Alpine Watersheds Research Articles

»Kisanje« severnega Jadrana

The present knowledge of the carbonate system in the northern Adriatic is described in this short overview. Its buffer capacity is rather high, due to riverine input of carbonates dissolved from Alpine and Karstic watersheds, and the waters should have a higher resilience to acidification. In the shallow eutrophic areas, the combined effect of rising atmospheric CO2, warming and river-induced anthropogenic CO2 with the associated decrease in buffer capacity could act to acidification process. Significant effect on calcifying organisms is expected in the future.

Runoff Projection from an Alpine Watershed in Western Canada: Application of a Snowmelt Runoff Model

The rising global temperature is shifting the runoff patterns of snowmelt-dominated alpine watersheds, resulting in increased cold season flows, earlier spring peak flows, and reduced summer runoff. Projections of future runoff are beneficial in preparing for the anticipated changes in streamflow regimes. This study applied the degree–day Snowmelt Runoff Model (SRM) in combination with the MODIS to remotely sense snow cover observations for modeling the snowmelt runoff response of the Upper Athabasca River Basin in western Canada. After assessing its ability to simulate the observed historical flows, the SRM was applied for projecting future runoff in the basin. The inclusion of a spatial and temporal variation in the degree–day factor (DDF) and separation of the DDF for glaciated and non-glaciated areas were found to be important for improved simulation of varying snow conditions over multiple years. The SRM simulations, driven by an ensemble of six statistically downscaled GCM runs under the RCP8.5 scenario for the future period (2070–2080), show a consistent pattern in projected runoff change, with substantial increases in May runoff, smaller increases over the winter months, and decreased runoff in the summer months (June–August). Despite the SRM’s relative simplicity and requirement of only a few input variables, the model performed well in simulating historical flows, and provides runoff projections consistent with historical trends and previous modeling studies.

Direct Observation of the Depth of Active Groundwater Circulation in an Alpine Watershed

AbstractThe depth of active groundwater circulation is a fundamental control on stream flows and chemistry in mountain watersheds, yet it remains challenging to characterize and is rarely well constrained. We collected hydraulic conductivity, hydraulic head, temperature, chemical, noble gas, and 3H/3He groundwater age data from discrete levels in two boreholes 46 and 81 m deep in an alpine watershed, in combination with chemical and age data from shallow groundwater discharge, to discern groundwater flow rates at different depths and directly observe active and inactive groundwater. Vertical head gradients are steep (average of 0.4) and thermal profiles are consistent with typical linear conductive continental geotherms. Groundwater deeper than ∼20 m is distinct from shallow groundwater and creek water in its chemistry, noble gas signature, and age (dominantly >65 years compared to <9 years). Together these results suggest low vertical groundwater flow velocities and a relatively shallow active circulation depth of ∼20 m. This hypothesis is tested with a simple 2‐D numerical fluid flow and heat transport model representing a hillslope transect through the two boreholes. The modeling indicates that the subhorizontally bedded sedimentary rocks underlying the basin are highly anisotropic with low vertical hydraulic conductivity, and at most ∼10% of bedrock recharge (equivalent to <2% of stream baseflow) flows below a depth of 20 m. The study demonstrates the considerable value of discrete‐depth hydrogeologic, chemical, and age data for determining active circulation depth, and illustrates an approach for maximizing the utility of individual boreholes drilled for mountain bedrock aquifer characterization.

Flexible user interface for machine learning techniques to enhance the complex geospatial hydro-climatic models with future perspective

Hydro-climatic (HC) models have complex environments due to the integration of hydrological processes and climate indices for the assessment of historical and future scenarios. The approximation of HC models leads to a major uncertainty in the selection of optimal methods for processing, enhancement, and assessment. The present work developed a User-Friendly Interface (UI) in the R programming platform to enhance the geospatial HC models using machine learning concepts. Here, UI complies with various technologies together to perform consistently with input control, processing, and visualization. To validate this interface, a snow-dominated alpine watershed was selected. The results showed that, (a) UI assisted to downscale of the future climatic data into finer resolution, (b) boosted the efficiency of the geospatial model by adaptive random forest regression with NSE = 0.92 and 0.84, respectively. Moreover, UI designed to apply for different geospatial optimization problems which assist academicians, scientists, decision-makers, planners, and stakeholders, etc.

