Glacier fragmentation effects on surface energy balance and runoff: field measurements and distributed modelling Hester Jiskoot* and Mark S. Mueller Department of Geography, University of Lethbridge, Lethbridge, Alberta, T1K 3 M4, Canada Abstract: In order to assess glacier runoff to the Upper Columbia River Basin (UCRB) and quantify energy balance effects of tributary-trunk detachment due to recession, we used field observations to develop a distributed melt model of Shackleton Glacier, Canadian Rockies. Field data were derived from meteorological stations, ablation and snowline measurements, and weather observations between 2004 and 2010. Katabatic wind speed and direction were linked to terrain heat advection and irradiance, potentially resulting in significant cross-glacier gradients in melt. A geographic information system-based distributed melt model, using standard energy balance components, was developed for the 2010 melt season. Benchmark model parameterisations were derived for clear, cloudy and overcast days. Novel model parameterisations include terrain irradiance using a sky view factor and an albedo mask, and a katabatic wind ‘switch’ with valley temperature thresholds. Modelled energy balance components suggest significant sensitivities to terrain irradiance and katabatic wind, in part related to cloudiness. Glacier-wide melt decreased by 10–15% when katabatic wind was turned off, with an interesting spatial pattern. Longwave radiation from valley walls increased local melt up to 30%, but net glacier-wide effects were <6%. Daily glacier melt was 0.1–0.8 million m 3 w.e. day 1 and peaked in early August. Net 2010 planar-area melt was 38–50 million m 3 w.e., depending on cold storage, whereas slope-corrected-area melt was ~4% higher. Our results indicate that katabatic wind and terrain are important in calculations of ablation in fragmenting glacier systems and that late-summer glacier contribution to UCRB runoff at Mica Dam is ~25%. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS distributed melt model; surface energy balance; katabatic wind; valley glacier; Canadian Rockies; runoff Received 20 August 2011; Accepted 21 February 2012 INTRODUCTION Runoff from the western slopes of the Canadian Rockies into the Upper Columbia River Basin (UCRB) is strongly dominated by winter snow accumulation and spring melt (Hamlet and Lettenmaier, 1999) and provides important water resources to both the Canadian and US reaches of the basin. Observed and predicted reductions in snowpack and earlier onsets of springtime snowmelt and streamflow create increased competition for water between spring and early fall (Mote et al., 2005; Stewart et al., 2005). With a glacier cover of ~3% in the entire UCRB and up to 8% in sub-basins along the western slopes of the Canadian Rockies (Schnorbus et al., 2011), glacier runoff in the basin can substantially affect summer flows. Glaciers generally regulate streamflow by behaving as large water reservoirs that delay most of their runoff to the late summer months when other streamflow sources (snow melt and precipitation) are at their minimum. In the UCRB, regional glacier area losses have been in the order of 14–28% since the 1980s (Jiskoot et al., 2009; Bolch et al., 2010). Continuing glacier retreat and changing runoff could ultimately reduce water availability. Although hydrological models using simplistic approaches (Schnorbus et al., 2011) and a temperature-index- based melt algorithm for glaciers GRUs (Jost et al., 2011) have recently been used to model glacier contribution to hydrologic response in the basin, neither mass balance measurements nor energy balance models have yet been applied. In contrast, glacier contribution to runoff from the adjoining eastern slopes of the Canadian Rockies is fairly well established and based on long-term energy and mass balance measurements (Marshall et al., 2011). The absence of glacier melt and runoff estimates in the UCRB severely constrains our ability to calibrate hydrologic models used to predict future streamflow levels and runoff timing (Moore et al., 2009). Worldwide, ongoing glacier recession has already resulted and will continue to result in glacier fragmentation, where tributaries detach from their trunks and contiguous glacier units separate into smaller units. This process of glacier fragmentation has in some cases, especially in larger glaciers, led to a faster retreat as compared with nearby glaciers that have not (yet) fragmented (Paul et al., 2004; Jiskoot et al., 2009). It has been suggested that this enhanced recession is due to positive feedback related to the changing radiation regime (i.e. through tributary-trunk detachment, emergence of rock outcrops, etc.), possibly in combination with changes in flow dynamics through tributary detachment (Paul et al., 2004; Kargel et al., 2005). Although net solar radiation is the major contributor to the energy balance of glaciers, the sensitivity to changing *Correspondence to: Hester Jiskoot, Department of Geography, University of Lethbridge, Lethbridge, Alberta, T1K 3 M4, Canada. E-mail: hester.jiskoot@uleth.ca Contract/grant sponsor: Natural Sciences and Engineering Research Council of Canada (NSERC). HYDROLOGICAL PROCESSES Hydrol. Process. 26, 1861–1875 (2012) Published online 22 April 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9288 Copyright © 2012 John Wiley & Sons, Ltd.