Glacier Meltwater Contributions and Glaciometeorological Regime of the Illecillewaet River Basin, British Columbia, Canada

J.M.R. Hirose* and S.J. Marshall

Department of Geography, University of Calgary, Calgary, Alberta, Canada

[Original manuscript received 3 March 2012; accepted 22 February 2013]

ABSTRACT This study characterizes the meteorological parameters influencing glacier runoff and quantifies recent glacier contributions to streamflow in the Illecillewaet River basin, British Columbia. The Illecillewaet is a glacierized catchment that feeds the Columbia River, with terrain, glacial cover, and topographic relief that are typical of Columbia River headwaters basins in southwestern Canada. Meteorological and mass balance data collected on Illecillewaet Glacier are used to develop and constrain a distributed model for glacier melt, based on temperature and absorbed solar radiation. The melt model is applied to all of the glaciers in the Illecillewaet River basin for the summers of 2009 to 2011. Modelled glacier runoff for the three years has an average value of 112 ± 12 × 106 m3, approximately 10% of Illecillewaet River yields for 2009 to 2011. Glaciers contributed 25% to August flows for the three years. On average, 66% of modelled glacial discharge is derived from the seasonal snowpack, with the remaining 34% resulting from the melting of glacier ice and firn. For the lowest flow year in the basin, 2009, snow and ice melt from glaciers in the basin contributed 14% and 33%, respectively; 81% of the August glacier runoff is derived from glacier storage (ice and firn). Climate sensitivity studies for Illecillewaet Glacier indicate that the glacier mass balance is strongly influenced by summer temperature, with a net balance change of −0.6 metres of water equivalent (m w.e.) under a 1°C warming. A 30% increase in winter precipitation is needed to offset this. Our values are initial estimates, and long-term monitoring is essential to characterize glacier and climate variability in the region better, to refine estimates of glacier runoff, and to quantify the sensitivity of runoff to glacier retreat.

RÉSUMÉ [Traduit par la rédaction] La présente étude caractérise les paramètres météorologiques qui influencent le ruissellement des glaciers et quantifie les contributions récentes des glaciers à l’écoulement fluvial dans le bassin de la rivière Illecillewaet, en Colombie-Britannique. L’Illecillewaet est un bassin hydrographique englacé qui alimente le fleuve Columbia, avec un terrain, une couverture de glace et des éléments topographiques caractéristiques des bassins du cours supérieur du fleuve Columbia dans le sud-ouest du Canada. Nous utilisons les données météorologiques et de bilan massique recueillies sur le glacier Illecillewaet pour mettre au point et contraindre un modèle distribué de fonte des glaciers, basé sur la température et le rayonnement solaire absorbé. Le modèle de fonte est appliqué à tous les glaciers situés dans le bassin de la rivière Illecillewaet pour les étés de 2009 à 2011. Le ruissellement modélisé des glaciers pour les trois années a une valeur moyenne de 112 ± 12 × 106 m3, approximativement 10% de l’écoulement de la rivière Illecillewaet pour 2009 à 2011. Les glaciers ont Downloaded by [University of Victoria] at 10:12 07 May 2013 contribué pour 25% de l’écoulement en août les trois années. En moyenne, 66% du ruissellement modélisé des glaciers provient de l’accumulation saisonnière de neige, les 34% restant provenant de la fonte de glace de glacier et de névé. Pour l’année du plus faible écoulement fluvial dans le bassin, 2009, la fonte de neige et de glace de glacier dans le bassin a contribué pour 14% et 33%, respectivement; 81% du ruissellement des glaciers en août est dérivé du stockage des glaciers (glace et névé). Des études de sensibilité climatique pour le glacier Illecillewaet indiquent que le bilan massique du glacier est fortement influencé par la température en été, avec une variation nette de −0,6 mètre d’équivalent en eau dans le bilan pour un réchauffement de 1°C. Un accroissement de 30% dans les précipitations hivernales est nécessaire pour annuler cet effet. Nos valeurs sont des estimations préliminaires, et une surveillance à long terme est essentielle pour mieux caractériser la variabilité des glaciers et du climat dans cette région, pour raffiner les estimations de ruissellement des glaciers et pour quantifier la sensibilité du ruissellement au retrait des glaciers.

KEYWORDS glacier; mass balance; glacier melt; glacier runoff; headwaters; Illecillewaet; Columbia River basin

*Corresponding author’s email: [email protected]

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 2 / J. M. R. Hirose and S. J. Marshall

1 Introduction observations and glacier mass balance models. Separation is The Columbia River basin (CRB) is the sixth largest river basin important because runoff from the seasonal snowpack is in North America, with a total basin area of 671,300 km2. intrinsically renewable and can be expected to persist, albeit About 15% of the basin lies within Canada, including numer- reduced, as glaciers recede from the landscape. Meltwater ous high-elevation catchments. Approximately 1760 km2 from glacier ice and firn, on the other hand, taps into a reser- (1.7%) of the Canadian CRB (CCRB) is glacierized, and its voir that is diminishing with time. In this paper we define upper headwaters receive high annual precipitation (e.g., glacier runoff to include all the meltwater that issues from gla- Cohen, Miller, Hamlet, & Avis, 2000; Hamlet, Mote, Clark, ciers, including snow, firn, and ice, but we report the runoff & Lettenmaier, 2005; Kite, 1997), with much of this accumu- from seasonal snow and that drawn from stored firn and ice lating in the mountain snowpack. An estimated 30–40% of separately. annual discharge in the Columbia River, as measured at It is unclear whether glacier contributions to streamflow The Dalles, Oregon, is derived from the Canadian portion of measured or modelled in other parts of western Canada are the basin (Cohen et al., 2000; FCRPS, 2001; Hamlet & representative of the CCRB. The eastern slopes of the Lettenmaier, 1999). Glacier contributions to annual discharge Rocky Mountains are in a more continental climate than are are unknown in the CRB. the Columbia Mountains, with relatively sparse glacier Glaciers are natural freshwater reservoirs on seasonal to cover. Rango, Martinec, and Roberts (2008) examine glacier centennial time scales. The retreat and thinning of glaciers contributions to runoff in the Illecillewaet River basin, but affects both water availability and water temperature in gla- with limited observational constraints and with a simplified cierized catchments (Moore, 2006). Much of the long-term treatment of glacier mass balance and melt processes. Jost storage (glacier ice) gets depleted in late summer and early et al. (2012) and Bürger, Schulla, and Werner (2011) apply fall (Fountain & Tangborn, 1985), and ice-melt runoff helps hydrological models to nearby basins of the CCRB and to maintain streamflows after the seasonal snow has melted provide preliminary assessments of the importance and (e.g., Déry et al., 2009; Huss, Farinotti, Bauder, & Funk, impact of glacier runoff, but these models treat the glaciers 2008; Moore & Demuth, 2001). Glacier melt also has a simply and also lack direct observations to constrain mass cooling effect on streams because of the colder meltwater balance gradients and high-elevation meteorological and higher flows; this reduces the sensitivity to energy conditions in the region. inputs and maintains habitat for cold-water species in down- Here we present observational and modelling results of stream rivers (Moore, 2006). There has been limited glaciolo- glacier meteorological conditions, mass balance, and runoff gical research in the CCRB, but modelling efforts in the Mica in the relatively undisturbed Illecillewaet River sub-basin sub-basin indicate that meltwater from glacier ice contributes (IRB), a high-elevation headwater catchment of the CCRB. up to 25–35% of streamflow in August and September (Jost, Our study has several objectives: i) to report direct mass Moore, Menounos, & Wheate, 2012). This represents a vital balance and meteorological observations from field studies freshwater resource to the Columbia River system, supporting at Illecillewaet Glacier; ii) to constrain mass balance gradients municipalities, industry, hydroelectricity generation, irriga- and high-elevation meteorological conditions needed for gla- tion, and ecosystems. ciological and hydrological models; iii) to develop and vali- Glacier contributions to streamflow have been assessed in date a distributed melt model for Illecillewaet Glacier mass the southern Coast Mountains (e.g., Moore, 1993; Moore & balance and meltwater runoff; iv) to explore the uncertainties Demuth, 2001) and on the eastern slopes of the Canadian and climate sensitivities of the model; and v) to estimate and

Downloaded by [University of Victoria] at 10:12 07 May 2013 Rocky Mountains (Comeau, Pietroniro, & Demuth, 2009; partition meltwater runoff contributions to Illecillewaet Demuth et al., 2008; Hopkinson & Young, 1998; Marshall River from glacier snow and ice. This preliminary study pro- et al., 2011). Hopkinson and Young (1998) conclude that vides a snapshot of recent glacier-melt contributions in a head- ice-melt from glaciers contributed 1.8% of the average waters catchment of the CCRB. annual discharge in the Bow River in Banff from 1951 to 1993, but as much as 15% of annual runoff and 50% of August flow in 1970, an extremely negative mass balance 2 Study area year. Comeau et al. (2009) apply a hydrological model to The climate regime of the CCRB is transitional between that of the glacierized headwater basins of the South and North Sas- the maritime environment of the Coast Mountains and the katchewan River systems and find that more than 60% of cool, dry conditions of the eastern slopes of the Canadian July to September streamflow is composed of glacial runoff Rocky Mountains, two extensively glacierized regions in in headwater catchments with over 10% glacier cover, but western Canada where mass balance records began in the this study does not distinguish between meltwater runoff mid-1960s (e.g., Demuth et al., 2008; Moore & Demuth, derived from seasonal snow and glacier ice. This is difficult 2001; Moore et al., 2009). Glaciological information in the to separate in hydrographs (or isotopically), because glacier CCRB is too scarce to know whether mass balance in the runoff contains a mixture of these two sources, derived simul- upper catchments of the CCRB is correlated with either taneously from a range of elevations on a glacier. However, it region. Mass balance records from Peyto Glacier in the is possible to separate the contributions using field Rocky Mountains have been taken as a proxy in CCRB

