Mass-Balance Measurement

The North Cascades mountains are host to approximately 725 glaciers (Fig.1) (Reference Post, Richardson, Tangborn and RosselotPost and others. 1971; Reference PeltoPelto. 1993). The North Cascade Glacier Climate Project (NGGCP) was founded Io identify the response of North Cascade glaciers to regional climate change. Reference Ebbesmeyer, Cayan, Mclain, Nichols, peterson, Redmond, Betancourt and TharpEbbesmeyer and others (1991) noted the broad impact of a regional climate change occurring in 1976. identifying a signifieant shift in 40 environmental factors that are sensitive to climate. Glacier response to alpine weather patterns and climate is complicated In local effects. Thus, to understand the causes of and nature of changes in glacier mass balance, it was necessary to monitor a significant number of glaciers. Since 1984. the North Cascade Glacier Climate Project has monitored annual net balance on at least eight North Cascade glaciers (Reference PeltoPelto, 1988.Reference Pelto1993) To monitor this Number of glaciers in the United Stales is not feasible financially or logisiically using standard mass-balance methods. A modified stratigraphic method based on an Observable summer surface is used to reduce logislical costs.

Fig. 1. Location of the nine glaciers on which annual balance measurements have been made by NCGCP from 1984 to 1993. C, Columbia; D, Daniels; F,Foss; L. Lower Curtis P. Yawing; R, Rainbows; S. Spider; Y. lynch.

Annual balance is the different between annual snow accumulation and snow firn ice melt (ablation). Mass-balance measureinenis are made on the same date each year in August and again in late September close to the of the ablation season. The late September date is considered the end of the hydrologic Year for that glacier. Any mass-balance changes occurring before the actual accumulation season begins is a measured mass loss or gain for the next hydrologic year. Annual ice and firn ablation (firn and ice net balance: Reference Mayo, Meir and TrngbornMayo and others, 1972) is determined using ablation stakes drilled into the glacier surface and simultaneously checked on the same date in late September. Residual mow accumulation (final late snow balance: Reference Mayo, Meir and TrngbornMayo and others, 1972) at the end ol the ablation season is determined using probing and erevasse Stratigraphy on the same date as ablation measurements are completed. The methods used are patterned after mass-balance studies on Lemon Greek Glacier, Alaska, and Blue Glacier, Washington (Reference Heusser and MarcusHeusser and Marcus 1964; Reference LaChapelleLaChapplle 1965; Reference Armstrong and OerlemansArmstrong 1989). The Only difference between of the methods used on Blue Glacier (Reference LaChapelleLaChapelle. 1965; Reference Armstrong and OerlemansArmstrong. l989) and in this study are: (1) snow density is not measured: 2 an order ol magnitude higher measurement density is used.

Only accumulation measurements are made above the snow line in the accumulation zone. annual accumulation-layer thickness is determined using crevasse stratigraphy and probing. Measurements are made in Aiignsi and again in late September. The August measurements are made to determine snowmelt run-off for the late summer period and are not used in the final annual balance assessment. The average density of measurements utilizedd in this study is 290 points km² while the average density of measurements used in assessing the mass balance in the accumulation zone of other Canadian. Norwegian. Swiss and United Stales glaciers is 33 points km² (Reference PyttePytte. 1969; Reference Meir, Thangborn, Aayo and PostMeier, and others. 1971) Table 1. This higher densitv is achieved In focusing on time-efficient measurement methods.

The accumulation-layer thickness is measured at each point to the nearest 0.01 m. Crevasse stratigraphy illeasnremenis are conducted only in vcrliiallv walled crevasses with distinguishable dirt bands. Crevasses lacking ventral walls yield inaccurate depth measure-ments. In the North Cascades the ablation surface Qf the previous year is always marked by 2-5 cm thick band of dirty firn or glacier ice. The depth to the top of this dirty band is measured at several points on each crevasse wall within a disianiee of several meters. The average thickness Of the several points is laken to be the accumulation-layer thickness at that location.

