Water Resources of Colorado
Water, Energy, and Biogeochemical Budgets (WEBB)
The hydrology and biogeochemistry of alpine watersheds has been shown to be more similar to forested watersheds than was previously believed. Although snowmelt dominates the hydrograph, there is a substantial reservoir of ground water, and significant contact with soil and soil-like material such as talus that provides a substrate for biogeochemical transformations (Bachmann, 1994; Campbell and others, 1995b; Ingersoll, 1995a; Mast and others, 1995).
The talus environment, which accounts for greater than 50 percent of the area in Loch Vale and is common throughout the Rockies, appears to be surprisingly important in terms of solute fluxes and ground-water storage. Lack of soil development and short residence times would suggest that biogeochemical reactions in the talus zone should be minimal; however, this does not appear to be the case. High concentrations of solutes, including nitrate, base cations, and alkalinity, in many talus waters indicate that biogeochemical reactions there are important. Moreover, there appears to be a close hydrologic connection between the talus environment and surface waters, so fluxes of solutes from talus to streams may be substantial. The importance of talus is supported by isotopic studies and recent hydrologic modeling efforts. Hydrograph separations based on oxygen isotopes and dissolved silica indicate that streamflow is dominated by snowmelt runoff that is routed through shallow subsurface reservoirs before discharging to the stream (Mast and others, 1995). Similarly, application of the Alpine Hydrochemical Model (AHM) to the Andrews Creek drainage, a subbasin in Loch Vale, indicated that talus is the dominant hydrologic unit controlling stream chemistry (Tom Meixner, University of Arizona, written comm., 1999).
Atmospheric deposition of nitrogen to Loch Vale is high compared to most other sites in the Rockies, although moderately high compared to sites in eastern North America and Europe. The alpine/subalpine ecosystem exhibits symptoms of advanced watershed nitrogen saturation, indicating extreme sensitivity to nitrogen deposition. Talus landscapes contribute substantially to nitrogen export in streamflow, and soil microbial processes are important in cycling nitrogen, even in areas such as talus that have little soil development. Nitrogen export is a function of both deposition and internal nitrogen-cycling processes that are affected by variability in climate, therefore, response of the ecosystem to changes in nitrogen deposition are not likely to be direct and immediate. Ecosystem response to nitrogen deposition is a topic of great interest to federal resource managers, and funding from NPS, USDA-FS, and NSF supplements the WEBB program. Isotopic techniques developed to identify sources of nitrate at the WEBB site are currently being tested for application to large river basins (Mississippi River/Gulf of Mexico Hypoxia Study). (Campbell and others, 1995a; Kendall and others, 1995a; Baron and Campbell, 1997; and three additional journal articles related to nitrogen cycling are in review.)
Stable and radioactive sulfur isotopes have been used to characterize sources and residence times of sulfur in the Loch Vale watershed. Use of sulfur isotopes in Loch Vale has required development of field methods appropriate for dilute lakes in remote locations. By working out problems in Loch Vale, it has been possible to use both techniques widely throughout the Rocky Mountains and Sierra Nevada. Application of these methods in Loch Vale has resulted in major changes in our understanding of solute budgets there. Stable sulfur isotopes indicated immediately that there is a watershed source of sulfate, whereas all previous research assumed there was no source other than atmospheric deposition (Turk and others, 1993a). This also allowed resolution of inconsistencies among other solute and hydrologic budgets. Radioactive sulfur isotopes indicate a very strong seasonality of sulfate sources in larger streams, with snowmelt dominating during early summer and older ground water becoming more important in late summer (Michel and Turk, 1995; Sueker and others, 1999). Samples from springs indicate their discharge is meteoric water of only a few months age.
Mineral weathering is the dominant source of alkalinity, silica, and base cations in surface water and ground water in Loch Vale. Fluxes from each of the primary mineral weathering reactions have been quantified using a combination of traditional mass-balance calculation methods and relatively new strontium isotope mixing-calculation techniques (Clow and others, 1997). Weathering of the dominant silicate minerals cannot account for cation ratios observed in surface water and ground water in Loch Vale (Mast, 1989; Mast and others, 1990; Clow, 1992; Mast, 1992). Mass-balance calculations indicate that weathering of plagioclase, the main silicate mineral source of Ca and Na, can account for approximately one quarter of the calcium flux in surface waters. Calcite weathering is the only other plausible mineral weathering source of calcium; however, traditional geochemical and mass-balance methods cannot be used to determine the relative importance of the two main types of calcite in Loch Vale, which are eolian calcite and microcrystalline calcite present in trace quantities in the bedrock. Mixing calculations using strontium isotopes enabled us to determine that approximately one-half of the annual flux of calcium in surface water was derived from weathering of bedrock calcite, and the remaining one-quarter of the calcium came from eolian calcite. These results provide the first quantitative evaluation of calcium sources to surface waters in the Rockies and confirm the importance of bedrock mineral weathering in controlling fluxes of calcium and, by implication, alkalinity. A pilot project has been funded by the University of Colorado to evaluate utility of the methodology developed in this study to characterize sources of calcium and alkalinity to other surface-water bodies in the Rocky Mountains.
Methane fluxes at Andrews wetland in the Loch Vale Watershed show strong seasonality with maximum emission to the atmosphere occurring shortly after snow melt in July. Flux to the atmosphere is hysteretic with wetland temperature, having flux rates that remain stable or tail off as temperature continues to rise into August. Methane flux through snowpack accounted for greater than 50 percent of annual flux in dry areas, where atmospheric CH4 is continually consumed, but was less than 20 percent of annual flux in inundated areas, where CH4 is continually emitted (Mast and others, 1998). Seasonal and spatial heterogeneity of flux along a moisture gradient at the wetland are summarized in Wickland and others (1999).
CO2, CH4, and productivity measurements indicate that the wetland had a net loss of carbon to the atmosphere during 1996 and 1997, despite net accumulation of Carex peat at the site over the last 7,500 years. Losses were much greater in 1996 than in 1997. This pattern appears to be consistent with recent observations at other ecosystems having seasonal snowpacks, where C accumulation rates have been small or negative in recent years.
Continued research at the wetland is focused on short- and long-term carbon accumulation and decomposition, processes that control the hysteresis of methane flux at the site, and on processes that control the variability of carbon flux throughout the year.
This page is at URL: http://co.water.usgs.gov/lochvale/results.html
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Last Modified: 01/29/2002