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 La Société canadienne de météorologie et d’océanographie Glacier Meltwater Contributions to Streamflow in the Illecillewaet River Basin / 3

studies (e.g., Bürger et al., 2011). However, glaciers in the Glacier hypsometry in a basin provides baseline data for CCRB may be more sensitive to interannual precipitation regional estimates of glacier mass balance and meteorologi- variability and North Pacific weather patterns than glaciers cal sensitivity (Marshall et al., 2011). A 20 m DEM was on the eastern slopes of the Rocky Mountains (e.g., Bitz & obtained from Parks Canada to assess glacier hypsometry Battisti, 1999). in the IRB. The DEM was derived from aerial photographs, The IRB, in the CCRB, extends from Rogers Pass to Revel- acquired in approximately 1997 from Terrain Resources stoke and is located in the Columbia Mountains where the Information Management (TRIM). The elevation range of snow climate is considered to be “transitional with a strong the Illecillewaet Glacier is 1993 to 2911 m, while the maritime influence” (Hägeli & McClung, 2003). The IRB elevation of the IRB glaciers ranges from 1592 to 2942 m covers 1150 km2 with an elevation range of 443 to 3284 m. (Fig. 2 and Table 2). The elevations of Illecillewaet Glacier Precipitation totals and orographic precipitation gradients are are reasonably representative of IRB glaciers because there high. For the 30-year climate normal from 1971 to 2000, is limited glacierized area below 2000 m, the lowest mean annual precipitation in Revelstoke (450 m) averaged reaches of Illecillewaet Glacier (Fig. 2). The median glacier 618 mm, compared with 1547 mm at Rogers Pass (1340 m), elevation for the IRB is 2385 m, 136 m below that of with 933 mm falling as snow at Rogers Pass (Environment Illecillewaet Glacier. Canada, 2012b). There are no long-term precipitation Given its close proximity to Rogers Pass, the Canadian measurements on the glaciers of the IRB, but snowfall and Pacific Railway line, and the TransCanada Highway passing precipitation totals can be expected to exceed those at through the area, Illecillewaet Glacier has been extensively Rogers Pass. photographed and visited. It was the site of several early twen- The IRB is a headwaters catchment of the Columbia River tieth century investigations of glacier motion and glacier and is undisturbed by regulated flows. Other anthropogenic retreat, and historical changes in Illecillewaet Glacier terminus disturbances are also minimal, because most of the basin lies position and area have been documented (Champoux & within the boundaries of Mt. Revelstoke and Glacier National Ommanney, 1986; Sidjak & Wheate, 1999). Loukas, Vasi- Parks. This makes it an excellent study basin to compare glacier liades, & Dalezios (2002) project future glacier-area evolution runoff with seasonal flows at Illecillewaet River’s Hydrometric in the IRB under different climate regimes, estimating a Station in Greeley, 08ND013 (51°0′49′′N, 118°4′57′′W; Fig. 1). glacier cover of 51 km2 by 2080–2100, 5 km2 smaller than Glacier meltwater runs off into Illecillewaet River from 79 gla- its current area. ciers in the IRB (Table 1). Hydrometric data from Illecillewaet Glaciological projections and hydrological modelling in the River have been gauged since 1963 and are available from basin do not benefit from regional glaciological data, however. HYDAT, the database of the Water Survey of Canada. Illecille- No mass balance measurements have been reported from Ille- waet River joins the Columbia River near Revelstoke, just cillewaet Glacier. The federal government carried out mass downstream of Greeley. balance studies on nearby Woolsey Glacier from 1966 to 1975, but these measurements were abandoned. Hence, there is little knowledge of glacier mass balance, high-elevation snowpack, and meteorological gradients in the IRB, or in a Glacier Area and Hypsometry other headwaters catchments of the CCRB. Through the Western Canadian Cryospheric Network (WC2N), Bolch, Menounos, and Wheate (2010) completed 3 Methods Downloaded by [University of Victoria] at 10:12 07 May 2013 an inventory of glaciers in British Columbia and Alberta. This work provides the areal extents for the glaciers used in a Mass Balance Measurements this study, based on Landsat Thematic Mapper (TM5) To address the lack of essential glaciological data in the imagery from 2004 to 2006. Glaciers are sub-divided into CCRB, we initiated mass balance studies on Illecillewaet watersheds based on a 25 m Digital Elevation Model Glacier in 2009. Mass balance measurements have sub- (DEM). Total glacierized area in the Illecillewaet River sequently been taken over by Parks Canada, in conjunction basin in 2005 was 56.0 km2, or 4.9% of the IRB. Glaciers in with the National Glaciology Program at Natural Resources the basin range from 0.05 to 5.94 km2. Most of the glaciers Canada, with winter balance data available from 2009 to in the basin are less than 1 km2 and most of the glacier area 2012 and summer balance data from 2009 to 2011. From is associated with the size class 1–2.99 km2 (Table 1). Illecil- late April until mid-September 2009, an intensive field study lewaet Glacier is the largest ice mass in the basin, covering was carried out on Illecillewaet Glacier to measure biweekly 5.94 km2 and representing 11% of the total glacierized area. snow accumulation, snow and ice melt, and meteorological The glacier is the largest outlet of the Illecillewaet Icefield, conditions along a centre line transect on Illecillewaet draining northward into the IRB; other outlets of the icefield Glacier (Fig. 3). drain into different hydrological catchments to the south and Point snowpack measurements were taken on 1 May 2009 southwest. Illecillewaet Glacier is the one of the most accessi- through a combination of probed snow depths and snow ble glaciers in the basin and was used to collect mass balance density measurements at several locations on the glacier, to and meteorological data (see Section 3). derive snow water equivalent (w.e.) or SWE. Snow density

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 4 / J. M. R. Hirose and S. J. Marshall Downloaded by [University of Victoria] at 10:12 07 May 2013 Fig. 1 Map of the Illecillewaet Basin with the Illecillewaet Glacier located to the northeast and the Greeley Hydrometric station located to the southwest (green dot). Glacier polygons from Tobias Bolch, UNBC and 20 m DEM from Parks Canada.

Table 1. Glacier area statistics for the glacierized portions of the Illecillewaet point measurements extrapolated to give the glacier-wide Basin. The estimated mapping error in glacier area is 3%. winter mass balance (m3 w.e. a−1). We express this as the −1 Size Range Glacier Glacier Glacier Area Glacier mean specific balance for the glacier, Bw (m w.e. a ) (km2) Count Count (%) (km2) Area (%) through normalization by the glacier area. In interior regions 0.00–0.99 63 80 16.6 30 of western Canada, seasonal melting at the elevation of the 1.00–2.99 11 14 18.7 33 glaciers typically begins in May and extends through Septem- 3.00–4.99 4 5 14.8 26 5.00–5.94 1 1 5.9 11 ber. Stake measurements provide data on summer mass Total 79 56.0 balance, with point values extrapolated to give glacier-wide surface mass balance, Bs. Combining the two measurements delivers the net balance, Bn = Bw +Bs. Any mass added was measured through a combination of snowpit profiling and through summer snow events is implicitly included in Bs. snow-core estimates. Snow w.e. on 1 May is taken to represent In 2009, a total of 13 stakes were drilled into the glacier the winter accumulation (bw) on Illecillewaet Glacier, with every 50 m in elevation along the centre line (NW–SE).

wind speed and direction, air pressure, and incoming and out- going shortwave and longwave radiation. Measurements at the AWS were taken every 10 seconds, with average values stored every 10 minutes on a Campbell Scientific datalogger (CR1000). Next to the AWS an ultrasonic ranger (SR50) was installed on a fixed pole to record snow or ice surface height every 30 minutes. To supplement the data gathered in summer 2009, we use hourly precipitation, humidity, and temperature data from an Environment Canada weather station at Rogers Pass, which has been operational since 1966. The station is at an elevation of 1340 m and is 9.3 km from the glacier AWS. Winter (September to May) precipitation at Rogers Pass averaged 1278 mm from 1966 to 2011, while mean summer (June, July, August) temperature averaged 12.0°C over this period. For our study period, winter precipitation deviated from the long-term mean by −18%, −16%, and +10% in 2008–09, Fig. 2 Percentage of glacierized area versus elevation for the Illecillewaet 2009–10, and 2010–11, respectively. Mean summer tempera- Glacier and in the Illecillewaet basin. ture anomalies from 2009 to 2011 were 1.2°C, −0.1°C, and −0.7°C, respectively. Mass balance year 2008–09 was therefore warm and dry, which should drive a more negative Table 2. Glacier area and elevation statistics for Illecillewaet Glacier and – – glaciers within the Illecillewaet River sub-basin (IRB), 2005. mass balance; 2009 10 was relatively normal, and 2010 11 was cool and wet in the region, conducive to positive mass Elevations (m) balance. Area Glacier cover in the Glaciers (km2) IRB (%) Min. Median Max. Illecillewaet 5.94 1.0 1993 2521 2911 Sub-basin 56.0 4.9 1592 2385 2942 c Illecillewaet Glacier Melt Model We develop and calibrate a local melt model at the AWS site and then apply it to Illecillewaet Glacier at 20 m resolution. Seven additional stakes were established along two transverse Spatially distributed energy balance investigations are lines 200 m apart (Fig. 3). Winter balance (snow accumu- necessary to understand and simulate glacier melt better lation) was measured at all stake locations in early May. (e.g., Hock, 1999; Wagnon, Ribstein, Kaser, & Berton, Beginning in 2010, mass balance surveys have been con- 1999). Although a complete energy budget approach is the ducted by Parks Canada, with a total of seven stakes spaced ideal way to model glacial melt, it demands many obser- approximately every 100 m in elevation along the same vations which are generally unavailable. Instead, tempera- centre-line transect. Measured winter and summer balances ture-index models that are based on cumulative positive are regressed against elevation (Fig. 4) and linearly extrapo- degree days (PDD) have proven to be useful in glacio-hydro- lated to the entire glacier. Error analysis for mass balance logical studies. The “classical” degree-day temperature-

Downloaded by [University of Victoria] at 10:12 07 May 2013 results is based on the estimated error in glacier area and index model captures seasonal patterns in runoff and is hypsometry (e.g., through non-contemporaneous imagery) likely sufficient for long-term analyses (e.g., Braithwaite, and measurement errors for snow depth, snow density, and 1995), and it has been applied in a wide variety of regions stake height. (e.g., Jóhannesson, Sigurdsson, Laumann, & Kennett, 1995; Rango et al., 2008). However, PDD models do not capture daily and seasonal b Meteorological Measurements runoff variability well (Hock, 1999), in part because degree- Meteorological input data needed to model glacier melt were day factors can vary considerably in space and time (e.g., collected from an automatic weather station (AWS) and sec- Hock, 1999; Singh and Kumar, 1997). Modified degree-day ondary weather stations positioned on and off the Illecillewaet models that include solar radiation (e.g., Hock, 1999; Pellic- Glacier from 1 May to 14 September 2009 (Fig. 3). Air temp- ciotti et al., 2005) capture daily cycles better, as well as erature, humidity, and precipitation, were recorded by second- spatial variations in melt resulting from topographic influences ary weather stations mounted on poles drilled into the ice and (i.e., effects of shading, slope, and aspect). Temperature-index spaced every 100 m in elevation. The centre-line transect of models that incorporate treatments of potential direct incom- temperature loggers was used to measure temperature lapse ing solar radiation are well established (e.g., Hock, 1999, rates on the glacier. 2003; Moore, 1993; Pellicciotti et al., 2005; Shea, Moore, & The AWS was mounted on a tripod at an elevation of 2435 m Stahl, 2009), because potential direct solar radiation is and was configured to record temperature, humidity, precipitation, readily calculated.

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 6 / J. M. R. Hirose and S. J. Marshall Downloaded by [University of Victoria] at 10:12 07 May 2013

Fig. 3 Topographic contour map of the Illecillewaet Icefield study area, with an elevation contour interval of 50 m (100 m on the glaciers). The field instrumenta- tion from summer 2009 is also indicated. AWS; automatic weathers station; RP: Rogers Pass; T and RH: temperature and relative humidity stations, respectively.

