Search

Background: Snow and ice melt provide sensitive indicators of climate change and serve as the primary source of stream flow in alpine basins.Aims: We synthesise the results of hydrological and hydrochemical studies during the period 1995–2014, building on a long history of earlier work focused on snow and water on Niwot Ridge and the adjacent Green Lakes Valley (GLV), which is part of the Niwot Ridge Long Term Ecological Research site (NWT LTER).Methods: These studies are discussed in the context of how snow, snowmelt and runoff reflect changing local climate. We review recent results of snow, snowmelt, hydrology and hydrochemistry from the plot to the basin scale, utilising new tools such as continuous global positioning system (GPS) measurements of snow depth, along with remotely-sensed measurements of snow-covered area and melt, combined with long-term measurements of snow properties, discharge and solute and isotopic content of water.Results and Conclusions: Surface–groundwater interactions are import...

Alpine ecosystems are particularly sus- ceptible to disturbance due to their short growing seasons, sparse vegetation and thin soils. Increased nitrogen deposition in wetfall and changes in climate currently affect Green Lakes Valley within the Col- orado Front Range. Research conducted within the alpine links chronic nitrogen inputs to a suite of ecological impacts, resulting in increased nitrate export. The atmospheric nitrogen flux decreased by 0.56 kg ha -1 year -1 between 2000 and 2009, due to decreased precipitation; however alpine nitrate yields increased by 40 % relative to the previous decade (1990-1999). Long term trends indicate that weath- ering products such as sulfate, calcium, and silica have also increased over the same period. The geochemical composition of thawing permafrost, as indicated by rock glacial and blockfield meltwater, suggests it is the source of these weathering products. Furthermore, mass balance models indicate the high ammonium loads within glacial meltwater are rapidly nitrified, contributing *0.5-1.4 kg N ha -1 to the growing season nitrate flux from the alpine watershed. The sustained export of these solutes during dry, summer months is likely facilitated by thawing cryosphere providing hydraulic connectivity late into the growing season. This mechanism is further supported by the lack of upward weathering or nitrogen solute trends in a neighboring catchment which lacks permafrost and glacial features. These findings suggest that reductions of atmospheric nitrogen deposition alone may not improve water quality, as cryospheric thaw exposes soils to biological and geochemical processes that may affect alpine nitrate concentrations as much as atmo- spheric deposition trends.

Hydrologic and hydrochemical studies have been conducted in Green Lakes Valley, Colorado Front Range, USA, above 3550 m since 1982. They show a classic seasonal hydrograph dominated by snowmelt and an earlier date for the start of spring flow and for peak flow over the period of record. This is consistent with patterns found at lower elevations in Colorado and throughout western North America. They also show an increasing trend in flows in September and October of 2.6 ±0.7 mm a−1 which is not found elsewhere and cannot be accounted for by increased autumn precipitation and the melting of surface ice. Because this late-season increase is not found at the highest elevations or in basins in which there is no evidence of permafrost, it seems best explained by the thawing of alpine permafrost at intermediate elevations. This is corroborated by an increase in the concentration of base cations and silica, and particularly in Ca2+ and SO4–, in the stream discharge starting in 2000. As with the physical hydrology, the geochemical signals have not been detected at the higher elevations in the basin, though they have previously been associated with streamflow from a small rock glacier in the valley. The combined evidence suggests the degradation of ice-rich permafrost on the north-facing slopes of the valley below 3700 m, where it has been detected at 3 m depth by geophysical surveys.

