Changes in climate and hydrochemical responses in a high‐elevation catchment in the Rocky Mountains, USA

Abstract

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.Many alpine regions are susceptible to environmental damage that will affect both their ecological health and the regional economies. Small changes in the flux of energy, chemicals, and water to high-elevation ecosystems may result in large changes in the climate, ecosystem dynamics, and water quality of these catchments (e.g. Baron 1992). For example, field and laboratory experiments have demonstrated that initial stages of snowmelt often have ionic concentrations many times higher than averages for the whole snowpack-an ionic pulse (e.g. Johannessen and Henriksen 1978; Colbeck 198 1). The magnitude of the ionic pulse may be increased or decreased by small changes in energy flux (Williams and Melack 199 1 b). In turn, the release of solutes from the seasonal snowpack may have a direct and large effect on the solute content of stream waters .

Climate change in the Colorado Rocky Mountains: free air versus surface temperature trends

“…Explanations refer to the fact that high-elevation stations are more directly in contact with the free troposphere and to the importance of snow-albedo positive feedback (common at high elevations). In contrast, other analyses of surface observations report an increase in surface lapse rates, especially on a local basis (Williams et al, 1996;Garnett et al, 1997;Pepin, 2000). For example, analyses in Colorado (Losleben, 1983Greenland, 1987Greenland, , 1989Brown et al, 1992;Pepin, 2000) have illustrated contrasting temperature trends at differing elevations in the Front Range, USA, with increased surface lapse Mountain Climate Program).…”

“…Our results fit alongside the global warming theory of Barry (1990), in which increases in precipitation and cloudiness, and hence cooling in high elevation regions, result from steeper lapse rates at the surface. In Barry's hypothesis, taken up by Williams et al (1996), increased atmospheric instability is thought to enhance convective processes, perhaps selectively over mountainous regions. However, the weaker free-air lapse rates we also observe imply enhanced stability and reduced convection.…”

“…Unfortunately, there are no high quality long-term (>40 years) observations of cloud cover over Niwot Ridge, although the recent installation of a new web camera on Niwot Ridge offers opportunities for archiving cloud photographs for further analysis (http://tundracam.colorado.edu/). Proxy data, including precipitation Williams et al, 1996Williams et al, , 1998, suggest a trend towards a moister regime since 1952, but there are difficulties with the measurement of precipitation during much of the year due to high snowfall and strong winds. Another potential resource is outward longwave radiation and specific humidity measurements from satellites, available from NOAA/CDC at the web site www.cdc.noaa.gov.…”

Potential impacts on Colorado Rocky Mountain weather due to land use changes on the adjacent Great Plains

“…Decreasing northeastern Colorado summer temperatures have been reported in the past [Greenland et al, 1995;Stohlgren et al, 1998]. Also decreased solar radiation was reported at some high-elevation sites [Williams et al, 1996]. Recent studies of plant successional trends and watershed discharge in the northeastern Colorado mountains also indicated a middle to high elevation cooling over the last 50-100 years [Stohlgren et al, 1998].…”

The Disappearing Cryosphere: Impacts and Ecosystem Responses to Rapid Cryosphere Loss

“…In the Northern Hemisphere, strong negative trends in the extent of snow cover have been observed over recent decades (Déry and Brown 2007). Increased snowfall and snow depth have been reported at the highest-elevation sites of the western United States (Williams et al 1996); however, most locations in the Mountain West have experienced snowpack declines, and concern has risen about streamflow, water yields, and water supply. In the Pacific Northwest, extensive snow-covered regions are now deemed at risk in terms of their capacity to provide reliable water yields because of atmospheric warming, altitudinal shifts in the distribution of snow and rain, and declining winter snowpacks (Nolin and Daly 2006).…”