1 MELT PARAMETERIZATION in our melt model. Pellicciotti et al. (2005) also Here we adapt the temperature-index melt models of consider absorbed solar radiation, in a bivariate relation Hock (1999) and Pellicciotti et al. (2005), using absorbed with PDD (i.e., two separate melt terms). Because absorbed solar radiation rather than potential direct solar radiation solar radiation and degree days are highly correlated, we

2 TEMPERATURE INPUTS For distributed modelling of snow and ice melt, hourly temperature is modelled as a function of elevation, using measured lapse rates on the glacier. For 2009 this is based on hourly mean temperatures recorded at the AWS, = + β − Tcell TAWS T (zAWS zcell), (2)

where zAWS and zcell are the AWS and grid cell elevations, respectively; TAWS is the AWS hourly temperature, βT is the surface temperature lapse rate, and Tcell is the computed cell temperature. Daily DD values are calculated from hourly temperatures, providing a measure of the heat energy available over a day in excess of 0°C (i.e., available for melting). For Fig. 4 Vertical profiles of Illecillewaet Glacier for winter (long dashed lines), other years, we adapt Eq. (2) so that it is based on hourly temp- summer (short dashed lines), and net mass balances (solid lines) for erature data from Rogers Pass (see Section 4b). 2009 (black), 2010 (dark grey), and 2011 (light grey). 3 RADIATION AND TRANSMISSIVITY take a different approach and combine these into a single Radiation and albedo data are available for the AWS site but term: must be estimated elsewhere on the glacier. Potential direct solar radiation, Iϕ, is calculated for each glacierized grid cell by running hourly ArcGIS Area Solar Radiation, which incor- = + − α M [TF SRF (1 )I]DD, (1) porates solar radiation at the top of the atmosphere, a specified clear-sky atmospheric transmissivity (τ), solar geometry, and − where M is the total daily melt (mm w.e. d 1); DD is the topographic characteristics (slope, aspect, and shading) (see, number of positive degrees in a day; α is the surface e.g., Oke, 1987). We find the optimal τ value by comparing albedo, and I is the average daily shortwave radiation measured daily incoming solar radiation for clear-sky days − (W m 2) received at the site. TF and SRF are empirical with modelled potential direct solar radiation. Diffuse pro- coefficients, the temperature factor and shortwave radiation portions of incoming radiation are assumed to be 20% − − − factor, expressed in mm w.e. °C 1 d 1 and mm w.e. W 1 (Arnold, Willis, Sharp, Richards, & Lawson, 1996). − m2 d 1, respectively. We treat these as constants. Instead Cloud cover reduces the solar radiation that reaches the of separate melt factors for snow and ice, which are typi- surface, relative to the potential direct value. From the AWS cally invoked in PDD models (e.g., Braithwaite, 1995; data, we calculate the fraction of daily mean radiation relative Hock, 1999; Jóhannesson et al., 1995), the solar radiation to the potential direct radiation in order to derive a clear-sky term embeds an evolving surface albedo, which varies in index: fcs = I/Iϕ. This is evaluated daily to provide an index space and time. fcs [ [0,1], with fcs = 1 indicating clear-sky (cloud-free) con- Equation (1) introduces additional complexity relative to ditions and fcs 0 for overcast skies. Daily values of fcs are Hock (1999) because of the need for information on surface assumed to apply glacier-wide, allowing an estimate of incom- albedo and atmospheric cloud conditions. Temperature, ing solar radiation I = fcs Iϕ to be applied in Eq. (1) for distrib-

Downloaded by [University of Victoria] at 10:12 07 May 2013 albedo, and incoming solar radiation all need to be measured uted melt modelling. In years when we lack radiation data, fcs or parameterized. Where AWS data are available, this can is parameterized as a function of temperature and humidity be evaluated directly. We determine TF and SRF based on conditions at Rogers Pass (see below). observations at the Illecillewaet Glacier AWS site and from our ablation stakes in summer 2009, using three-day averaged 4 ALBEDO melt data from the AWS ultrasonic depth gauge (SR50) and Albedo also needs to be estimated at all points on the glacier. biweekly melt data from stake ablation measurements. To Snow albedo increases after a fresh snow event and declines convert SR50 and stake measurements to water equivalence, through the summer melt season as a result of grain recrystal- initial snowpack density was measured from four snowpits lization, liquid water content, and increasing concentrations of (2000, 2200, 2400, and 2600 m) in 2009 and three snowpits aerosols and debris in a melting snowpack (e.g., Brock, Willis, (2200, 2400, and 2600 m) in 2010 and 2011. Ice lenses and Sharp, & Arnold, 2000; Klok & Oerlemans, 2004). Based on glacier ice are assumed to have a density of 917 kg m−3.We observations at the AWS site, we parameterize the summer α neglect the effects of snow densification through the snow-albedo evolution, s(t) as a function of cumulative posi- summer. Half of the days from our study period, 1 May to tive degree days, Σ DD (t), following a logarithmic fit: 14 September 2009, are used to calibrate the model, with the αs(t) = a + b ln [Σ DD(t)]. (3) other days reserved for model evaluation. Once calibrated, the melt model can be applied to other points on the glacier Parameters a and b are fit to the data and are assumed to apply as well as to other years. at all locations on the glacier. Once the seasonal snowpack

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 8 / J. M. R. Hirose and S. J. Marshall

melts away, fixed albedo values for firn or ice are assigned, Illecillewaet Glacier cannot be expected to apply to the based on measurements of old snow and bare ice at the whole basin, but it may be “statistically” reasonable over the AWS site. course of a summer. That is, the number of clear-sky and Fresh snowfall events in the summer have a large effect on heavily overcast days may be similar across the IRB, the surface albedo, reducing summer melt (Brock et al., 2000; because these are generally dictated by synoptic weather pat- Oerlemans & Klok, 2004). These were measured in summer terns during the summer season. Furthermore, the subset of 2009 by direct observations during biweekly site visits, glacierized IRB locations occupy a similar range of elevation, SR50 snow-surface heights, and the AWS albedo data. mostly above 2000 m, and are likely to experience common Snow-water equivalence (SWE) is estimated for fresh snow sky conditions (e.g., similar transmissivity and immunity to events based on the depth of the fresh snow and measurements valley fogs). of snow density from summer snowfall events. When a Nevertheless, cloud conditions in summer 2009 are not summer snowfall occurs, surface albedo is reset to a fresh necessarily typical of other summers, so we develop a proxy snow value until the fresh snow melts away, at which time for the clear-sky index, fcs, based on observed diurnal tempera- the previous underlying surface value, from Eq. (3) or for ture patterns and atmospheric humidity. Bristow and Camp- bare ice or firn, is restored. Snow events at the AWS site bell (1984) parameterize mean daily cloud cover and/or may correspond to rainfall at lower elevations. Based on atmospheric transmittance as a function of daily temperature observations from the AWS site, we adopt a local (grid cell) range, Tr = Tmax – Tmin. Cloudy days reduce daytime tempera- DD threshold of 4°C d−1 to determine whether a summer pre- ture maxima and increase the overnight minima, giving low cipitation event manifests as snow or rain. values of Tr. Other parameterizations relate transmittance to the relative or absolute humidity, which are also rough proxies for cloud cover (e.g., Richardson & Reddy, 2004). d Illecillewaet Basin Model We examine different combinations of temperature and Winter mass balance for other glaciers in the IRB is estimated humidity variables to develop an empirical relation for fcs, cali- through extrapolation from Illecillewaet Glacier, based on a brated against observations from summer 2009. linear regression of May bw against elevation on Illecillewaet. Summer snowfall events are well characterized for 2009 but We therefore assume that the observed snowpack, its variation are not known for other years. We explore two approaches to with altitude, and their interannual variability are representa- modelling summer snow events: (i) introducing summer tive of other glaciers in the basin. snowfall as a stochastic variable and (ii) estimating summer Application of the melt model to the IRB requires data for snow events based on daily precipitation at Rogers Pass. For temperature, lapse rates, cloud cover, and summer snow the stochastic approach, the number and frequency of events. This is available for Illecillewaet Glacier in 2009, summer snow events is specified, then randomly generated but for 2010 and 2011 we use hourly weather data from in MATLAB to determine event timing and magnitude. Mag- Rogers Pass. This includes temperature data for DD estimates, nitudes are sampled from a uniform distribution between 1 and lapsed to different elevations in the IRB, along with parame- 10 cm, based on observations in summer 2009. We introduce terizations of cloud cover and summer snowfall based on this approach as a way of including summer snow events in Rogers Pass temperature, humidity, and precipitation data. mass balance models where local station data are unavailable. These adaptations to the melt model are explained below. For the second method, summer precipitation at Rogers The mass balance and runoff models are then extended to all Pass is assumed to be representative of precipitation on the

Downloaded by [University of Victoria] at 10:12 07 May 2013 the glaciers in the IRB. glaciers, and local grid-cell temperatures (daily DD totals) The observed relation between hourly temperature at determine whether precipitation falls as snow or rain. We Rogers Pass and the glacier AWS is used to extrapolate measured vertical precipitation gradients in Illecillewaet high-elevation temperatures. This is not a simple matter of valley in summer 2009 and found no significant relationship adopting a fixed lapse rate, because near-surface air tempera- between precipitation totals and elevation. Hence, we do not tures are determined by local surface energy balance (i.e., heat apply a precipitation lapse rate. Note that this may be appropri- absorption in a forested versus glaciated environment) and by ate for summer precipitation events but is not expected to hold local air movements, such as cold air drainage patterns true in winter when orographic precipitation is the dominant (see, e.g., Pepin & Losleben, 2002; Petersen & Pellicciotti, mechanism of snow accumulation. 2011; Shea & Moore, 2010). As discussed in Section 4, we Meltwater runoff from the IRB glaciers is our primary inter- observe a strong diurnal relation in the temperature difference est in this study. Snowpack and melt models are applied to between the Rogers Pass AWS and the glacier AWS and use each 20 m × 20 m grid cell to determine the distributed melt, this as the basis for temperature extrapolation to the glaciers. and all meltwater is assumed to run off to the Greeley gauge Glaciers in the IRB occupy different aspects and experience at the outlet of the basin. We do not apply a hydrological different degrees of topographic shading, so the radiation- model to route the runoff. Our simplistic approach neglects based melt model is essential for basin-scale distributed mod- delays and storage in the glaciers and the groundwater elling. Potential direct solar radiation is readily modelled over system but may be reasonable for total glacier discharge on the entire IRB. Effective daily cloud cover (fcs) measured at a monthly and seasonal time scale.