Method for assigning hydrological computational units in alpine watersheds

Study regionThe Nagqu River Watershed in the Qinghai–Tibet Plateau. Study focusA computational unit is the basic spatial unit employed in distributed hydrological models (DHMs), which combines the spatiotemporal distributions of meteorological variables and physical parameters denoted within a region to reveal the characteristics affecting the water cycle of watersheds. However, the current methods developed for spatially divided regions into computational units fail to consider the physical meaning and impact of climate change in montane altitudinal zones (MAZs), which significantly affects alpine watersheds. This study proposes a method for assigning the computational units of alpine DHMs according to one-period MAZ that explicitly account for physical meaning and climate change. New hydrological insights for the regionThe benefits of the proposed altitudinal discretization method are verified by applying it to construct a DHM based on the water and energy transfer processes in large river basins (WEP-L) models. The results obtained using the WEP-L, SWAT model and the new regional division method (denoted as the WEP-C model) that applies the proposed altitudinal discretization method are compared. The results demonstrate that the simulation accuracy of the daily and monthly streamflows and daily soil moisture from the alpine watershed is improved by the use of the proposed altitudinal discretization method, which provide guidance for further DHM development and can be applied to other hydrological models.

Climatic, land cover, and anthropogenic controls on dissolved organic matter quantity and quality from major alpine rivers across the Himalayan-Tibetan Plateau

Alpine rivers in mountainous regions are crucial not only for land-ocean transfer of chemical species and sediments, but also for water, food, and energy security. Here, we examined dissolved organic matter (DOM) from the major alpine waters on the Tibetan Plateau. Our results revealed a decreasing trend of DOM quantity juxtaposed to an increasing trend of aromaticity from the northern to southern plateau. This is potentially caused by a general decreasing gradient of dust load combined with an increasing gradient of precipitation and vegetation from the NW to SE plateau. Furthermore, most proglacial streams and smaller tributaries were found to be relatively dominated by tyrosine-like fluorescent DOM from glaciers. In contrast, most main stems of rivers and tributaries within larger catchment basins were more controlled by humic-like fluorescent DOM from terrestrial origins. Condensed aromatics accounts for 14–21% of molecular formulas for riverine DOM, much higher than the world's average of ~11%, which indicated anthropogenic black soot pollution. In addition, there is a higher level of DOM amount in the monsoon season than in winter, and DOM characteristics varied more widely (dissolved organic carbon concentration: 0.2-37 mg-C L−1, Fluorescence Index: 1.2-1.8) on the Tibetan Plateau in comparison to other global alpine watersheds. This suggests heterogeneous land cover, anthropogenic, and climatic factors at play, which is reflected in DOM quantity and quality, over the highest plateau on Earth.

Alpine hydrogeology

Groundwater discharge in alpine headwaters sustains baseflow in rivers originating in mountain ranges of the world, which is critically important for aquatic habitats, run-of-river hydropower generation, and downstream water supply. Groundwater storage in alpine watersheds was long considered negligible, but recent field-based studies have shown that aquifers are ubiquitous in the alpine zone with no soil and vegetation. Talus, moraine, and rock glacier aquifers are common in many alpine regions of the world, although bedrock aquifers occur in some geological settings. Alpine aquifers consisting of coarse sediments have a fast recession of discharge after the recharge season (e.g., snowmelt) or rainfall events, followed by a slow recession that sustains discharge over a long period. The two-phase recession is likely controlled by the internal structure of the aquifers. Spatial extent and distribution of individual aquifers determine the groundwater storage-discharge characteristics in first- and second-order watersheds in the alpine zone, which in turn govern baseflow characteristics in major rivers. Similar alpine landforms appear to have similar hydrogeological characteristics in many mountain ranges across the world, suggesting that a common conceptual framework can be used to understand alpine aquifers based on geological and geomorphological settings. Such a framework will be useful for parameterizing storage-discharge characteristics in large river hydrological models.

