Knowles, John F 1 ; Cowie , Rory 2 ; Zeliff, Morgan 3 ; Harpold, Adrian A 4 ;Barnard, Holly P 5 ; Burns, Sean P 6 ; Morse, Jennifer F 7 ; Williams, Mark W 8
1 University of Colorado
2 University of Colorado
3 University of Colorado
4 University of Colorado
5 University of Colorado
6 University of Colorado and National Center for Atmospheric Research
7 University of Colorado
8 University of Colorado
Climate change is affecting the hydrology of high-elevation mountain ecosystems, with implications for ecosystem functioning and water availability to downstream consumers. To quantify the water balance of a mixed alpine tundra/subalpine forest headwater catchment in Colorado, we directly and continuously measured precipitation (P), discharge (Q) and evapotranspiration (ET) fluxes from both ecosystems, and then integrated the measurements by water year from 2008 to 2012. Overall, water balance closure averaged 90.7%. Mean annual catchment P was 929 mm, and the relationship between P and elevation differed by season: winter P increased an average of 74 mm per 100 m elevation gain, while summer P was evenly distributed. In the subalpine forest, peak snow water equivalent (SWE) varied by 153 mm and 78 days between the wettest and driest winters. Across the catchment, the majority of P was dispelled as ET, and ET peaked during the summer and winter over forested and alpine areas, respectively. This discrepancy resulted in relatively constant intra- and inter-annual ET fluxes. In all, the ET accounted for 66% of P. Budyko analysis indicated that the subalpine ET was generally moisture limited, whereas energy inputs mainly governed the alpine and catchment scale ET. The Q efficiency was higher in alpine relative to subalpine areas, and mean annual Q efficiency was 23% for the catchment as a whole. Between years, peak Q differed by an order of magnitude, while total Q varied by a factor of only 2.5. The peak Q coincided with the snowmelt pulse during all years excluding 2012, when Q peaked in response to summer rainfall. Although both alpine and subalpine ecosystems under-generated Q relative to the Budyko relationship, the catchment did not. These results provide insight as to how the water balance of high-elevation, mountain catchments that span the alpine treeline may change as a result of climate change.