e Sensitivity Tests Table 3 summarizes the observed mass balance data on We assess model uncertainties by exploring the parameter Illecillewaet Glacier from 2009 to 2011. Error estimates are space. In particular, we examine the sensitivity of modelled based on the combined uncertainty in measurements and glacier runoff to our assumptions about melt model par- input datasets used to derive the glacier-wide mass balance. ameters, cloud conditions, summer-snow frequencies, and We assess these to be 22% for winter balance and 23% for initial (1 May) snowpack. There is uncertainty about the summer balance (Hirose, 2012). Average specific mass winter mass balance gradient and even greater uncertainty balances from 2009 to 2011 were Bw = 1.43, Bs = −1.91, and −1 with respect to snowpack estimates on other (unmeasured) gla- Bn = −0.48 m w.e. a . All three years had negative net mass ciers in the IRB. balances, with the most extensive melting and runoff in 2009. In addition to numerical experiments to evaluate uncertain- ’ ties, we examine Illecillewaet Glacier s sensitivity to climate b Meteorological Measurements variability through systematic perturbations in temperature, Monthly mean meteorological data collected at the glacier precipitation, and the clear-sky index. Using the mass AWS are summarized in Table 4 and calibrated parameter balance and meteorological measurements from 2009 as a values from field measurements are found in Table 5. reference climatology, we explore the effects of temperature perturbations from −3°C to +3°C, winter precipitation variations from 20% to 220% of the observed 2009 snowpack, 1 TEMPERATURE A mean summer temperature lapse rate of β = −0.0067°C m−1 and clear-sky index (f ) variations from 0.65 to 0.90 on T cs (R2 = 0.94) was measured from the transect of temperature mass balance and glacier runoff. stations on Illecillewaet Glacier, with high correlations between all glacier stations. Temperatures at Rogers Pass 4 Results have a complex relationship with those on the glacier, a Mass Balance Measurements however. For 1 May to 14 September 2009, the mean tempera- May snowpack (bw) data are available from 2000 m to 2600 m ture difference between the glacier AWS and the AWS at Δ − on Illecillewaet Glacier. Linear regression of these data in Rogers Pass, Tm, was 7.8°C. There is a strong diurnal 2009, for which we have the most detailed snowpack infor- pattern to the temperature differences (Fig. 5), with mean −1 − − mation, gives a balance gradient of βP = 0.92 mm w.e. m midday and overnight differences of about 13°C and 3°C, (R2 = 0.61; Fig. 4). Winter mass balance measurements from respectively (i.e., a 10°C diurnal cycle about the mean temp- May 2010 to 2012 indicate a weaker elevation gradient, erature offset of −7.8°C). Daytime heating at Rogers Pass, −1 with a mean value of βP = 0.73 mm w.e. m for 2009 to 2012. These values are comparable to precipitation lapse Table 3. Observed and modelled mass balance and glacier runoff, 2008 to rates adopted in other studies (e.g., Stahl, Moore, Shea, Hutch- 2011 (m w.e.) of Illecillewaet (Ille) Glacier and Illecillewaet River Basin inson, & Cannon, 2008). (IRB). Uncertainty estimates are discussed in the text. Observations indicate that winter accumulation on Illecille- Season 2008–09 2009–10 2010–11 Average waet Glacier increases approximately linearly with elevation Observations up to the last measured stake at 2600 m, the altitude of the Winter 1.48 ± 0.33 1.43 ± 0.31 1.39 ± 0.31 1.43 ± 0.18 main icefield plateau. Above this elevation we assume that Summer −2.08 ± 0.48 −2.00 ± 0.46 −1.65 ± 0.38 −1.91 ± 0.25 winter accumulation levels off at the value measured at Net −0.61 ± 0.58 −0.57 ± 0.56 −0.26 ± 0.49 −0.48 ± 0.31 Runoff (106 m3) 12.4 ± 2.8 11.9 ± 2.7 9.8 ± 1.9 11.3 ± 1.5 2600 m. This may overestimate the snow at the higher Downloaded by [University of Victoria] at 10:12 07 May 2013 elevations, because slopes above this tend to be steep and Model, Ille Glacier Winter 1.55 ± 0.23 1.38 ± 0.21 1.44 ± 0.22 1.46 ± 0.13 wind scoured. Winter balances at elevations below 2000 m Summer −2.32 ± 0.42 −1.60 ± 0.29 −1.18 ± 0.21 −1.70 ± 0.18 and above 2600 m are uncertain because of the lack of Net −0.77 ± 0.48 −0.22 ± 0.35 0.26 ± 0.30 −0.24 ± 0.22 Runoff (106 m3) 13.8 ± 2.5 9.5 ± 1.7 7.0 ± 1.3 10.1 ± 1.1 direct measurements. The regression functions for bw(z) provide winter snowpack estimates that can be extrapolated Model, IRB Winter 1.45 ± 0.36 1.30 ± 0.33 1.39 ± 0.35 1.38 ± 0.20 to all grid cells on Illecillewaet Glacier and for the IRB, Summer −2.68 ± 0.48 −1.91 ± 0.34 −1.43 ± 0.26 −2.01 ± 0.22 providing Bw and the initial snowpack that is input to the Net −1.23 ± 0.60 −0.61 ± 0.47 −0.04 ± 0.43 −0.63 ± 0.29 melt model. Runoff (106 m3) 150 ± 27 107 ± 19 80 ± 14 112 ± 12

Table 4. Monthly mean weather at the Illecillewaet Glacier AWS site, summer 2009.

W −2 Period Tmin (°C) T ( C) Tmax (°C) PDD (°C d) RH (%) I (W m ) α fcs May −12.7 −1.3 10.2 40 72 308 0.80 0.85 June −6.6 2.4 10.4 93 71 312 0.70 0.79 July −6.2 5.7 15.4 191 69 300 0.62 0.80 August −2.4 5.5 14.8 181 71 232 0.40 0.77 1–14 Sept. −2.9 3.8 13.3 62 73 184 0.56 0.78 Season −12.7 2.6 15.4 566 71 296 0.62 0.80

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 10 / J. M. R. Hirose and S. J. Marshall

Table 5. Parameter values calibrated from field measurements.

Parameter Description Value Units −1 βP Winter snowpack lapse rate, 2009 0.92 mm w.e. m Winter snowpack lapse rate, 2009–11 0.73 mm w.e. m−1 −1 βT Temperature lapse rate −0.0067 °C m TF Temperature factor 0.9308 mm w.e. °C−1 d−1 SRF Solar radiation factor 0.02974 mm w.e. m2 W−1 °C−1 d−1 τ Atmospheric transmissivity 0.73 fcs Average summer clear-sky index 0.80 αs Fresh snow albedo 0.86 αi Ice albedo 0.20 αf Firn albedo 0.40 −1 kdT Regression parameter for fcs 3.08 % °C −1 ke Regression parameter for fcs −2.07 % mbar DDt Degree day threshold for snow 4.0 °C d A Half amplitude, diurnal ΔTd 5.8 °C tlag Lag time, diurnal ΔTd 2.9 hr

and midnight for the peak temperature difference. Because daily DD totals are the essential variable of interest for glacier melt modelling, we determine A and tlag based on the best fit to observed summer DD totals. Resulting values are A = 5.8° C and tlag = 2.8 hr, shown by the dashed line in Fig. 5. Temperatures at Rogers Pass are then used to drive the melt model with a three-stage correction: (i) a uniform temperature shift to the reference elevation of the glacier AWS site, ΔTm, (ii) a correction for the diurnal temperature pattern, ΔTd, and (iii) a lapse rate correction for the elevation of the glacier position of interest, after Eq. (2):

= + Δ + Δ + β − Tcell TRP Tm Td T (zAWS zcell), (5)

where TRP is the hourly mean temperature at Rogers Pass. This temperature parameterization is used for basin-wide melt modelling and for driving Illecillewaet Glacier mass balance Fig. 5 Hourly temperature differences between the Rogers Pass AWS and models for 2010 and 2011. Illecillewaet Glacier AWS for all days, 1 May to 14 September 2009 (asterisks), after correcting for the mean summer temperature difference between the sites (−7.8°C). The solid line shows the average temperature difference for each hour and the dashed line 2 SOLAR RADIATION shows the best-fit sine wave introduced to correct for this diurnal Measurements of average daily incoming solar radiation on pattern.

Downloaded by [University of Victoria] at 10:12 07 May 2013 clear-sky days, relative to modelled potential direct solar radiation, allow an estimate of atmospheric transmissivity for the τ 2 relative to the glacier, may be caused by the lower albedo in site. Fits to the data indicate a value for of 0.73 (R = 0.94), the valley-floor environment, resulting in increased solar within the range of other studies which recommend values of absorption. Closure of the overnight temperature difference 0.6–0.9 (Hock, 1999; Oke, 1987). Radiation modelling for the τ is consistent with cold air drainage, driving weak or inverted basin was carried out treating as a constant. Daily mean −2 overnight lapse rates. This systematic diurnal cycle means potential solar radiation in the IRB averages 315 W m for that a constant offset (i.e., static lapse rate adjustment) from the summer melt season, 1 May to 14 September, ranging −2 −2 Rogers Pass to the glaciers would give daytime temperatures from 22 W m on a northern exposed slope to 433 W m that are too high, leading to excessive modelled melt. on a southern aspect. Based on these observations, we introduce a sinusoidal, In summer 2009, clouds and atmospheric scattering reduced diurnal temperature correction to make Rogers Pass data solar radiation on the glacier by 20%, on average, a clear-sky more representative of the glacier, factor of fcs = 0.80. Daily values for fcs ranged from 0.28 to 1.03. Monthly mean f values are relatively consistent, Δ =− π − / cs Td A cos (2 (t tlag) 24), (4) varying from 0.77 to 0.85 (Table 4). These mean values could be applied to melt modelling in other years for which where A is the half-amplitude of the temperature difference; t is we lack radiation data. Alternatively, we examine the relation the hour of interest, and tlag is the time lag from local solar noon between mean daily fcs values, daily temperature range, Tr, and

humidity variables at Rogers Pass. Of several different combi- through a combination of SR50 records and fits to the nations of variables, a bivariate combination of Tr and the albedo data. mean daily vapour pressure, ev, proves to be the strongest pre- To apply this to the rest of the glacier in 2009, we assume dictor of the clear-sky index: the same timing for snow events but determine an elevation threshold where rain transitions to snow based on the local fcs = 0.646 + 0.0308Tr − 0.0297ev. (6) daily DD total. Based on the fresh-snow and meteorological observations at the AWS site, rain is assumed at locations This relation is statistically significant (R2 = 0.65), with a with a daily DD in excess of 4°C d−1; below this, precipitation standard error in fcs of 0.01. This relation allows regional is assumed to fall as snow. cloud conditions to be parameterized from historical The total amount of SWE contained in summer snowfall is temperature and humidity data at Rogers Pass, or from near- relatively minor, but its influence on albedo is important. A surface temperature and humidity fields in climate model stochastic model of summer snowfall events is developed projections. based on the frequency of snow events (approximately 10 per summer) and their observed magnitude (up to 10 cm of snow accumulation) in summer 2009. These parameters are 3 ALBEDO varied in sensitivity tests to explore their relative importance Seasonal melt had not yet begun at the time of our initial snow to seasonal melt and runoff models. Alternatively, precipi- surveys and AWS setup in late April 2009. Measurements of tation at Rogers Pass can be extrapolated to the glacier, early season and fresh summer snow indicate an albedo along with temperature adjustments following Eq. (5), to esti- of 0.85 to 0.9 (Fig. 6). This is borne out by regressions of mate summer snowfalls. For summer 2009, this method gives snow albedo against cumulative DD, which give the relation similar results to the melt model runs with observed snowfall and with stochastic (N = 10) snow events: B values of −2.32, s −2.35, and −2.33 m w.e., respectively. Because this is readily α = . − . . s 0 86 0 05668 ln DD (7) applied to other seasons when we lack direct observations of summer snowfall, we adopt the Rogers Pass precipitation The mean measured albedo of ice at our site was 0.20. Fits extrapolation as our “reference model” for 2010 to 2011 and of the albedo model to the measured AWS albedo are good for basin-scale modelling. (Fig. 6), but the detailed fit is contingent on capturing the fresh-snow events, which briefly boost the albedo back to c Illecillewaet Glacier Melt Model fresh-snow values. Snow-depth gauge and albedo measure- Using measured mass balance and AWS meteorological data, ments identify the timing of fresh-snow events, but the the melt model parameters are calibrated (Tables 2 and 4). amount of snowfall and the altitude of the snowline (i.e., Using multiple linear regression the melt model’s coefficients − − where the glacier received snow rather than rain) in each are optimized with TF = 0.9308 mm w.e. °C 1 d 1 and SRF = − − − snowfall event are less certain. We use a fresh-snow 0.02974 mm w.e. W 1 m2 °C 1 d 1 ((Eq.) 1), significant at the density of 145 kg m−3 for summer snow accumulations, 95% and 99.9% levels of confidence, respectively. Standard − − based on measurements, and estimate SWE at the AWS errors for the two coefficients are 0.46 mm w.e. °C 1 d 1 and 0.0026 mm w.e. W−1 m2 °C−1 d−1, respectively. The melt model is independently validated at a subset of the ablation stakes and selected biweekly periods of the SR50 data to Downloaded by [University of Victoria] at 10:12 07 May 2013 represent the seasonal, elevation, and surface variability. For the independent test data, R2 = 0.88 and the mean absolute error |ε|=0.4 mm w.e. (Fig. 7). Driving the distributed model with these parameters and with the temperature, radiation, and albedo models described above, glacier-wide melt can be modelled for the summers of 2009 to 2011. Distributed radiation and DD fields for July 2009 are illustrated in Fig. 8. Glacier melt can also be partitioned into that associated with seasonal snow and that caused by melting of glacier ice and firn. Modelled mass balance and Illecillewaet Glacier runoff for 2009 to 2011 are given in Table 3. We discuss uncertainty estimates in Section 5b.

d Illecillewaet Basin Melt Model Fig. 6 Observed (solid) and modelled (dashed) daily mean albedo at the Ille- Basin-wide glacier snow accumulation and melt from 2009 to cillewaet Glacier AWS (2450 m elevation). 2011 can be calculated by extension of the methods used for

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 12 / J. M. R. Hirose and S. J. Marshall

Fig. 7 Observed versus modelled melt (m w.e.) at the AWS and ablation stakes during the calibration and verification periods.