Here we characterize the nutrient content in the outflow of the Green Lake 5 rock glacier, located in the Green Lakes Valley of the Colorado Front Range. Dissolved organic carbon (DOC) was present in all samples with a mean concentration of 0·85 mg L−1 . A one-way analysis of variance test shows no statistical difference in DOC amounts among surface waters (p = 0·42). Average nitrate concentrations were 69 µmoles L−1 in the outflow of the rock glacier, compared to 7 µmoles L−1 in snow and 25 µmoles L−1 in rain. Nitrate concentrations from the rock glacier generally increased with time, with maximum concentrations of 135 µmoles L−1 in October, among the highest nitrate concentrations reported for high-elevation surface waters. These high nitrate concentrations appear to be characteristic of rock glacier outflow in the Rocky Mountains, as a paired-difference t-test shows that nitrate concentrations from the outflow of 7 additional rock glaciers were significantly greater compared to their reference streams (p = 0·003). End-member mixing analysis suggest that snow was the dominant source of nitrate in June, ‘soil’ solution was the dominant nitrate source in July, and base flow was the dominant source in September. Fluoresence index values and PARAFAC analyses of dissolved organic matter (DOM) are also consistent with a switch from terrestrial DOM in the summer time period to an increasing aquatic-like microbial source during the autumn months. Copyright © 2006 John Wiley & Sons, Ltd.

We characterize the seasonal variation in the geochemical and isotopic content of the outflow of the Green Lake 5 rock glacier (RG5), located in the Green Lakes Valley of the Colorado Front Range, USA. Between June and August, the geochemical content of rock glacier outflow does not appear to differ substantially from that of other surface waters in the Green Lakes Valley. Thus, for this alpine ecosystem at this time of year there does not appear to be large differences in water quality among rock glacier outflow, glacier and blockslope discharge, and discharge from small alpine catchments. However, in September concentrations of Mg2+ in the outflow of the rock glacier increased to more than 900 µeq L−1 compared to values of less than 40 µeq L−1 at all the other sites, concentrations of Ca2+ were greater than 4,000 µeq L−1 compared to maximum values of less than 200 µeq L−1 at all other sites, and concentrations of SO reached 7,000 µeq L−1, compared to maximum concentrations below 120 µeq L−1 at the other sites. Inverse geochemical modelling suggests that dissolution of pyrite, epidote, chlorite and minor calcite as well as the precipitation of silica and goethite best explain these elevated concentrations of solutes in the outflow of the rock glacier. Three component hydrograph separation using end–member mixing analysis shows that melted snow comprised an average of 30% of RG5 outflow, soil water 32%, and base flow 38%. Snow was the dominant source water in June, soil water was the dominant water source in July, and base flow was the dominant source in September. Enrichment of δ18O from −10‰ in the outflow of the rock glacier compared to −20‰ in snow and enrichment of deuterium excess from + 17.5‰ in rock glacier outflow compared to + 11‰ in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow. Copyright © 2005 John Wiley & Sons, Ltd.

Snow surface roughness is an important variable in the study of surface–atmosphere exchanges, including the investigation of snow melt at several scales, meltwater production and meltwater flux, wind transport and erosion in winter. In this paper the morphogenesis of the snow surface in winter and in summer is investigated both phenomenologically and quantitatively. For the first time, snow surface roughness and microtopography is measured spatially, using the Glacier Roughness Sensor (GRS), an instrument developed especially for this purpose. Data are analysed by application of geostatistical characterization and classification methods. Parameters that are useful as snow surface descriptors are defined and extracted from vario functions of first and second order, which are calculated for snow-surface-roughness data. As a result of the geostatistical analysis, characteristics of morphogenetic processes of the winter and summer snow surface are derived. Characteristic parameters are given for suncups (summer) and sastrugi (winter) and their development stages, as well as quantitative discriminators between these forms, which may facilitate automation of snow surface classification. As an application, the interaction of environmentally induced processes and self-organizational processes is analysed. Copyright © 2003 John Wiley & Sons, Ltd.

