Lakes and Watersheds in the Sierra Nevada (California): Responses to Environmental Change

Abstract:

California’s Sierra Nevada harbors thousands of remote, high- elevations lakes and watersheds. We have synthesized over three decades of research on these systems in a book to be published by UC Press (December 2020). Measurements of precipitation and outflow discharge and estimates of evaporation allow calculation of annual water budgets for the Emerald Lake watershed for three decades and seven other watersheds for several years. Most precipitation falls as snow with 10-fold interannual variability in snow water equivalent (SWE). The majority of outflow occurs via streams during snowmelt which usually extends over several months and varies depending on SWE. Atmospheric deposition of chemical species occurs via snow, rain and dry deposition of particles and gases; ion concentrations are much greater in summer rains than in winter snow. Outflow solute concentrations are strongly influenced by melting of the seasonal snowpack; nitrate pulses are common during snowmelt. The Emerald Lake watershed consumes over 90% of the atmospheric loading of hydrogen ion and ammonium, and about half of nitrate deposition; the catchment generates 2 to 5 times more cations than are deposited via atmospheric deposition. In Emerald Lake, episodic depletion of acid neutralizing capacity occurs mainly through dilution by snowmelt rather than by addition of strong acids. Long term environmental change in the Sierra Nevada is linked to variability and trends in snowfall and solute deposition. Despite high rates of warming of air temperatures in the Emerald watershed, no long-term warming trend in lake temperature was evident because interannual variability in SWE drives the majority of variation in lake temperature. During years with large SWE, lakes have higher peak nitrate concentrations and lower acid neutralizing capacity.