Illecillewaet Glacier. Because glacier hypsometries and solar Average Illecillewaet Glacier mass balance was negative radiation fields differ between Illecillewaet Glacier and the from 2009 to 2011, but there is interesting interannual varia- IRB, different specific mass balances are predicted. To esti- bility in the seasonal balances and summer runoff. The mate Bw, we extrapolate from the observed Illecillewaet 2008–09 balance year was the most negative of the three Glacier winter snowpack, based on the observed altitude gra- years, 2009–10 was slightly negative, and mass balance in dient, βP, for each year. The melt model can be applied directly 2010–11 was close to a state of balance (Table 3). Summer to the basin. Model simulations indicate more negative runoff from the glaciers decreased in each of the three years. summer and annual mass balances for the IRB as a whole, These results are consistent with available meteorological relative to Illecillewaet Glacier (Table 3). For the 2008–09 observations at Rogers Pass, which indicate that summer balance year, modelling of the IRB gives Bw = 1.45, Bs = 2009 was warm relative to 2010 and 2011. Mean temperature −2.68, and Bn = −1.25 m w.e. Snow accumulation is 7% less from 1 May to 14 September 2009 was 10.4°C, compared with than on Illecillewaet Glacier, and there is 16% more summer values of 9.3°C and 9.1°C in 2010 and 2011, respectively. melt, with both factors contributing to the more negative Relative humidities for this period were 70.8, 73.6, and mass balance. Modelled glacier runoff for the basin is 74.8% for 2009, 2010, and 2011, respectively. Precipitation 150 × 106 m3 in 2009, with 45% of this derived from ice was recorded at Rogers Pass on 65 days in summer 2009, com- and firn melt. pared with 78 and 71 days in the two subsequent summers. For the three-year composite, 2009 to 2011, IRB mass bal- This indicates relatively cool, overcast summer weather in Downloaded by [University of Victoria] at 10:12 07 May 2013 ances are Bw = 1.38, Bs = −2.01, and Bn = −0.63 m w.e. (Table 2010 and 2011. 3), and modelled glacier runoff averages 112 × 106 m3, Regional snowpack was above normal in the La Niña winter of which 34% comes from glacial ice and firn. Differences of 2010–11 (Anslow & Roddenhuis, 2011). September through in annual mass balance are primarily driven by summer April precipitation at Rogers Pass was 1332 mm in 2010–11, temperature and cloud cover, with winter snowpack having 10.4% above the 1965–2011 average of 1206 mm. In contrast, a secondary influence for these years. Frequent summer precipitation in 2008–09 and 2009–10 was below normal, with snowfalls also contribute to the reduced melting and runoff 990 and 1042 mm of September–April precipitation, respect- in summer 2011. ively. It is unlikely that Illecillewaet Glacier winter mass balance was as low as reported in 2010–11, Bw =1390mm; this is comparable to the winter precipitation at Rogers Pass 5 Discussion and less than the winter mass balance that was measured in a Observed and Modelled Mass Balance 2009. We suspect that high elevation snow depth measurements Here we discuss the observed and modelled mass balance and on the glacier may have been misread because of ice layers or total glacier runoff from Illecillewaet Glacier. There are no denser snow that was mistaken as firn. observations to evaluate this on the scale of the IRB, but we Modelled winter balance in 2008–09 overestimates the discuss basin-scale mass balance in the context of glacier balance by 5% relative to the observations (Table 3). The discharge in Section 5d. linear accumulation gradient that we adopt oversimplifies the

Fig. 8 Modelled (a) positive degree days (°C d), (b) melt (m w.e.), (c) potential direct solar radiation (W m−2), and (d) absorbed solar radiation (W m−2)on Illecillewaet Glacier, July 2009.

snowpack variability. A portion of the glacier also lies above The model overestimates summer mass balance by 12% 2600 m, where we do not have measurements. The highest compared with the observed stake data, when extrapolated stake data are assumed to apply to all elevations above to the glacier area (Table 3). We suspect that the model may 2600 m. Winter balance data need to be measured at higher be closer to the truth, because it parameterizes snow and ice elevations to assess this assumption. These areas on the glacier melt based on energy constraints (i.e., solar variability, are steep, avalanche prone, and heavily crevassed, however, albedo, elevation, and aspect), while high-elevation ablation making this difficult. These steep upper slopes may retain less estimates in the “observed” mass balance are from the snow accumulation, so that both our measured and modelled highest stake data (2600 m). The stake data do not capture winter balances overestimate bw at elevations above 2600 m. important differences in energy inputs on the upper glacier

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 14 / J. M. R. Hirose and S. J. Marshall

slopes, which have southwest aspects. The highest elevations Table 6. Sensitivity of modelled Illecillewaet Glacier 2008–09 mass balance on the glacier therefore receive greater amounts of solar and runoff to parameter uncertainties and climatic conditions. The final column is the percentage of glacier runoff associated with ice and firn melt as radiation (Fig. 8c), which will contribute to higher melt rates. opposed to seasonal snow. In combination, observed and modelled 2008–09 net mass balances are comparable and well within our uncertainties, Mass balance (m w.e.) Runoff (106 m3) at −0.61 and −0.77 m w.e. Discrepancies between observed and modelled net balance are also within the uncertainties in Experiment Bw Bs Bn Total Snow Ice Ice (%) 2009–10 and 2010–11 (Table 3), but the melt model underes- Reference (2009) 1.55 −2.32 −0.77 13.8 9.4 4.4 32 timates summer melt and annual mass losses in these years, in TF −1 s.e. 1.55 −2.03 −0.48 12.0 9.2 2.8 23 TF +1 s.e. 1.55 −2.59 −1.05 15.4 9.5 5.9 39 contrast with 2009. On average, for the three years, observed SRF −1 s.e. 1.55 −2.11 −0.56 12.6 9.3 3.2 26 − − Bn = −0.48 m w.e. and modelled Bn = −0.24 m w.e. Most of the SRF +1 s.e. 1.55 2.52 0.97 15.0 9.5 5.5 37 difference can be attributed to underestimated melt in the TF, SRF −1 s.e. 1.55 −1.83 −0.28 10.9 9.0 1.9 17 TF, SRF +1 s.e. 1.55 −2.80 −1.25 16.7 9.5 7.2 43 model; mass balance observations from 2009 to 2011 indicate fcs = 0.72 1.55 −2.07 −0.52 12.3 9.3 3.0 25 6 3 − − a mean summer runoff of 11.3 × 10 m compared with a mean fcs = 0.88 1.55 2.52 0.97 15.0 9.4 5.6 37 modelled runoff of 10.1 × 106 m3. Average B and runoff in No summer snow 1.55 −2.43 −0.88 14.4 9.1 5.3 37 s Random snows, N = 10 1.55 −2.33 −0.78 13.8 9.3 4.6 33 the model are 11% less than the observational estimate, Random snows, N = 20 1.55 −2.25 −0.70 13.3 9.4 3.9 29 − − although values fall within the uncertainties. No Bw plateau 1.57 2.30 0.73 13.7 9.5 4.2 31 −1 βP = 0.5 mm w.e. m 1.35 −2.37 −1.02 14.1 8.3 5.8 41 −1 βP = 1.3 mm w.e. m 1.74 −2.27 −0.52 13.5 10.2 3.3 24 b Model Uncertainties Climate perturbations ΔT = −2°C 1.55 −1.38 0.17 8.2 7.5 0.7 8 Uncertainties in the model can be explored through pertur- ΔT = +2°C 1.55 −3.57 −2.02 21.2 9.4 11.8 56 bation experiments. We examine the sensitivity of modelled ΔP = −20% 1.24 −2.43 −1.19 14.4 7.6 6.8 47 Δ − − runoff and mass balance to the melt model coefficients, P = +20% 1.86 2.24 0.38 13.3 10.8 2.5 19 ΔT = +2°C, ΔP = +20% 1.86 −3.46 −1.60 20.6 11.3 9.3 45 clear-sky parameter, and summer snowfall. Additional uncertainties in albedo and temperature values can be considered to be embedded in the melt model coefficients (i.e., they intro- to our reference value of 10 snow events in summer 2009. This duce errors similar to having higher or lower values of the introduces less uncertainty than the melt parameters, with Bs coefficients TF and SRF). We vary these parameters individu- varying from −2.43 to −2.25 m w.e. (+5 to −3%) for 0–20 ally and in combination, based on the standard error estimates summer snowfall events, although it is important to emphasize for TF and SRF reported above. For the clear-sky index, fcs,we that summer snows have a non-negligible impact on summer examine the impact of assuming a constant value for the mass balance. summer, mean monthly values, or through varying the mean Taking uncertainties in TF, SRF, fcs, and summer snow summer value of fcs by ±10%, from 0.72 to 0.88. This frequency as additive (using the root mean square error), exceeds the range of mean monthly values observed at the modelled Bs = −2.32 ±0.42 m w.e. (±18%). Modelled summer site. We also explore the importance and sensitivity of runoff from the glacier has the same uncertainty: Q = 13.9 modelled runoff to summer snowfall by varying the number ±2.5 × 106 m3. of randomly selected snow events from 0 to 20. Accurate estimates of the winter snowpack also represent a Results of these sensitivity tests for Illecillewaet Glacier are major source of uncertainty in our model, particularly at the

