Water, Energy, and Biogeochemical Budgets (WEBB): Loch Vale Watershed

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Climate Change

Climate change studies in Loch Vale have documented increasing air temperatures, earlier snowmelt, and earlier runoff. These changes may have important impacts on water availability, water quality, and ecosystem function.

Water Resources

High-elevation and high-latitude watersheds in North America receive the majority of their annual precipitation as winter and spring snow [Serreze et al., 1999]. Most of this snow accumulates in seasonal snowpacks, which represent a natural storage reservoir whose size exceeds that of manmade reservoirs in many river basins of the world [Mote, 2006; Nijssen et al., 2001]. Arid regions, such as western United States, that receive relatively little precipitation during summer months are heavily dependent on natural and manmade storage to provide water for agriculture, industry, and drinking during the dry summer and fall seasons [Barnett et al., 2005].



Loch Vale typically receives 70-80% of annual precipitation in the form of snow; investigation of hydroclimatic processes that influence the timing of snowmelt is an important area of research. Recently, WEBB researchers used a new statistical method, the Regional Kendall Test (RKT), to document changes in the timing of snowmelt and streamflow runoff in Loch Vale and elsewhere in Colorado [Clow, submitted 2008]. Snowmelt and streamflow are occurring approximately two weeks earlier now than in the late 1970s, and the changes were positively correlated with increasing springtime air temperatures, and negatively correlated with maximum snow water content of the snowpack [Clow, submitted 2008]. Use of the RKT allowed identificaton of trends that had previously gone undetected due to large interannual variability and the relatively short period of record of the SNOTEL data sets [Stewart et al., 2005].



Analyses of temporal and spatial variations in the depth and water content of seasonal snowpacks was another area of extensive research at the Loch Vale WEBB site. Spatial variations in snowpack depth and water content were monitored starting in 1991 through the WEBB program, permitting development of snowpack distribution models using a combination of binary decision tree and geostatistical techniques [Balk and Elder, 2000; Cline, 1995]. Using data from Loch Vale and 65 additional sites, the methods have been applied to the Rocky Mountain region, and solute chemistry was added to the analysis to develop maps of nitrogen, sulfur, and acidity deposition in snow [Nanus et al., 2003]. Long-term, repeat sampling at these sites permitted an evaluation of temporal trends in solute deposition that accounted for climate variability [Ingersoll et al., 2008]. This allowed separation of trends attributable to climate versus those due to variations in nitrogen and sulfur emissions [Ingersoll et al., 2008].



Analysis of streamflow records for climate-induced trends in cold climates had hampered by the poor quality of discharge records during winter months, which is due to the presence of in-channel ice that causes variable backwater conditions and alters the stage-discharge relationship. WEBB research contributed to the recent development of an automated dye-dilution gaging system, which can provide high-quality, real-time discharge data at 8 hour intervals in ice-affected streams [Clow and Fleming, 2008].

Carbon Cycling

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Carbon cycling has been studied in Loch Vale since the 1980s. Research indicated that wetlands in Loch Vale are now releasing more carbon than they are storing, and respiration occurs even during winter through deep snowpacks.

Landscapes at northern latitudes and high elevations account for a significant percentage of the Earth's land surface. Soils in many of these landscapes have been identified as important sources and sinks of atmospheric CO2 that maybe particularly sensitive to climate change [Oechel et al., 1997] Wetlands are the largest natural source of CH4 to the atmosphere, and northern and high-elevation wetlands may account for a third of this natural source [Moosavi and Crill, 1996]. Understanding of carbon sources and sinks in these landscape types is needed to reduce uncertainties about the North American carbon budget and underlying processes controlling carbon dynamics.

As much as half of northern latitude and high-elevation landscapes are snow covered for most of the year; yet, most attention has been focused on soil gas emissions in these environments during the growing season. Mast et al. [1998] measured CO2 and CH4 fluxes through snowpacks in Loch Vale to investigate processes controlling the exchange of gas between the soil and atmosphere during winter. The snowpack insulated soils from cold midwinter air temperatures allowing microbial activity to continue through the winter. Subalpine soils were net sources of CO2 through the winter, whereas saturated soils were net emitters of CH4 and dry soils were net CH4 consumers. Winter accounted for 8 to 23% of annual soil CO2 flux and 12 to 58% of the CH4 flux. These results indicate that soil gas fluxes during winter are significant and should be included in annual carbon budgets for seasonally snow-covered terrains.