Mapping Snowmelt Progression in the Upper Indus Basin With Synthetic Aperture Radar

The Indus River is a vital resource for food security, ecosystem services, hydropower, and economy for millions of people living in Pakistan, India, China, and Afghanistan. Glacier and snowmelt from the high altitude Himalaya, Karakoram, and Hindu Kush mountain ranges are the largest drivers of discharge in the Upper Indus Basin, and contribute significantly to Indus flows. Complex climatology and topography, coupled with the challenges of field study and meteorological measurement in these rugged ranges, elicit notable uncertainties in predicting seasonal runoff as well as cryospheric response to changes in climate. Here we utilize Sentinel-1 Synthetic Aperture Radar (SAR) imagery to track ablation season development of wet snow in the Shigar Watershed of the Karakoram Mountains in Pakistan from 2015-2018. We exploit opportune local image acquisition times to highlight diurnal differences in radar indications of wet snow, and examine the spatial and temporal contexts of radar diurnal differences for 2015, 2017, and 2018 ablation seasons. Radar classifications for each ablation season show spatial and temporal patterns that indicate a dry winter snowpack undergoing diurnal surface melt-refreeze cycles, transitioning to surface snow that remains wet both day and night, and finally snow free conditions following melt out. Diurnally differing SAR signals may offer insights into important snowpack energy balance processes that precede melt out, which could provide useful constraints for both glacier mass balance modeling and runoff forecasting in remote alpine watersheds.

Alpine Hydrogeology: The Critical Role of Groundwater in Sourcing the Headwaters of the World.

Groundwater discharge in alpine headwaters sustains baseflow in rivers originating in mountain ranges of the world, which is critically important for aquatic habitats, run‐of‐river hydropower generation, and downstream water supply. Groundwater storage in alpine watersheds was long considered negligible, but recent field‐based studies have shown that aquifers are ubiquitous in the alpine zone with no soil and vegetation. Talus, moraine, and rock glacier aquifers are common in many alpine regions of the world, although bedrock aquifers occur in some geological settings. Alpine aquifers consisting of coarse sediments have a fast recession of discharge after the recharge season (e.g., snowmelt) or rainfall events, followed by a slow recession that sustains discharge over a long period. The two‐phase recession is likely controlled by the internal structure of the aquifers. Spatial extent and distribution of individual aquifers determine the groundwater storage‐discharge characteristics in first‐ and second‐order watersheds in the alpine zone, which in turn govern baseflow characteristics in major rivers. Similar alpine landforms appear to have similar hydrogeological characteristics in many mountain ranges across the world, suggesting that a common conceptual framework can be used to understand alpine aquifers based on geological and geomorphological settings. Such a framework will be useful for parameterizing storage‐discharge characteristics in large river hydrological models.

Heterogeneity in Hyporheic Flow, Pore Water Chemistry, and Microbial Community Composition in an Alpine Streambed

AbstractThe hyporheic zone, where surface water and groundwater mix, is an important microbial habitat where biogeochemical reactions influence water quality. We show that spatial variability in hyporheic flow in the East River near Crested Butte, CO, drives heterogeneity in streambed geochemical conditions and microbial community assemblages, but the diversity of microbial assemblages remains nearly constant throughout the reach. In July 2018, we collected approximately 100 pore water samples at 20‐cm depth and analyzed them for anions, cations, dissolved organic carbon, dissolved organic matter (DOM) quality, and basic water quality parameters. Vertical hydraulic head gradients were also measured to assess the potential for upward or downward flow, and heat tracing was used to quantify vertical flux rates at a subset of locations. We found that regions of the streambed that are more groundwater‐dominated contain less dissolved oxygen, higher concentrations of reduced metals, and more microbially processed, recalcitrant DOM, while more surface water‐dominated locations contain higher dissolved oxygen concentrations and terrestrially derived, labile DOM. 16S rRNA gene sequencing of extracted DNA revealed that microbial community composition varies with geochemical gradients related to hyporheic flow. These findings provide a better understanding of hyporheic controls on streambed biogeochemistry during the baseflow season, which is expected to lengthen with climate change in alpine watersheds due to earlier snowmelt onset and reduced snowpack.