Downloaded by [University of Victoria] at 10:12 07 May 2013 compiled in Table 6. Sensitivity tests are reported for summer basin scale. Experiments in which we allow snow accumulation 2009 and the reference model, with Bs = −2.32 m w.e., is based to increase with altitude above 2600 m, rather than levelling out on our optimized parameters and observed temperature, clear- at this altitude, have only a small effect on Bw (+1%; Table 6). sky index, and summer snow events. The most sensitive free We also carry out experiments in which the accumulation −1 parameter in the melt model is the temperature melt factor gradient βP ranges from 0.5 to 1.3 mm w.e. m , relative to − TF, for which variation by ±1 standard error gives a range the reference value of 0.92 mm w.e. m 1. This range of in modelled Bs from −2.59 to −2.03 m w.e. (i.e., ±12%). values in the accumulation lapse rate brackets the interannual Uncertainty in SRF gives a variation in summer balance of variability observed from 2009 to 2012. Resulting winter ±9%, and the sensitivity of summer balance to the clear-sky balance for 2009 ranges from 1.35 to 1.74 m w.e. (Table 6), index is ±10% for fcs ranging from 0.72 to 0.88. 13% of the reference model value. Based on these numerical We have observations of the timing and extent (water equiv- experiments, we adopt 15% as the uncertainty for modelled alent) of summer snow events in 2009, but this needs to be winter balance (i.e., Bw = 1.55 ± 0.23 m w.e.). parameterized for other years. Our two main methods for Net balance, Bn, has a greater uncertainty because of the this, randomly generated snow events and extrapolation of compounding uncertainties in Bw and Bs. We assess Bn for daily precipitation at Rogers Pass, give similar results for the 2008–09 at −0.77 ± 0.48 m w.e., with an average value of − summer 2009 mass balance: Bs = −2.33 and −2.32 m w.e., Bn = 0.24 ± 0.22 m w.e. from 2009 to 2011. Our best esti- respectively. The sensitivity of Bs to summer snowfall is eval- mates predict cumulative mass loss, with Illecillewaet uated by varying the number of summer snow events, relative Glacier near a state of balance in 2010–11. Within confidence

limits, modelled mass balance is consistent with observations extrapolated to the basin scale, because it is calibrated for a for this period. range of weather conditions, and the distributed pattern and Our error estimates are conservative relative to those typi- intensity of incoming solar radiation are physically based. cally reported in the literature but are realistic given our Finally, the chosen DEM and glacier mask can also be short period of study at the site. Direct measurements of expected to influence our results. We reran the analysis using stream discharge and additional summer albedo and melt a 95 m DEM available from the Shuttle Radar Topography observations would improve the confidence in summer melt Mission (SRTM; Farr et al., 2007). This represents the modelling. Targeted observations and a more sophisticated glacier topography and outlines in somewhat less detail, snow-accumulation model are also needed to help reduce the giving a glacierized area of 54.4 km2 in the IRB watershed 2 uncertainty in Bw. Development of a detailed snow distribution compared with 56.0 km using the 20 m DEM. Modelled model would be valuable (e.g., Dadic, Mott, Lehning, & Ber- IRB glacier runoff in 2009 with the SRTM topography is lando, 2010) and is recommended for future study. Snowpack 148 × 106 m3, compared with 150 × 106 m3 using the 20 m extent varies from year to year, but patterns of snow deposition DEM—1.3% less, mostly explained by the reduced glacier on glaciers typically recur, because snow redistribution cover. Specific discharge for the IRB is 1.5% higher with the through processes such as avalanching and wind scouring is 95 m DEM; Bs = −2.72 m w.e. compared with −2.68 m w.e. primarily a function of the terrain. However, for the same for the 20 m DEM. Hence, there are some differences, but reason, patterns of snow redistribution and measured altitudi- these are not significant relative to our overall uncertainly nal gradients on Illecillewaet Glacier are not necessarily repre- level of approximately 20%. sentative of the broader IRB. We therefore increase our uncertainty estimates in basin-scale estimates of Bw from 15 to 25%, in order to bracket available snowpack measurements c Climatic Sensitivity of Illecillewaet Glacier at Rogers Pass and Mt. Fidelity. Sensitivity of Illecillewaet Glacier mass balance to Basin-wide uncertainty in the melt model is also difficult to perturbations in temperature and precipitation is shown in assess because there is structural uncertainty associated with Fig. 9. This plots Bn and summer runoff for temperature extrapolation of the Illecillewaet Glacier melt model and perturbations, ΔT, from −3 to +3°C, relative to observed temp- Rogers Pass weather conditions to the entire basin. We eratures in summer 2009 and for winter accumulation (Bw) adopt the confidence limits derived for Illecillewaet Glacier scaled from 20 to 220% of the observed 2008–09 snowpack. to the IRB simulations (±18%) but acknowledge that this Model parameters are set to the reference values for the may underestimate the model uncertainty. We have more con- 2008–09 balance year. Simulated mass balance values and fidence in the ability of our empirical melt model to be runoff totals for illustrative experiments are given in Table 6. Downloaded by [University of Victoria] at 10:12 07 May 2013

Fig. 9 Sensitivity of modelled Illecillewaet Glacier runoff to perturbations in (a, c) summer temperature and (b, d) winter mass balance. (a, b). Winter (blue), summer (red), and net (black) mass balance, m w.e. (c, d). Glacier runoff (106 m3) from 1 May to 14 September for total runoff (black), snow melt (red), and ice melt (blue).

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 16 / J. M. R. Hirose and S. J. Marshall

The strong sensitivity to summer temperature is consistent Table 7. Modelled glacier runoff, Qg, which includes meltwater from the with mass balance observations from 2009 to 2011. A linear seasonal snowpack on the glaciers, Qs, and meltwater from glacier ice and firn, Qi. IRB yield is the total basin discharge measured at the Greeley Hydrometric fit to the Bn(T) curve in Fig. 9a gives a slope ∂Bn /∂T = −0.55 station for the period of study. The proportions of basin yield resulting from −1 m w.e. °C . A quadratic equation fits the Bn (T) curve better, total glacier melt, snowmelt, and ice melt are expressed as percentages, fg, fs, 2 and fi. with Bn = −0.7725 − 0.5458ΔT − 0.0374ΔT . This gives a temperature sensitivity ∂Bn/∂T = −0.5458 − 0.0748ΔT, which Modelled Runoff is equivalent to the linear line of best fit when evaluated at (106 m3) IRB yield fg fs fi the reference temperature (ΔT = 0). However, modelled temp- 6 3 Period Qi Qg Qs (10 m ) (%) (%) (%) erature sensitivity increases with the extent of warming as a 2009 149.8 82.4 67.4 1037 14.4 7.9 6.5 result of the increasing exposure of low-albedo glacial ice. May 2.2 2.2 0.0 180 1.2 1.2 0.0 An increase in the summer temperature anomaly ΔT from June 23.3 23.1 0.2 364 6.4 6.3 0.1 0 to 1°C corresponds to ∂B /∂T = −0.58 m w.e. °C−1, while July 62.7 46.5 16.2 271 23.1 17.2 6.0 n August 54.0 10.2 43.8 166 32.5 6.1 26.4 warming from ΔT = 1°C to ΔT = 2°C exhibits greater September 7.6 0.4 7.2 55 13.8 0.7 13.1 sensitivity: ∂B /∂T = −0.68 m w.e. °C−1. Hence, we can con- n 2010 107.2 71.2 35.9 1013 10.6 7.0 3.5 clude that the net balance impact of summer temperature May 1.8 1.8 0.0 185 1.0 1.0 0.0 − change is about −0.6 m w.e. °C 1, with a non-linear increase June 18.2 18.1 0.1 342 5.3 5.3 0.0 under greater warming. July 46.1 37.9 8.2 289 16.0 13.1 2.8 August 39.7 13.2 26.5 159 24.9 8.3 16.6 Mountain glacier sensitivity to summer temperature gener- September 1.4 0.3 1.1 38 3.7 0.7 3.0 ally exceeds that associated with variability in precipitation 2011 80.1 68.5 11.5 1380 5.8 5.0 0.8 (Braithwaite, Zhang, & Raper, 2002; Oerlemans and Reichert, May 0.8 0.8 0.0 242 0.3 0.3 0.0 2000). For Illecillewaet Glacier, a linear fit to the Bn (P) curve June 10.4 10.4 0.0 464 2.2 2.2 0.0 ∂ ∂ July 24.4 24.1 0.3 421 5.8 5.7 0.1 in Fig. 9b gives a slope Bn / P = +1.91 m w.e., indicating a August 35.1 28.2 6.8 202 17.4 14.0 3.4 net balance change of +0.19 m w.e. for a 10% increase in pre- September 9.4 5.0 4.4 51 18.4 9.8 8.5 cipitation. A quadratic fit to Bn (P) is again slightly stronger. Average, 112.3 74.1 38.3 1143 10.3 6.6 3.6 For a 30% increase in accumulation relative to the 2008–09 2009–11 winter balance, dBn = +0.58 m w.e., Illecillewaet Glacier requires a precipitation increase of 30% to offset a warming season were obtained from the Greeley gauging station of 1°C (i.e., to overcome a net balance loss of approximately (Environment Canada, 2012a). Mean annual discharge from − 0.58 m w.e.). This is consistent with the predictions of 1963 to 2011 at Greeley was 52.6 m3 s 1; mean annual dis- Oerlemans and Reichert (2000) for a continental, mid- charge rates for 2009, 2010, and 2011 were 41.2, 44.8, and − latitude mountain glacier but contrary to the conclusions of 54.8 m3 s 1, respectively, corresponding to a total runoff of − Bürger et al. (2011) at an adjacent basin in the 1300, 1412, and 1735 × 106 m3 a 1, respectively. Hence, dis- CCRB. Bürger et al. (2011) argue that a 10% increase in charge rates for 2009 and 2010 were below normal but slightly precipitation could offset a 1°C warming in the region, but above normal for 2011, in good accord with the winter precipi- precipitation in the IRB appears to be considerably less than tation observations in this snowmelt-dominated catchment. they report (in excess of 3000 mm). Our estimated summer The Greeley station records natural flows; we assume that temperature sensitivity is also about twice that of Bürger glacier meltwater will discharge through Greeley within the et al. (2011). month in order to compare monthly proportions of glacier

Downloaded by [University of Victoria] at 10:12 07 May 2013 We do not carry out future climate change assessments in runoff to the river yield for each ablation year (Table 7). this analysis because our objective is to characterize the con- We estimate that average glacier volume losses from 2009 temporary basin, but Table 6 includes mass balance and to 2011 were equivalent to 112 ± 12 × 106 m3, or 10% of the runoff estimates from Illecillewaet Glacier for projected total May to September discharge in the Illecillewaet River. future climate conditions in the basin (PCIC, 2010), a 2°C Glacier runoff contributions decreased from 2009 to 2011, summer warming and increases in winter snowpack of up to accounting for 14, 11, and 6% of basin yield (Fig. 10). The 20%. Summer balance is severely affected by the warming, highest discharges for 2009 and 2010 occurred in July, with with Bs = −3.46 m w.e. and Bn = −1.60 m w.e. for this scen- snowmelt being the primary contributor. As a result of ario. Increased snow cover delays the transition to bare ice, cooler temperatures, peak snowmelt did not occur until but the effect is minor relative to the increase in melt August 2011. As a proportion of Illecillewaet River flows, energy. Summer runoff is 20.6 × 106 m3 under this climate glacier runoff contributes most heavily in August, constituting scenario, 49% higher than the reference model (Table 3), 25% of basin yield from 2009 to 2011 and 33% in August although the proportion of runoff associated with seasonal 2009. Glacier runoff derived from ice and firn melt represents snow is similar at 55%. 16% of the total August yield in the basin. Glacial runoff can be separated into the fraction derived d Glacier Contributions to Discharge in Illecillewaet Basin from melting of the seasonal snowpack and that associated To compare modelled glacier runoff to Illecillewaet River dis- with melting of glacier ice and firn. Seasonal snow runoff charge, monthly streamflows during the 2009–11 glacier melt accounts for 66% of the glacier runoff for the three years,