Wickland et al. [1999; 2001] expanded this research by developing annual budgets for gas exchange in a subalpine wetland system in Loch Vale. Annual respiration and CH4 emission were modeled by applying the flux-temperature relationships to a continuous soil temperature record. Gross photosynthesis was modeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland was a net source of carbon gas to the atmosphere during the 3-year study period. This contrasts with estimates from an age-dated peat core, which indicate the wetland has been a sink of carbon for the past 7,100 years. Wickland et al. [2001] suggested the switch from sink to source may indicate these wetland systems are sensitive to even minor variations in climate.

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Mineral Weathering

Mineral weathering is a natural process that occurs in soil and helps neutralize acid rain. Thin, patchy soils in Loch Vale make this area sensitive to acid deposition. Mineral weathering also consumes atmospheric carbon dioxide, and over geologic time scales, it is the principal mechanism for regulating climate.

Solute fluxes in streams, which are the product of concentration times discharge, are strongly affected by variations in climate. Research in Loch Vale, subsequently confirmed at many other headwater catchments in the United States, has documented that concentrations of weathering products exhibit relatively little variability in relation to changes in precipitation, discharge, or annual runoff [Clow, 1992; Clow and Drever, 1996; Godsey et al., submitted 2008]. As a result, fluxes of weathering products increase almost in direct proportion to annual variations in precipitation. This has important implications for carbon budgets because weathering of silicate minerals in pristine catchments is driven primarily by carbonic-acid dissolution reactions, which consume CO2. Exploring the mechanisms for this "chemostatic" behavior is an area of active research for the WEBB program [Clow and Drever, 1996; Godsey et al., submitted 2008; White and Blum, 1995a; White and Blum, 1995b; White et al., 1999]. A hydrograph separation study by Mast [1995] using water isotopes and silica documented that fast reactions in the soil must play an important role. Dilution of weathering products during wet periods with concomitant increases in weathering rates also may be important [Drever and Clow, 1995].

In contrast to fluxes of weathering products, nitrogen fluxes in Loch Vale are not strongly related to annual precipitation amount [Campbell et al., 2000]. Currently, nitrogen assimilation in alpine terrain is limited by cold temperatures; however, increasing trends in air temperatures and lengthening growing seasons may increase nitrogen assimilation rates.

Nitrogen Deposition

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Nitrogen emissions from automobiles, industry, and agriculture are transported to Loch Vale through the atmosphere. When deposited on vegetation, soil, and water, excess nitrogen can lead to increased nutrient concentrations in lakes and streams, and changes in the type and abundance of aquatic life and vegetation.

Mercury Deposition

Mercury is emitted to the atmosphere by a variety of natural processes and anthropogenic activities. Mercury deposition to alpine environments can be large, and is bioaccumulated in fish.

Groundwater in Alpine Terrain

Alpine/subalpine catchements like Loch Vale tend to have thin, patchy soils and little alluvial material. However, talus cones and permafrost may provide substantial underground storage capacity for water.

Permafrost

In a study of groundwater occurrence and contributions to streamflow in Loch Vale, it was determined that ice stored in permafrost (including rock glaciers) represented the second largest ground water reservoir in the basin [Clow et al., 2003b]. Rock glaciers were mapped and the depth to ice was measured using seismic refraction methods. The extent of potential permafrost was modeled based on remotely-sensed snow cover data and climatic data from the 3 weather stations in Loch Vale. Mean annual air temperatures (MATs) were sufficiently cold to support permafrost above 3460 m (50% of the watershed); however, MATs increased by 2.6°C between 1983 and 2007 [Clow, submitted 2008]. If other climatic factors remain constant, the increase in air temperatures at Loch Vale is sufficient to increase the lower elevational limit of permafrost by approximately 300 m [Clow et al., 2003b; Clow, submitted 2008]. Additional evidence for melting permafrost in Loch Vale is provided by unexpectedly long average transit times for water and sulfur, based on analyses of CFCs, sulfur-35 isotopes, and tritium [Clow et al., 2003a; Clow et al., 2005; Michel et al., 2000].



Earlier research in Loch Vale documented that glaciers contained water approximately equivalent to mean annual runoff for the basin [Ingersoll, 1995]. During drought conditions in 2002-2003, the runoff:precipitation ratio increased from approximately 0.7 to almost 1.0, suggesting that glacier and permafrost melt may have subsidized runoff during late summer [Baron et al., submitted 2008]. Melting glacial ice and permafrost has been implicated as a driving force for increasing nitrate concentrations in streams in Loch Vale [Baron et al., submitted 2008].