High elevation ecosystems are like water towers that store water as snow during the fall and winter and then release it as snow melt runoff in the spring and summer. The runoff provides a large quantity of high quality water, which largely drives the ecology and economy of the western US.
While atmospheric N flux decreased by 0.56 kg ha-1 yr-1 between 2000 and 2009 due to decreased precipitation, alpine nitrate yields increased by 40% relative to the previous decade (1990-1999) (Barnes et al. 2014). Long-term trends indicate that weathering 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 meltwater, suggests it is the source of these weathering products. Furthermore, mass balance models indicate that the high ammonium loads within glacial meltwater are rapidly nitrified, contributing approximately 0.5 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 N solute trends in a neighboring catchment that lacks permafrost and glacial features. These findings suggest that reductions of atmospheric N 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 atmospheric deposition trends.
Correctly accounting for dissimilar hydrological cycling above and below alpine treeline is critical to quantify the water balance of high-elevation mountain catchments over periods of meteorological variability. Measurements made between 2008 and 2012 in the alpine tundra and subalpine forest in the Como Creek catchment, located on and below the southeast flank of Niwot Ridge, indicate that alpine areas generate the majority of the catchment water discharge, despite covering a smaller proportion (31%) of the catchment area than the forest (Fig. 1; Knowles et al. 2015). Alpine and catchment water yields appear to be relatively unaffected by annual meteorological variability, but subalpine runoff efficiency is more sensitive to changes in precipitation. Inter-annual analysis of the evaporative and dryness indices show persistent moisture limitations at the catchment scale, with the alpine alternating between energy-limited and water-limited states in wet and dry years.
This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1637686. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.
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