Analysis of suspended sediment dynamics at event scale: comparison between a Mediterranean and an Alpine basin

ABSTRACTSuspended sediment dynamics are complex processes resulting from the interactions between different factors. To better comprehend these dynamics at event scale, we investigated two large datasets produced in two instrumented Italian catchments: a Mediterranean (Carapelle) and an Alpine watershed (Rio Cordon). An extensive set of variables concerning climatic, hydrological and suspended transport characteristics was used to explore the predictability of sediment dynamics. The differences in climate and size (499.40 vs 5.07 km2) led to different rainfall-runoff responses, while the discharge peak resulted as being a good descriptor for the hydrological and sediment dynamics. The sediment supply conditions influenced the hysteresis patterns while the principal component analysis explained 69.9% and 69.4% of variance in the Carapelle and Rio Cordon events, respectively. Using data produced by two long-term monitoring programmes, this study permits a better understanding of the sediment dynamics in Mediterranean and Alpine environments, also stressing their high complexity.

The importance of and need for rapid hydrologic assessments in Latin America

AbstractLong‐term observations are critical in hydrology to understand the dynamics of biological and physicochemical processes involved in and affected by the flux of water. Long‐term observations have been employed to provide basic understanding of the water cycle (e.g., infiltration, evaporation, run‐off generation, and groundwater–surface water interactions), but they are lacking in hydrologically relevant regions such as the Andes Mountains, including alpine watersheds. Although the call for long‐term data acquisition in Latin America has been made, the establishment of long‐term data collection centres remains logistically challenging. This ever‐growing scientific gap hinders our understanding of differences and similarities in hydrological processes of tropical and temperate regions. Furthermore, technological advances such as in situ optical sensors for water quantity and quality remain cost‐prohibitive for both short and long deployment at most existing research sites in Latin America, restricting researchers pursuing research funding or developing meaningful, intersite comparisons and syntheses. Here, we emphasize the importance of and need for rapid assessments (i.e., field campaigns conducted over a few days) for improved hypothesis development and mechanistic understanding of hydrological dynamics in Latin America. We report on rapid assessments conducted in the high‐elevation mountains (>3,000 m) of Colombia. Our results highlight rapidly changing dynamics in nutrient retention potential and dissolved CO2 (pCO2), as well as highly variable spatial distribution of water quality parameters (N, C, P, Cl) in areas with varying land use. We present an initial examination of the effects of land‐use change on stream nutrient dynamics in one of the most biodiverse and threatened ecosystems on Earth. We conclude that rapid assessments not only are necessary but also represent a cost‐effective way to develop clear, testable hypotheses to advance a hydrologic research agenda in Latin America and work towards long‐term hydrological knowledge and information for use by other scientists.

Groundwater dynamics in a hydrologically-modified alpine watershed from an ancient managed recharge system (Sierra Nevada National Park, Southern Spain): Insights from hydrogeochemical and isotopic information

In many of the alpine watersheds of Sierra Nevada (Southern Spain) exists an ancient network of dug canals that collect, transport and facilitate the recharge the snowmelt in the underlying aquifer during the spring season. This practice, known as careos, in the lower part of the watersheds supply drinking water as spring discharge during the dry season. To study how this managed recharge technique modifies the natural response of these basins this work focuses on characterizing the hydrological behavior of one of the sites, the Berchules watershed. The mechanisms for mineralization of groundwater are based on geochemical processes such as evapo-concentration in the soil layer and silicate mineral weathering due to dissolved CO2 originated from both soil biogenic processes and the atmosphere. Groundwater presents a main hydrogeochemical calcium‑magnesium-bicarbonate type facies, which is associated to groundwater flowing through the upper weathered silicates and quickly drained through springs located in the uplands and in the intermediate altitude catchment zone. Additionally, in the lower part of the basin some springs discharge mineralized groundwater with a sodium–calcium–bicarbonate composition associated to regional groundwater flow. In natural conditions, this hydrogeological system behaves as a sloping aquifer, occurring recharge between 1400 and 2500 m a.s.l. The springs discharge groundwater with an isotopic content and temperature in coherence with the local rainfall isotopic and thermal atmospheric altitudinal lines. Nevertheless, once the careo recharge begins the affected springs reveal the fingerprint of the concentrated recharge system by blurring the fingerprint of both the isotopic and thermal altitudinal dependence in the springs discharge. This validates the previous conceptual model and supports average recharge values of 141 ± 140 mm/yr and total average water resources of 181 ± 111 mm/yr which include a 40% increase in the study period due to the effect of the acequias de careo.