increase would come from ice melt, concentrated from July to September. Our comparison with IRB yields at Greeley station neglects meltwater losses to evaporation, delays that may be introduced by glacier or groundwater storage, and the potential ecological influences of the river’s downstream hydrological balance. Hydrological routing through the glacier and through the basin can introduce delays from days to weeks (Willis, Arnold, & Brock, 2002), particularly during the early melt season when runoff is delayed by the snowpack and the subglacial drainage system can be inefficient (Fountain & Walder, 1998). Our estimated proportions of glacier runoff to streamflow should, therefore, be treated cautiously with respect to timing of meltwater delivery to the Illecillewaet River. Terrain in the IRB is steep and well drained, particularly in the late summer when the seasonal snowpack has receded. We therefore expect that monthly and total summer runoff estimates are representative. However, daily discharges cannot be compared to basin values without a model of hydrological routing that considers delays and storage terms (e.g., Moore, 1993; Stahl et al., 2008). Our mass balance and runoff studies are short term, and we Fig. 10 Modelled glacier runoff in the IRB, 1 May to 14 September, 2009 to have not sampled a full range of interannual variability in the 2011, partitioned into snow (red), ice (blue), and total (black) runoff. IRB. The conditions during the three years assessed in this study differed: dry, warm, and clear in 2009; cool and cloudy in 2010; wet, cool, and cloudy in 2011. Meteorological while melting of glacier ice and firn makes up the remaining conditions from 2009 to 2011 were typical with respect to the 34%. The percentage of glacier runoff resulting from ice and climate normals for Rogers Pass (within one standard devi- firn melt increases continually from May to September, aver- ation), the most proximal site with long-term meteorological aging 0, 1, 20, 62, and 69%, in the respective months. This observations. Our results are likely to be representative of essentially tracks the depletion of the seasonal snowpack “normal years” in the basin, for the current glacier extent and exposure of glacial ice and emphasizes the importance and hypsometry; past and future magnitudes of glacier of August and September for the streamflow that is derived runoff will be sensitive to the evolution of the glacierized from glacier storage. area in the basin. A heavy winter snowpack or cool summer can dramatically reduce late-summer ice melt, as evidenced in 2011 (Table 7) and several of the sensitivity tests summarized in Table 6. In e Modelling of Other Time Periods in Illecillewaet Basin these cases, total runoff is reduced and the fraction of snow- Further research is needed to extend the model to historical

Downloaded by [University of Victoria] at 10:12 07 May 2013 melt to total glacier runoff increases. In contrast, a low runoff reconstructions and future projections of glacier and winter snowpack or warm summer causes ice to be exposed hydrological change in the IRB. The distributed mass earlier in the season, increasing meltwater runoff and glacier balance and runoff models can be driven by historical (e.g., depletion. Rogers Pass) station data. This works well for summer mass Rango et al. (2008) assess glacier storage contributions to balance in our study period, with results comparable to the Illecillewaet River using a simplified hydrological model. direct observations on Illecillewaet Glacier (statistically equiv- They conclude that for a temperature increase of 4°C, glacier alent Bs and runoff estimates, within uncertainty bounds). − storage (ice) will contribute 134 × 106 m3 a 1 to the IRB. In Extrapolation of winter mass balance from historical station the warm, dry year of 2009, glaciers in the IRB generated records to high-elevation glacial environments is less 150 × 106 m3 of runoff in our model, with 67 × 106 m3 certain, and methods for this need to be developed. derived from glacier ice. Sensitivity analysis indicates a rate In general, the method of estimating summer snow events − of increase in total glacier discharge of 3.7 × 106 m3 °C 1 for from Rogers Pass daily precipitation (with appropriate temp- − Illecillewaet Glacier (Fig. 9c) or 27% °C 1. If warming is erature adjustments to the glacier) is favourable because this − accompanied by an increase in precipitation of 10% °C 1 the provides a more deterministic treatment of the influence of − − sensitivity is reduced to 3.4 × 106 m3 °C 1 or 24% °C 1. Apply- interannual temperature and precipitation variability. It may ing this temperature sensitivity to the basin, it is expected that not be as helpful for future projections, however, where runoff will approximately double under a warming of 4°C, station data are unavailable and climate models are not gener- equivalent to the results of Rango et al. (2008). Most of this ally reliable with respect to precipitation frequency (e.g.,

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 18 / J. M. R. Hirose and S. J. Marshall

Bader et al., 2008). We recommend the random snow-event Average modelled mass balance from 2009 to 2011 for parameterization where the mass balance model is driven by Illecillewaet Glacier is Bw = 1.46, Bs = −1.70, and Bn = output from climate models. −0.24 m w.e. a−1. This compares reasonably well with Ice thickness and volume also need to be known to simu- average observed mass balances of Bw = 1.43, Bs = −1.91, −1 late ice dynamics and glacier evolution. Slope-thickness and and Bn = −0.48 m w.e. a . A composite of our observed volume-area scaling calculations can be used to estimate ice and modelled balances gives an average glacier runoff of volume, but these are highly uncertain and not applicable for 10.9 ± 2.5 × 106 m3 for the three summers, with 24% of this local ice thickness data (Clarke, Berthier, Schoof, & Jarosch, derived from glacier ice and firn. The remaining 76% of Illecil- 2009; Farinotti, Huss, Bauder, Funk, & Truffer, 2009). To lewaet Glacier runoff comes from melting of the seasonal snow- our knowledge, there are no measurements of glacier ice pack, including a small contribution from summer snows. thickness within the CCRB. As glacier area and volume are Extended to the basin scale, modelled mass balances are depleted, glacier contributions to runoff will decline, which lower, specific discharges are higher, and the fraction of mod- will be of particular importance in late summer. It is elled glacier runoff derived from ice melt is higher, 34% on necessary to quantify glacier area and volume changes to average. In a heavy melt year such as 2009, more than 45% understand the regional impacts of diminishing glacier of glacier runoff in the basin is attributed to ice and firn melt. runoff. We recommend that ice thickness characterization One of the largest uncertainties in our observations and our be part of a sustained monitoring program within the IRB model is associated with winter snowpack and its distribution or other headwaters catchments of the CCRB. Future work to the basin scale. Our melt model is calibrated for a range of should extend the mass balance and high-elevation weather conditions measured throughout the ablation season, meteorological observations in the region and aim to refine improving our confidence in the summer mass balance esti- the snowmelt and accumulation models at the basin mates. Nevertheless, additional melt data (i.e., a repeat of scale. Historical weather data are available in the region to the intensive observations conducted in summer 2009) characterize glacier runoff over the last several decades, would improve calibration and confidence in the melt model but such an analysis requires a treatment of evolving parameters, reducing uncertainties in our mass balance and glacier geometry. melt model estimates. Based on climate sensitivity analysis, we conclude that a 1°C warming on Illecillewaet Glacier requires an approximate 30% 6 Conclusions increase in winter precipitation, to overcome a net balance per- Through combined field and modelling studies, we examine turbation of −0.58 m w.e. Under an increase in temperature of the meteorological and mass balance regime of Illecillewaet 2°C accompanied by a precipitation increase of 20%, relative to Glacier. Observations are used to develop and validate an our reference summer (2009), runoff increases by 49% while empirical melt model to estimate glacier runoff contributions net mass balance declines from −0.77 to −1.60 m w.e. Future to Illecillewaet basin, a headwaters catchment of the Columbia projections and climate change studies require a dynamic ice River. Required inputs for our model are a DEM, glacier mask, model to assess the impacts of geometric and hypsometric hourly temperature, and winter mass balance (initial May changes on glacier mass balance and runoff. snowpack). Where available, daily humidity and precipitation Our results provide preliminary estimates of glacier contri- data can also be applied in estimation of daily cloud cover butions to streamflow in the basin, broadly consistent with (incident solar radiation) and summer snowfall. the hydrological modelling results of Jost et al. (2012),

Downloaded by [University of Victoria] at 10:12 07 May 2013 We find little degradation in model performance for Illecil- which assess a neighbouring basin with similar glacier lewaet Glacier when we drive the melt model with nearby extent. Glacier contributions to Illecillewaet Basin averaged valley-bottom weather records from Rogers Pass, rather than 112 ± 12 × 106 m3 from 2009 to 2011, with 66% of this using local AWS temperature and radiation data. This requires from seasonal snow and 34% from glacier ice and firn. This a two-stage adjustment of Rogers Pass temperature data to represents 10% of the summer Illecillewaet River yield from make it applicable on the glacier: (i) an elevation correction 2009 to 2011. The most significant glacier contributions and (ii) a diurnal temperature correction, to account for occur in August, when glacier runoff constituted 25% of the daytime heating and overnight cold air drainage in the Illecillewaet River from 2009 to 2011. Glacial runoff valley bottom. This was accommodated by applying a sinusoi- exceeded 32% of Illecillewaet River yield in August 2009, dal temperature correction to Rogers Pass temperatures. with more than 80% of this derived from glacier ice. Potential direct solar radiation is scaled through a clear-sky Winter precipitation and summer temperature conditions for index (fraction of transmitted radiation), which is calibrated 2009 to 2011 fall within one standard deviation of the from AWS data for summer 2009. Daily temperature ampli- “normal” conditions for the region, based on records at tude and vapour pressure at Rogers Pass provide a good Rogers Pass that date to 1966. Hence, these years are represen- proxy for the clear-sky index, and we use this to estimate tative of the past 46 years. The IRB is a typical headwaters incoming solar radiation in 2010 and 2011, when AWS data catchment of the CCRB with respect to its terrain, glacier are not available. The melt model is then applied to Illecille- cover, and climate regime. Our results emphasize the impor- waet Glacier and the IRB. tance of glacier contributions to late-summer streamflow in

the region and the need to characterize the sensitivity of this meteorological data collection by Environment Canada has water resource to ongoing climate change and glacier retreat. been essential to this study. The Western Canadian Cryo- spheric Network, funded by the Canadian Foundation for Climate and Atmospheric Sciences, created a comprehensive Acknowledgements glacier inventory of western Canada and sponsored work- We thank the Natural Sciences and Engineering Research shops that enabled networking and collaboration. Roger Council (NSERC) of Canada and Parks Canada for support Wheate provided additional assistance. Mt. Revelstoke and of the Illecillewaet Glacier fieldwork. The University of Glacier National Parks and the glaciology group at Natural Calgary and the Canadian Institute for Advanced Research Resources Canada have been instrumental in taking on Illecil- provided additional support. Ongoing hydrological and lewaet Glacier as a long-term mass balance monitoring site.