A 20-year record of ice thickness in late-March on an alpine lake in the Colorado Front Range shows a consistent thinning of the ice cover at 2.0 cm/yr(r = −0.931; N = 16; p < 0.01) during a period of increasing precipitation. The correlation between ice thickness and precipitation in the preceding Oct.–Mar. period is –0.439 (N = 16; p < 0.10),reflecting the insulating effect of an increase in snow cover on the lake. A second effect derived from increased precipitation occurs through additional water storage in the lake's catchment during the previous summer. This leads to a corresponding increase in groundwater flow to the lake during the winter, with a correlation between ice thickness and groundwater storage at the end of the preceding September of –0.656 (N = 16; p < 0.01). Whencombined, these two effects account for 50% of the variability in ice thickness(R = 0.714; N = 16; p < 0.01). Thus, much of the decline in ice thicknessin March is explained by two influences derived from an increase in winter precipitation: one associated with the current winter's precipitation and one with that of the previous winter.

Here we report on 3 years (1996 -1998) of measurements of organic and inorganic nitrogen (N) fluxes to and from Green Lakes Valley, a high-elevation ecosystem in the Colorado Front Range of the Rocky Mountains. Nitrate-N (NO3-N) was the dominant form of N in both precipitation and stream water. Annual precipitation contained 52% NO3-N, 32% ammonium-N (NH4-N), 9% dissolved organic N (DON), and 7% particulate organic N (PON). Annual export of N in streamflow was composed of 70% NO3-N, 4% NH4-N, 14% DON, and 12% PON. Thus the percentage of organic N increased from 16% of total N in precipitation to 26% of total N in streamflow. Subtracting inputs from outputs, Green Lakes Valley always shows net retention of inorganic N. The only form of N that showed net export was DON. DON export was low (0.18 to 0.13 kg ha 1 yr 1 ), with net export recorded in 2 years and basin retention recorded in 1 year. There was a seasonal pattern in the concentrations of inorganic N (NO3-N NH4-N) and organic N (DON PON). Concentrations of inorganic N were 15-25 mol L 1 during base flow, increased to 30 mol L 1 on the rising limb of the hydrograph during snowmelt runoff, then decreased to 5 mol L 1 on the recession limb of late summer, with a return to base flow values in the autumn. In contrast, organic N was 7-15 mol L 1 during base flow and decreased to near or below detection limits on the rising limb of the hydrograph, with a gradual but consistent increase on the recession limb and on into the autumn. The amount of N in dissolved organic matter changed over time, with the dissolved organic carbon (DOC):DON ratio decreasing from 45 on the rising limb of the hydrograph to 20 in the autumn. Spatially, there was a striking difference in the ratios of NO3-N and DON between talus and tundra areas. Nitrate concentrations in surface water draining talus areas were always greater than DON. In contrast, DON concentrations in surface water draining tundra areas were always greater than NO3-N. Concentrations of DON were not significantly correlated with DOC (R 2 0.04, p 0.05), indicating that controls on DON export may be different than controls on DOC export. Our results suggest that the ratio of the annual mass flux of inorganic N to organic N in stream waters may provide a novel index to evaluate the N status of terrestrial ecosystems from various biomes.

A continuous climate record since 195 1 at Niwot Ridge in the Colorado Front Range shows a decline in mean annual temperature, an increase in annual precipitation amount, and a decrease in mean daily solar radiation for the summer months. The increase in precipitation amount explains about half of the 200% increase in annual wet deposition of NO,- to Niwot Ridge over the last decade. Differences in climate parameters between 1994 and 1995 (increased snow depth and decreased net energy flux to the snowpack) resulted in a 4-5-fold increase in the magnitude of solute release from the snowpack in the form of an ionic pulse. In turn, the high chemical loading of strong acid anions in the seasonal snowpack and release of these solutes from the seasonal snowpack in the form of an ionic pulse is causing episodic acidification (ANC < 0 peq liter-l) in headwater catchments at present deposition levels. Small changes in climate parameters may cause large changes in the hydrochemistry of alpine streams. The changes in climate at Niwot Ridge are not in synchrony with lowland warming in the Great Plains to the east and serve as a reminder that climate in alpine areas is driven by local conditions and may be asynchronous with regional and global climate trends.

