Energy balance analysis of the temperature dynamics of a steep proglacial stream

Abstract

The application of physically based models to assess the e↵ect of glacier retreat is complicated by the heterogeneity of proglacial stream morphology, the diurnal fluctuations of stream flow, and the e↵ects of aeration on turbulent exchange and stream albedo. Current energy balance parameter values derived in shallow streams provide unreliable predictions by incorporating incorrect representations of the underlying physical processes. In proglacial streams, diurnal flow produces transient channel beds from daily drying and re-wetting, which can e↵ect transient heat storage from pre-exposed boulders and hyporheic exchange. However, no available model predicts the dependence of stream aeration and complex hydraulic geometry on both turbulent exchange and residual heating inputs of steep streams. This study quantified the dependence of steep stream morphology and turbulence on the wind function and residual heating parameters in order to improve stream temperature predictions in proglacial alpine and headwater streams. The study focused on a 1-km-long reach of South Creek, a steep glacier-fed stream in the Bridge Glacier valley of British Columbia. Energy balance input variables were acquired from measured and calculated field data using above-stream and above-land weather stations, salt dilution gauging, UAV photogrammetry (SfM), and water temperature monitoring from the observed two-month-long melt season. Following model optimization, derived wind function coe cient ranges were an order of magnitude greater than the literature values for low gradient streams (c = 20-975 W·m ·kPa , d = 25-200 W·s·m ·kPa ). An added stability term to the wind function further improved results. Nighttime optimization yielded smaller wind function coe cient values and greater residual warming to the stream, revealing a diurnal thermal regime shift. Adjustments to albedo and net radiation improved the predictive power relative to models with the standard literature inputs. The results from this study contribute to our understanding of steep stream heat dynamics from a morphodynamic standpoint, which is of increasing importance for headwater streams in the context of continued glacial retreat.