References Anslow, F. S., & Roddenhuis, D. (2011). Why was this spring and early summer Demuth, M., Pinard, V., Pietroniro, A., Luckman, B., Hopkinson, C., Dornes, so cool in British Columbia. PCIC Update. Retrieved from http:// P., & Comeau, L. (2008). Recent and past-century variations in the glacier pacificclimate.org/sites/default/files/publications/PCIC.UpdateSpecial.12Sep resources of the Canadian Rocky Mountains: Nelson River system. Special 2011.pdf Issue: Mountain glaciers and climate changes of the last century. Terra Arnold, N. S., Willis, I. C., Sharp, M. J., Richards, K. S., & Lawson, W. J. Glacialis, 11,27–52. (1996). A distributed surface energy-balance model for a small valley Déry, S. J., Stahl, K., Moore, R. D., Whitfield, P. H., Menounos, B., & glacier. I. Development and testing for Haut Glacier d’Arolla, Valais, Burford, J. E. (2009). Detection of runoff timing changes in pluvial, nival Switzerland. Journal of Glaciology, 42(140), 77–89. and glacial rivers of western Canada. Water Resources Research, 45, Bader, D. C., Covey, C., Gutowski, W. J., Held, I. M., Kunkel, K. E., Miller, W04426. 1–11. R. L., … Zhang, M. H. (2008). Climate models: An assessment of strengths Environment Canada. (2012a) Water Survey of Canada HYDAT data. and limitations. Synthesis and Assessment Product 3.1, U.S. Climate Retrieved from http://www.wsc.ec.gc.ca/products Change Science Program. Department of Energy, Washington, DC. Environment Canada. (2012b) National climate data and information archive. Bitz, C. M., & Battisti, D. (1999). Interannual to decadal variability in climate Retrieved from http://www.climate.weatheroffice.gc.ca/ClimateData and the glacier mass balance in Washington, western Canada and Alaska. Farinotti, D., Huss, M., Bauder, A., Funk, M., & Truffer, M. (2009). A method Journal of Climate, 12, 3181–3196. to estimate ice volume and ice thickness distribution of alpine glaciers. Bolch, T., Menounos, B., & Wheate, R. (2010). Landsat-based inventory of Journal of Glaciology, 55(191), 422–430. glaciers in western Canada, 1985–2005. Remote Sensing of Environment, Farr, T. G., Rosen, A. P., Caro, E., Crippen, R., Duren, R., Hensley, S., … 114, 127–137. Alsdorf, D. (2007). The Shuttle Radar Topography Mission. Reviews of Braithwaite, R. J. (1995). Positive degree-day factors for ablation on the Geophysics, 45, RG2004. doi:10.1029/2005RG000183 Greenland ice sheet studied by energy-balance modeling. Journal of FCRPS (Federal Columbia River Power System). 2001. The Columbia River Glaciology, 41(137), 153–160. system inside story. Retrieved from http://www.bpa.gov/power/pg/ Braithwaite, R. J., Zhang, Y., & Raper, S. C. B. (2002). Temperature sensi- columbia_river_inside_story.pdf tivity of the mass balance of mountain glaciers and ice caps as a climatolo- Fountain, A. G., & Tangborn, W. V. (1985). The effect of glaciers on stream- gical characteristic. Zeitschrift fur Gletscherkunde und Glazialgeologie, 38 flow variations. Water Resources Research, 21(4), 579–586. (1), 35–61. Fountain, A. G., & Walder, J. S. (1998). Water flow through temperate gla- Bristow, K., & Campbell, G. S. (1984). On the relationship between incoming ciers. Reviews of Geophysics, 36, 299–328. solar radiation and daily maximum and minimum temperature. Agriculture Hägeli, P., & McClung, D. M. (2003). Avalanche characteristics of a transi- and Forest Meteorology, 31(2), 159–166. tional snow climate—Columbia Mountains, British Columbia, Canada. Brock, B. W., Willis, I. C., Sharp, M. J., & Arnold, N. S. (2000). Modelling Cold Regions Science and Technology, 37, 255–276. seasonal and spatial variations in the surface energy balance of Haut Glacier Hamlet, A. F., & Lettenmaier, D. P. (1999). Effects of climate change on Downloaded by [University of Victoria] at 10:12 07 May 2013 d’Arolla, Switzerland. Annals of Glaciology, 36,53–62. hydrology and water resources in the Columbia River Basin. Journal of Bürger, G., Schulla, J., & Werner, A. T. (2011). Estimates of future flow, the American Water Resources Association, 35, 1597–1623. including extremes, of the Columbia River headwaters. Water Resources Hamlet, A. F., Mote, P. W., Clark, M. P., & Lettenmaier, D. P. (2005). Effects Research, 47, W10520. doi:10.1029/2010WR009716 of temperature and precipitation variability on snowpack trends in the Champoux, A., & Ommanney, C. S. L. (1986). Evolution of the western United States. Journal of Climate, 18, 4545–4561. Illecillewaet Glacier, Glacier National Park, B.C., using historical data, Hirose, J. M. R. (2012). Glacier meltwater contributions and glaciometeoro- aerial photography and satellite image analysis. Annals of Glaciology, logical regime of the Illecillewaet River Basin, a headwaters catchment of 8,31–33. the Upper Columbia River Basin, British Columbia, Canada. (M.Sc. Clarke, G. K. C., Berthier, E., Schoof, C. G., & Jarosch, A. H. (2009). Neural thesis). University of Calgary, Alberta, Canada. networks applied to estimating subglacial topography and glacier volume. Hock, R. (1999). A distributed temperature-index ice- and snowmelt model Journal of Climate, 22, 2146–2160. including potential direct solar radiation. Journal of Glaciology, 45(149), Cohen, S. J., Miller, K. A., Hamlet, A., & Avis, W. (2000). Climate change 101–111. and resource management in the Columbia River Basin. Water Hock, R. (2003). Temperature index melt modeling in mountain areas. International, 25(2), 253–272. Journal of Hydrology, 282(1–4), 104–115. Comeau, L. E. L., Pietroniro, A., & Demuth, M. N. (2009). Glacier contri- Hopkinson, C., & Young, G. J. (1998). The effect of glacier wastage on the bution to the North and South Saskatchewan Rivers. Hydrological flow of the Bow River at Banff, Alberta. Hydrological Processes, 12, Processes, 23, 2640–2653. 1745–1762. Dadic, R., Mott, R., Lehning, M., & Berlando, P. (2010). Wind influence on Huss, M., Farinotti, D., Bauder, A., & Funk, M. (2008). Modelling runoff snow depth distribution and accumulation over glaciers. Journal of from highly glacierized alpine drainage basins in a changing climate. Geophysical Research, 115, F01012. doi:10.1029/2009JF001261 Hydrological Processes, 22, 3888–3902.

ATMOSPHERE-OCEAN iFirst article, 2013, 1–20 http://dx.doi.org/10.1080/07055900.2013.791614 Canadian Meteorological and Oceanographic Society 20 / J. M. R. Hirose and S. J. Marshall

Jóhannesson, T., Sigurdsson, O., Laumann, T., & Kennett, M. (1995). Pellicciotti, F., Brock, B., Strasser, U., Burlando, P., Funk, M., & Corripio, J. Degree-day glacier mass-balance modelling with applications to glaciers (2005). An enhanced temperature-index glacier melt model including the in Iceland, Norway and Greenland. Journal of Glaciology, 41(138), shortwave radiation balance: Development and testing for Haut Glacier 345–358. d’Arolla, Switzerland. Journal of Glaciology, 51(175), 573–587. Jost, G., Moore, R. D., Menounos, B., & Wheate, R. (2012). Quantifying the Pepin, N., & Losleben, M. (2002). Climate change in the Colorado Rocky contribution of glacier runoff to streamflow in the upper Columbia River Mountains: Free air versus surface temperature trends. International basin, Canada. Hydrology and Earth System Sciences, 16, 849–860. Journal of Climatology, 22, 311–329. Kite, G. W. (1997). Simulating Columbia River flows with data from regional- Petersen, L., & Pellicciotti, F. (2011). Spatial and temporal variability of air scale climate models. Water Resources Research, 33(6), 1275–1285. temperature on a melting glacier: Atmospheric controls, extrapolation Klok, E. J., & Oerlemans, J. (2004). Modelled climate sensitivity of the mass methods and their effect on melt modeling, Juncal Norte Glacier, Chile. balance of Morteratschgletscher and its dependence on albedo parameteri- Journal of Geophysical Research, 116, D23109. doi:10.1029/ zation. International Journal of Climatology, 24, 231–245. 2011JD015842 Loukas, A., Vasiliades, L., & Dalezios, N. R. (2002). Climatic impacts on the Rango, A., Martinec, J., & Roberts, R. (2008). Relative importance of glacier runoff generation processes in British Columbia, Canada. Hydrology of contributions to water supply in a changing climate. World Resource Earth System Sciences, 6(2), 211–227. Review, 20, 487–503. Marshall, S. J., White, E. C., Demuth, M. N., Bolch, T., Wheate, R., Richardson, A. G., & Reddy, K. R. (2004). Assessment of solar radiation Menounos, B., … Shea, J. M. (2011). Glacier water resources on the models and temporal averaging schemes in predicting radiation and cotton eastern slopes of the Canadian Rocky Mountains. Canadian Water production in the southern United States. Climate Research, 27,85–103. Resources Journal, 36(2), 109–133. Shea, J. M., & Moore, R. D. (2010). Prediction of spatially distributed Moore, R. D. (1993). Application of a conceptual streamflow model in a regional-scale fields of air temperature and vapour pressure over mountain glacierized drainage basin. Journal of Hydrology, 150, 151–168. glaciers. Journal of Geophysical Research–Atmospheres, 115, D23107. Moore, R. D. (2006). Stream temperature patterns in British Columbia, doi:10.1029/2010JD014351 Canada, based on routine spot measurements. Canadian Water Resources Shea, J. M., Moore, R. D., & Stahl, K. (2009). Derivation of melt factors from Journal, 31,41–56. glacier mass-balance records in western Canada. Journal of Glaciology, 55 Moore, R. D., & Demuth, M. N. (2001). Mass balance and streamflow varia- (189), 123–130. bility at Place Glacier, Canada, in relation to recent climate fluctuations. Sidjak, R. W., & Wheate, R. D. (1999). Glacier mapping of the Illecillewaet Hydrological Processes, 15, 3473–3486. ice field, British Columbia, Canada, using Landsat TM and digital elevation Moore, R. D., Fleming, S. W., Menounos, B., Wheate, R., Fountain, A., Stahl, data. International Journal of Remote Sensing, 20(2), 273–284. K., … Jakob, M. (2009). Glacier change in western North America: Singh, P., & Kumar, N. (1997). Impact assessment of climate change on the Implications for hydrology, geomorphic hazards and water quality. hydrological response of a snow and glacier melt runoff dominated Hydrological Processes, 23,42–61. Himalayan river. Journal of Hydrology, 193, 316–350. Oerlemans, J., & Klok, E. J. (2004). Effect of summer snowfall on glacier Stahl, K., Moore, R. D., Shea, J. M., Hutchinson, D. G., & Cannon, A. (2008). mass balance. Annals of Glaciology, 38,97–100. Coupled modeling of glacier and streamflow response to future climate Oerlemans, J., & Reichert, B. K. (2000). Relating glacier mass balance to scenarios. Water Resources Research, 44, W02422. doi:10.1029/ meteorological data by using a seasonal sensitivity characteristic. Journal 2007WR005956 of Glaciology, 46(152), 1–6. Wagnon, P., Ribstein, P., Kaser, G., & Berton, P. (1999). Energy balance and Oke, T. R. (1987). Boundary layer climates (2nd ed.). London: Routledge runoff seasonality of a Bolivian glacier. Global and Planetary Change, 22, Press. 49–58. PCIC (Pacific Climate Impacts Consortium). (2010). Summary of climate Willis, I., Arnold, N., & Brock, B. (2002). Effect of snowpack removal on change for Columbia-Shuswap in the 2050s. Retrieved from http://www. energy balance, melt and runoff in a small supraglacial catchment. plan2adapt.ca/tools/planners?pr=8&ts=8&toy=16 Hydrological Processes, 16, 2721–2749. Downloaded by [University of Victoria] at 10:12 07 May 2013

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