Nitrogen saturation is occurring throughout high-elevation catchments of the Colorado Front Range. Annual inorganic N loading in wet deposition to the Front Range of ∼4 kg ha-1 yr-1 is about twice that of the Pacific States and similar to many sites in the northeastern United States. In the last ten years at Niwot Ridge/Green Lakes Valley and Glacier Lakes, annual minimum concentrations of NO3- in surface waters during the growing season have increased from below detection limits to ∼10 μequiv L-1, indicating that these two catchments are at the threshold of N saturation. The Loch Vale watershed is N saturated, with annual minimum concentrations of NO3- in surface waters generally above 10 μequiv L-1; annual volume-weighted mean (VWM) concentrations of 16 μequiv L-1 in surface waters are greater than that of ∼11 μequiv L-1 NO3- in wet deposition. At these high-elevation catchments, there has been a shift in ecosystem dynamics from an N-limited system to an N-saturated system as a result of anthropogenical...

Dry deposition of atmospheric nitrogen gas and aerosol species was estimated for the alpine tundra of Niwot Ridge, 3525 m elevation in the Colorado Rockies. Comparisons, for the 4 month long growing season and the remaining 8 months of the year, were made with wet deposition and throughfall incident measurements taken during 1987-1989. Dry deposition of N to the tundra is estimated to be equal to or slightly greater than its wet deposition. During the mid-May to mid-September growing season, atmospheric N deposition is >1.0 mg N m−2d−1 directly from the atmosphere with a similar amount contributed indirectly as NO3-N in snowmelt water as a result of dry and wet deposition to the winter snowpack. The total N deposition to Niwot Ridge tundra during the growing season of about 2 mg N m−2d−1 may be compared to an earlier measurement of dry plus fog deposition (1 - 2 mg N m−2d−1) to a subalpine coniferous canopy at Niwot Ridge. Nitrate yields from two small drainage basins at Niwot Ridge match these fluxes. Seven years of record from an unvegetated glacial cirque suggest an average yield from the alpine of 0.7 mg N m−2d−1. The equivalent estimate for a basin with 50% tundra vegetation cover is 0.4 mg N m−2d−1. The contrast in these two estimates of daily averaged N yields for the entire year suggests the retention of nearly 1 mg N m−2d−1 during the growing season in the more vegetated basin. The sink for this N could be the tundra soil and vegetation where biological activity is often limited by the availability of N.

A 10 year record of streamflow from the 8 ha Martinelli Basin in the Colorado Front Range is dominated by snowmelt-produced flows. During the main part of the May-September flow season, the flows developed a marked 24-h cycle which often accounts for more than 50% of their variance, after removal of the seasonal trend. The amplitude and phase of this cycle vary in a similar fashion each year, showing a reduced amplitude and a forward phase shift as the summer progresses. These changes are explained by the reduction in the area of snow cover and its depth during the season. The phase shift may be used to estimate a catchment-scale hydraulic conductivity for a melting snow cover, yielding values in the range 40–80 cm h −1 . These are consistent with other estimates based upon plot and laboratory studies.

Seven years of discharge and water quality records define temporal and spatial patterns of solute movement in a Colorado alpine stream system. Dissolved solids concentrations are low, generally less than 30 mg 1−1 and occasionally less than 3 mg 1−1 at the highest elevations. Calcium is the dominant cation and bicarbonate and sulfate are the main anions. Temporal changes in solute concentrations are dominated by an annual cycle with high values in late winter and spring that decrease rapidly during early summer and then return more slowly through fall. This pattern corresponds to the seasonal streamflow regime and reflects differential elution of the snowpack by meltwater and changing proportions of surface and subsurface water in the streamflow. The amplitude of the annual cycle of solute concentration is reduced with increasing catchment area and where the groundwater contribution to flow is relatively high. In general, solute concentrations increase down valley but this trend is reversed in the case of biologically important solutes, such as nitrate and potassium. Rates of geochemical denudation are dominated by the volume of water discharge and thus are highest in the parts of the basin that accumulate the greatest depths of winter snow. They vary between 5 and 26 g m−2 yr−1 for different parts of the catchment and average less than 9 g m−2 yr−1. These rates are low compared to those from high-elevation catchments elsewhere but are an order of magnitude higher than rates of sediment removal from the basin.