The Role of Frozen Soil in Groundwater Discharge Predictions for Warming Alpine Watersheds

AbstractClimate warming may alter the quantity and timing of groundwater discharge to streams in high alpine watersheds due to changes in the timing of the duration of seasonal freezing in the subsurface and snowmelt recharge. It is imperative to understand the effects of seasonal freezing and recharge on groundwater discharge to streams in warming alpine watersheds as streamflow originating from these watersheds is a critical water resource for downstream users. This study evaluates how climate warming may alter groundwater discharge due to changes in seasonally frozen ground and snowmelt using a 2‐D coupled flow and heat transport model with freeze and thaw capabilities for variably saturated media. The model is applied to a representative snowmelt‐dominated watershed in the Rocky Mountains of central Colorado, USA, with snowmelt time series reconstructed from a 12 year data set of hydrometeorological records and satellite‐derived snow covered area. Model analyses indicate that the duration of seasonal freezing in the subsurface controls groundwater discharge to streams, while snowmelt timing controls groundwater discharge to hillslope faces. Climate warming causes changes to subsurface ice content and duration, rerouting groundwater flow paths but not altering the total magnitude of future groundwater discharge outside of the bounds of hydrologic parameter uncertainties. These findings suggest that frozen soil routines play an important role for predicting the future location of groundwater discharge in watersheds underlain by seasonally frozen ground.

Variability and trends in runoff in the rivers of British Columbia's Coast and Insular Mountains

AbstractThis study examines the 1914–2015 runoff trends and variability for 136 rivers draining British Columbia's Coast and Insular Mountains. Rivers are partitioned into eastward and westward flowing rivers based on flow direction from the Coast Mountains. Thus, eastward and westward runoff trends and influence of topography on runoff are explored. Our findings indicate that rivers flowing eastward to the Nechako and Chilcotin plateaus contribute the lowest annual runoff compared to westward rivers where runoff is high. Low interannual runoff variability is evident in westward rivers and their alpine watersheds, whereas eastward rivers exhibit high interannual runoff variability. On Vancouver Island, some of the rivers with the highest annual runoff exhibit high interannual variability. A significant (p < .05) negative correlation exists between mean annual runoff (Rm) and latitude, gauged area, mean elevation, and its corresponding coefficient of variation. However, a significant positive correlation was found between the glacierized area of mountainous regions and Rm. The mean coefficient of variation in annual runoff is significantly negatively correlated with latitude and glacierized area, but significantly positively correlated with longitude. Annual and seasonal runoff trend analyses of each river were performed for an early (1936–2015), a middle (1966–2015), and a late (1986–2015) period using the Mann–Kendall test. Trend analyses revealed a shift towards more positive detectable (signal‐to‐noise ratio > 1) trends in annual and seasonal runoff from the middle to the late period across the study domain. Most positive detectable seasonal runoff trends in the middle period occur in spring in glacierized westward rivers located >1,200 m, whereas in the late period, they all occur in fall and are regionally coherent around Vancouver Island and south coastal BC. Rivers draining eastward exhibit more positive trends over 1986–2015 compared to westward rivers. This study provides crucial information on the hydrology of mountain watersheds across British Columbia's coast in response to Pacific Decadal Oscillation phase changes, the elevational amplification of regional climate change, and their influences on precipitation and glacier retreat.

