO3 (71.7%) and exhibited spatial heterogeneity, which was related to the lithologic compositions and water source across Tajikistan. A significant negative correlation of river water δ18O with elevation was observed with a vertical lapse rate of 0.17‰/100 m. The more negative δ18O values in rivers from eastern Tajikistan were scattered in the lower left corner of the δ18O-δ2H plot, implying that the rivers were primarily supplied by snow/glacier meltwater because of the substantial number of glaciers and high elevation mountain in eastern regions. The drinking and irrigation suitability from ionic compositions revealed that the water quality of Tajikistan rivers was naturally good, though some sites posed a safety concern. These findings can provide new insights into sustainable management of water quality in the climatically and lithologically distinct segments of headwater regions in the Tajikistan Pamirs.

Climate Change May Increase the Drought Stress of Mesophytic Trees Downslope With Ongoing Forest Mesophication Under a History of Fire Suppression

In mountainous headwater catchments, downslope flow of subsurface water could buffer downslope forest communities from soil moisture stress during drought. Here we investigated changes in landscape-scale vegetation patterns at five forested headwater catchments in the Coweeta Hydrologic Laboratory in the southern Appalachians. We used a ca. 30-year Landsat Thematic Mapper (TM) image record of normalized difference vegetation index (NDVI), spanning a period of recorded warming since the mid-1970. We then, related spatial and temporal canopy patterns to seasonal water balance, streamflow recession behavior, and low flow dynamics from the long-term hydrologic records. All hydrologic metrics indicated increasing evapotranspiration, decreasing streamflow given precipitation, and potentially decreasing downslope subsidy at the watershed scale over time, especially during low-flow periods. Contrary to expectations, leaf area index (LAI) and basal area increased more upslope compared to downslope over time, coincident with warming. Trends in the ratio of NDVI in upslope and downslope topographic positions were also supported by long-term tree basal area increment, litterfall, and sap flux data in one of the reference watersheds. Mesophytic trees downslope appeared to respond more to frequent droughts and experience lower growth than xerophytic trees upslope, closely mediated by the isohydric/anisohydric continuum along hydrologic flow paths. Considering ongoing forest “mesophication” under a history of fire suppression across the eastern US deciduous forests, this study suggests that mesophytic trees downslope may be more vulnerable than xerophytic trees upslope under ongoing climate change due to an apparent dependence on upslope water subsidy.

Ubiquitous Fractal Scaling and Filtering Behavior of Hydrologic Fluxes and Storages from A Mountain Headwater Catchment

We used the weighted wavelet method to perform spectral analysis of observed long-term precipitation, streamflow, actual evapotranspiration, and soil water storage at a sub-humid mountain catchment near Tucson, Arizona, USA. Fractal scaling in precipitation and the daily change in soil water storage occurred up to a period of 14 days and corresponded to the typical duration of relatively wet and dry intervals. In contrast, fractal scaling could be observed up to a period of 0.5 years in streamflow and actual evapotranspiration. By considering long-term observations of hydrologic fluxes and storages, we show that, in contrast to previous findings, the phase relationships between water balance components changed with component period and were not perfectly in or out of phase at all periods. Self-averaging behavior was apparent, but the temporal scales over which this behavior was applicable differed among the various water balance components. Conservative tracer analysis showed that this catchment acted as a fractal filter by transforming white noise in the precipitation input signal to a 1/f flicker in the streamflow output signal by means of both spatial and temporal subsurface advection and dispersion processes and soil wetting properties. This study provides an improved understanding of hydrological filtering behavior in mountain critical zones that are critical sources of water and ecosystem services throughout the world.

Stormflow threshold behaviour in a subtropical mountainous headwater catchment during forest recovery period

AbstractForest ecohydrological feedbacks complicate the threshold behaviour of stormflow response to precipitation or wetting conditions on a long‐term scale (e.g. several years). In this study, the threshold behaviours in an evergreen‐deciduous mixed forested headwater catchment in southern China were examined during 2009–2015, when damaged vegetation was recovering after the great 2008 Chinese ice and snowstorm. The non‐uniqueness of the thresholds and the slow and rapid responses of stormflow at the outlet of the catchment in different hydro‐climate datasets with different maximum values of gross precipitation (P) and sums of precipitation and antecedent soil moisture index (P + ASI) were assessed. The thresholds of P and P + ASI required to trigger stormflows (i.e. ‘generation thresholds’) and the transition from slow to rapid responses of stormflow (i.e. ‘rise thresholds’) were compared both seasonally and annually. The results indicated significant differences in the analysed datasets, highlighting the need to compare thresholds with care to avoid misinterpretation. Seasonal variations in threshold behaviours in the catchment suggested that vegetation canopy interception contributed to higher rise thresholds, and wetter conditions resulted in higher runoff sensitivity to precipitation during the growing and rainy seasons. Furthermore, the generation thresholds were higher in the dormant season, possibly due to drier soil moisture conditions in the near‐channel areas. During the vegetation recovery period, the annual generation thresholds increased, however the rise thresholds did not exhibit a similar trend. The rapid stormflow response above the threshold decreased, possibly due to transpiration and interception of the recovered vegetation. However, the slow stormflow response to small rainfall events below the thresholds was higher in wetter years but lower in drier years, suggesting that the total water input dominated the stormflow response during small rainfall events. In conclusion, the seasonal and annual variations in threshold behaviours highlight that vegetation recovery and hydro‐climatic conditions had a notable impact on the stormflow response.