[1] Source waters and flow paths of streamflow draining high-elevation catchments of the Colorado Rocky Mountains were determined using isotopic and geochemical tracers during the 1996 snowmelt runoff season at two subcatchments of the Green Lakes Valley, Colorado Front Range. A two-component hydrograph separation using δ18O indicates that new water dominated (82 ± 6%) streamflow at the 8-ha Martinelli catchment and old water dominated (64 ± 2%) at the 225-ha Green Lake 4 (GL4) catchment. Snowmelt became isotopically enriched as the melt season progressed, complicating the interpretation of source water models. Thus old water may be underestimated if the temporal variation in δ18O of snowmelt is ignored or extrapolated from point measurements to the catchment. Two-component hydrograph separations for unreacted and reacted waters using a single geochemical tracer were not always meaningful. Three-component hydrograph separations using end-member mixing analysis indicated that subsurface flow contributed more than two thirds to the streamflow at both catchments. Talus fields contributed more than 40% of the total discharge during summer at the GL4 catchment. A conceptual model was established for flow generation based on these results. It is suggested that surface water and groundwater interactions are much more important to the quantity and quality of surface water in high-elevation catchments than previously thought.

Seasonally snow-covered areas of Earth’s mountain ranges are important components of the global hydrologic cycle. Although their area is limited, the snowpacks of these areas are a major source of the water supply for runoff and ground water recharge over wide areas of the mid-latitudes. They are also sensitive indicators of climatic change. The release of ions from the snowpack is an important component in the biogeochemistry of alpine areas and may also function as a sensitive indicator of changes in atmospheric chemistry. The demand for water in the semiarid areas of the western United States is reflected in extensive systems of reservoirs, canals, and flow diversions that have been constructed over the past century. Most of the water resources tapped by these systems derives from the mountain environments of the Rocky Mountains, where contributions of the alpine have long been recognized (Martinelli 1975). In Colorado, 9000 km2 of alpine terrain, less than 4% of the state’s area, provide more than 20% of the state’s streamflow and is especially important in maintaining late-summer flows (Martinelli 1975). Lakes in the Rocky Mountains are relatively uncontaminated compared with many other high-elevation lakes in the world, with the median value of NO-3 concentrations less than 1 μeq L-1 (Psenner 1989). However, in comparison with downstream ecosystems, these high-elevation ecosystems are relatively sensitive to changes in the flux of energy, chemicals, and water because of extensive areas of exposed and unreactive bedrock, rapid hydrologic flushing rates during snowmelt, limited extent of vegetation and soils, and short growing seasons (Williams 1993). Hence, even small changes in atmospheric deposition have the potential to result in large changes in ecosystem dynamics and water quality (Williams et al. 1996a). Furthermore, these ecosystem changes may occur in alpine areas before they occur in downstream ecosystems (Williams et al. 1996b). Apart from its use in municipal supply, agriculture, recreation, and power generation, this water also mediates transfers of geomorphic and biological materials. For this reason, the drainage basin, or catchment, has long been recognized as a basic geomorphic unit in environmental research (e.g., Chorley 1967; Bormann and Likens 1969).

In the alpine catchment of Green Lakes Valley, the great variability in snow distribution is an important influence on environmental conditions. The snow cover has relatively little direct effect on geomorphic processes: avalanches of wet snow in Spring transport small volumes of debris to the valley floor and boulders are shifted locally in small sub-nival pavements. Indirectly, the influence of snow patterns on erosion is much greater. By changing the surface topography, it facilitates sediment transport across the tundra; through its control of hydrologic responses, its meltwater mediates sediment and solute movement; and by controlling the biota, it defines the source areas from which most sediments and solutes are derived.

Green Lakes Valley is a high (3500 m elevation), mid-continental drainage at relatively low latitude (40°N) which appears to have been influenced by a variety of environmental changes in the past h...