Inferring hydraulic properties of alpine aquifers from the propagation of diurnal snowmelt signals

AbstractAlpine watersheds source major rivers throughout the world and supply essential water for irrigation, human consumption, and hydroelectricity. Coarse depositional units in alpine watersheds can store and transmit significant volumes of groundwater and thus augment stream discharge during the dry season. These environments are typically data scarce, which has limited the application of physically based models to investigate hydrologic sensitivity to environmental change. This study focuses on a coarse alpine talus unit within the Lake O'Hara watershed in the Canadian Rockies. We investigate processes controlling the hydrologic functioning of the talus unit using field observations and a numerical groundwater flow model driven with a distributed snowmelt model. The model hydraulic parameters are adjusted to investigate how these properties influence the propagation of snowmelt‐induced diurnal signals. The model results expectedly demonstrate that diurnal signals at the talus outlet are progressively damped and lagged with lower hydraulic conductivity and higher specific yield. The simulations further indicate that the lag can be primarily controlled by a higher hydraulic conductivity upper layer, whereas the damping can be strongly influenced by a lower hydraulic conductivity layer along the base of the talus. The simulations specifically suggest that the talus slope can be represented as a two layer system with a high conductivity zone (0.02 m s−1) overlying a 10 cm thick lower conductivity zone (0.002 m s−1). This study demonstrates that diurnal signals can be used to elucidate the hydrologic functioning and hydraulic properties of shallow aquifers and thus aid in the parameterization of hydrological models.

Multi-scale analysis of alpine landscapes with different intensities of abandonment reveals similar spatial pattern changes: Implications for habitat conservation

The abandonment of traditional anthropogenic activities is an important driver shaping landscape patterns. Therefore, multi-scale pattern analysis over time is needed to identify appropriate scales for biodiversity conservation and monitoring of abandoned landscapes. We compared spatial and temporal changes in a pair of alpine watersheds in Italy (Cajada and Tovanella), which are similar in size, geo-climatic conditions, and land-use histories; but have had divergent anthropogenic abandonment processes since the 1950s. We hypothesize that this divergence has led to corresponding dissimilarities in multi-scale patterns of landscape change. To examine this hypothesis, we analyzed land cover maps from three years (1954, 1980/83, 2006) and described the changes using transition matrices. For each year and watershed, landscape heterogeneity and a set of class-level metrics (i.e. percentage of the landscape, area-weighted mean patch size, patch density, area-weighted mean shape index, edge density, and aggregation index) were also measured at different scales using random sampling techniques, and the results were summarized by using scalograms. Woodland expansion occurred mainly at the expenses of grasslands, meadows, and shrublands. These changes were greater during the first time-period (1954-80/83) than in the more recent period (1980/83-2006), with a mean annual value that decreased from +5.18 to +1.33ha/year and from +4.08 to +1.96ha/year in the abandoned and managed watersheds, respectively. Landscape heterogeneity decreased over time with a similar pattern in both watersheds, which indicates a general process of homogenization. Management regime affected the spatial-scale response of class-level metrics; these metrics showed a variety of multi-scalar responses, which were not always consistent over time and under different management regimes. When considering the response of the indices across spatial-scales for both watersheds, certain historical curves showed a scale break, representing a significant change in the shape and slope of the curve (i.e. scale divergence). The presence of scale breaks in the scalograms can potentially reveal important thresholds for biodiversity. For example, grassland and meadow patch density at small spatial scales (<200m radius), which was found to be important for protected butterfly species, had a greater reduction over time in the managed watershed when compared to the abandoned watershed. In conclusion, the findings of this study indicate that there is good potential for understanding changes in landscape patterns under different management abandonment regimes by combining spatial and temporal analysis of class-level metrics.

