Knowles, John F 1 ; Blanken, Peter D 2 ; Williams, Mark W 3 ; Chowanski, Kurt M 4
1 University of Colorado, Department of Geography and Institute of Arctic and Alpine Research
2 University of Colorado, Department of Geography
3 University of Colorado, Department of Geography and Institute of Arctic and Alpine Research
4 South Dakota School of Mines and Technology, Institute of Atmospheric Sciences
This study highlights the importance of landscape position and resultant snow accumulation to the hydrologic balance of snow-free alpine tundra, and suggests that modeling studies must account for seasonally dissimilar partitioning of the energy balance in order to accurately predict evaporation and/or sublimation. The eddy covariance method was used to measure the surface energy balance above high-elevation (3502 m above sea level) alpine tundra at Niwot Ridge, CO, over 3 years from 2007 to 2009. During the winter the site was characterized by wind scour, with little snow accumulation (Fig. 1). Two co-located towers afforded the opportunity to constrain the influence of complex mountain topography on measurement uncertainty, and overall errors were comparable to other FLUXNET sites. Random measurement uncertainty for the turbulent fluxes was approximately 10% of midday summertime values. The 0.5-h mean energy balance closure was 81% over the entire measurement period, and improved to 91% during the summer when the magnitude of the turbulent fluxes was larger. In spite of 955 mm mean annual precipitation, the 24-h mean evaporative fraction was 0.39, typical of dry grassland or rangeland ecosystems. These low values were attributed to rapid, efficient removal of snow by prevailing windy conditions throughout the winter. During the summer when rainfall provided moisture, evaporation was principally limited by available energy. Overall, an average of 39% of annual precipitation was evaporated or sublimated back to the atmosphere. We conclude that the annual distribution of precipitation is an essential control on evaporation and sublimation from this ecosystem (Fig. 2).