Snowmelt hydrographs for the 8 ha Martinelli Basin on Niwot Ridge, Colorado Front Range, show a well marked diurnal periodicity for much of the summer melt season. The discharges have been separated into ground-surface and subsurface components on the basis of the major ion concentrations and specific conductance of the streamwater. The results suggest a high (up to 50%) contribution by subsurface routing through the superficial mantle of the basin, even during the highest flows of the melt season. Both surface and subsurface components retain the diurnal pattern of the hydrograph, with the latter lagged by only about one hour. This suggests that even subsurface routing gives a quick hydrologic response from this basin.

The results of short-interval sampling for water quality over seven diurnal flow cycles from the Martinelli basin, site of a long-lasting snowpatch in the Colorado Front Range, are reported here. They show an inverse relationship between discharge and the concentration of most solute species which reflects varying proportions of melt water transferred to the channel through different routes. This suggests that variations in water quality over the diurnal cycle might be used to partition the flow hydrograph. It also indicates a source of bias in mass yield estimates that are based upon water samples taken with a daily (or longer) interval at the same point in the flow cycle. On the Martinelli site, this bias is less than 5% for most solutes but is much greater than that in the case of chloride, silicon and suspended sediment.

When estimated from measurements of introduced tracer particles, the rate of surface soil movement tends to be greater than the natural rate for equivalent particles on the same site. This consistent overestimation is greatest in the period following tracer introduction and leads to a measurement bias that may be as high as 300 per cent. The magnitude of the bias decreases with time, as the tracer is incorporated into the surface material, but remains detectable statistically for more than a year on some low-angle sites.

As part of a hazard mapping and evaluation study of the Kathmandu-Kakani area in the Middle Hills, Nepal, conducted as a United Nations/Unesco project, channel conditions at 43 stations on Thulo Khola and Ghatte Khola have been measured and peak discharges for the 1979 monsoon (a season of approximately normal precipitation) have been estimated. The results show that variations in the channels and the flows they carry in this tropical, high-relief area conform to the patterns established by other studies. This should allow hydrologic concepts and models that are derived from the long records and widespread studies of mid-latitude regions to be used in tropical mountains, where there are few direct observations. Flood hazards in the Kathmandu-Kakani area appear to be limited to those riparian zones which consist of constructed agricultural terraces. Damage to these terraces by channel expansion could provide a large influx of sediment to the fluvial system with consequent downstream siltation.

It is suggested that observations of contemporary sediment movement in subaerial situations be estimated by the physical work (in joules) that they represent. This can be conveniently done by estimating the change due to erosion in the potential energy of the landscape. An example from an alpine area in southwestern Colorado is used to illustrate the use of geomorphic work as a measure of erosion and to show how it may be compared to other environmental exchanges of material and energy.

Published records of the rainfall intensities and durations associated with shallow landsliding and debris flow activity suggests a limiting threshold for this type of slope instability. The limit ...

Precipitation acidity was monitored continuously from July to September 1983 at a well-instrumented research site located near the Continental Divide in central Colorado. Rainfall amount, temperature, pH, and specific conductance were determined using a prototype device consisting of a modified wet-only precipitation collector with appropriate sensors. Data were recorded automatically with microprocessor control by preset time intervals. Weekly wet-only composite precipitation samples were also collected. Precipitation totaled 190 mm during the period, all as rain. Sixty-five percent of the continuous pH measurements were between 3.40 and 4.10, with an average of pH 3.80. Values of pH greater than 5.00 were rare. Composite samples were significantly less acidic, and averaged pH 4.36. Precipitation pH typically decreased rapidly as a storm began, was lowest near or slightly after the time of maximum rainfall intensity, then increased slightly. Paradoxical low pH and low specific conductance were recorded, and it is possible that some initial continuous pH measurements were too low. Acid loading varied widely among storms, and may have resulted largely from short bursts of high-intensity, low-pH rainfall.