Influences of Frozen Ground and Climate Change on Hydrological Processes in an Alpine Watershed: A Case Study in the Upstream Area of the Hei'he River, Northwest China

In cold regions, the occurrence of frozen ground has a fundamental control over the character of the water cycle. To investigate the impact of changing ground temperature conditions on hydrological processes in the context of climate change, a distributed hydrological model with an explicit frozen ground module was applied to an alpine watershed in the upstream area of the Hei'he River in the Qilian Mountains, northwest China. After evaluating the base model, we considered scenarios of frost-free ground and climate change. Results showed that the base model with a frozen ground module successfully captured the water balance and thermal regimes in the basin. When the frozen ground module was turned off, the simulated groundwater recharge and base flow increased by a factor of two to three because surface runoff caused by exceeding infiltration capacities at high elevations, which occurred in the base model, was eliminated. Consequently, the river hydrograph became smoother and flatter, with summer flood peaks delayed and reduced in volume. The annual mean depth where subsurface runoff was generated, was about 2.4 m compared to 1.1 m in the base model. For a warming climate, a combination of increasing evapotranspiration and reducing permafrost area results in smoother and flatter hydrographs, and a reduction in total river discharge. Although our analysis using numerical models has its limitations, it still provides new quantitative understanding of the influences of frozen ground and climate change on hydrological processes in an alpine watershed. Copyright © 2016 John Wiley & Sons, Ltd.

Pedogenetic interpretations of particle-size distribution curves for an alpine environment

The Qilian Mountains are generally capped by a productive silty soil layer wherever the environment allows. As Holocene loess is prevalent across the Qilian Mountains, we assume that at least some of these fine sediments are derived from loess deposition and that soils across the region may be genetically linked. To test these hypotheses, nine pedons in different landscapes within a typical alpine watershed of the Qilian Mountains were sampled. We also collected fine earth samples from glacier surfaces and crevices of glacial debris in the moraine zone. The particle-size distribution (PSD) is used as a proxy for identifying aeolian fractions in soils. The PSD curves of all of the fine-earth fractions examined are polymodal, although three modal sizes of roughly 16μm, 35μm and 80μm are found in almost all sediments in varying proportions. These modal sizes have been identified as three sources of aeolian sediments in different transport systems. The composition and thickness of loess sediments are related to local site conditions and may be related to the paleoenvironmental history of the site. Clastic debris and vegetation cover serve as dust traps during different stages of soil formation in this alpine environment, and soils grow upward with accumulating loess sediment. Our study demonstrates that PSD curves can be used to determine the origins (and especially aeolian origins) of soils. Furthermore, this study provides insight into the pedology, ecology and paleoenvironmental history of loess-affected ecosystems in alpine areas of the Qinghai-Tibetan Plateau.

Determination and impact factor analysis of hydrodynamic dispersion coefficient within a gravel layer using an electrolyte tracer method

Hydrodynamic dispersion is a measure for describing the process of solute transport in porous media. Characterizing the dispersion of water flow within gravel is essential for the prediction of solute transport especially nonpoint source pollutants migration in alpine watersheds where the land surface is typically covered with gravel. In this study, an integrated model and experimental method using an electrolyte tracer is proposed for determination of the hydrodynamic dispersion coefficient. Two experimental scenarios were designed to measure electrolyte tracer transport processes in both free water flow and gravel layer flow under different slope gradients and transport distances. Subsequently, the measured data were used to simultaneously calculate both the hydrodynamic dispersion coefficient and flow velocity by fitting the experimental data with the mathematical model. Dispersivity, as a critical feature of hydrodynamic dispersion, was determined as well under the two specified scenarios. Finally, the impact mechanisms of the gravel layer and factors related to the dispersion processes were comprehensively analyzed. The results indicate that the presence of a gravel layer significantly reduces flow velocity and the hydrodynamic dispersion coefficient, but increases solute dispersivity. For the flow within gravel layers, with much lower velocity, the positive effect of the gravel layer on dispersivity may be neutralized or even surpassed by the negative effect of flow velocity. The results should be helpful in characterizing the dispersion processes of water flow within gravel layer and hence in predicting solute transport, especially in nonpoint source pollutants migration in alpine watersheds where the land surface is richly covered with gravel.