We completed more than 100 snow pits lora 1984 to 1986, the tauge in mean accumulation-layer density observed was 0.58 0.63 Mg m^-3. This narrow range indicates thai late in the ablation season the density of snowpaek on North Cascade glaciers is uniform and need not be measured to determine mass balance. For this reason, snow pits an- no longer utilized. The lack of density variation has been observed in the two other mass-balance programs in Washington on IiUic Glacier and South Cascade Glacier (Reference Meir and ThangbornMeier and Tangborn. 1965; Reference Armstrong and OerlemansArmstrong, 1989). Of equal importance is that the range of density variation is of the same order as the density-measurment error, determined through repeat measurements. Since North Cascade glaciers rarely have ice lenses an indicator of little internal accumulation), probing is an accurate method of measuring accumulation-layer thickness. The lack of ice lenses is also crucial to having a constant snow density. The probe is driven through the snowpack until the previous ablation surface is reached; this suirface of glacier ice or dirty hard firn cannot be penetrated. The probing instrument is a 1/2in [l.27 cm] thick type I. copper tube which is driven through using a 1 kg weight.

The accuracy of crevasse stratigraphy and probing measurements is cross-cheeked by comparing for Consistency. On each glacier, at least 25% of the accumulation area is zone of overlap where both probing a nd Crevasse stratigraphy are used. This cross-checking identifies measurement points that either represent an ice lens and not die previous summer surface in the case of probing or areas where crevasses do not yield representative accumulation depth in the case of crevasse stratigraphy.

The standard deviation in snow depth obtained in cross-checking and duplicate measurements is smallest for crevasses stratigraphy. 0.02 m. and 0.03 m Ior probing. Ihe narrow range of deviation in vertically walled crevasses indicates that they do vield Consistent and representative accumulation depths late in ihc summer. After a decade of predominantly negative mass balance, the number of crevasses that are suffciently open to complete measurments has declined significantly.

To ensure that mass-balance measurements are consistent from year to year, measurements are made at a spatially fixed network of points using the same methods on the same date each year. The network is fixed spatially with respect to the surroundings bedrock walls. A typical measurement network is shown in Figure 2. Each measurement network covers a glaciei S entire accumulation zone with a reasonably consistent density of measurements, Because all of the sites are accessed by backpacking, it has not been a problem to reach each glacier at the right date.

Fig. 2. Mass-balance measurement network on Columbia Glacier in 1991. Measurments are made at each of these points at the same time each year. Annual balanace contours are then constructed.

Mass-balance measurements are in water equivalent The product of the accumulation-layer thickness and density of the snowpack (0.60 Mg M⁻³) yields the water equivalent Errors in depth measurement are less than ±0.05 m and ±0.02-0.03 Mg³ is the error due to density variation. I resulting error in annual assessment for the accumulation zone ranges from ±0.10 to ±0.15 m.

Below the snow line, ablation stakes emplaced in a triangular pattern are used lo determine annual ablation. An ablation triangle consists of three stakes driven or drilled into the ice at 3 m spacing. forming an equilateral triangle. Three to four triangles are emplaced on each glacier. Ablation stakes our white wooden pules 3.3 m long. This length was chosen as longer stakes are too cumbersome to transport and emplace, and shorter stakes tend to meltsout Ablation measurements are made at nine points on the triangle periphery from a cross bar resting upon two of the ablation stakes to the firn or ice surface. Measurements are made in late july and early August, recording the ablation season during the 3 months of the ablation season. After re-drilling if necessary in August, ablation measurements in- repeated in late September at the designated conclution of the hydrologic year.

Abalation triangles are placed in a sequence from regions that first lose their snow cover to regions where snow cover to persists for a significant part of the ablation season. Each ablation triangle is lheu representative of ablation for other parts ol the glacier that lose their snowpack simultaneously. Ablation variation over the entire ablation season at the difierent points on the periphery of a single ablation triangle are insignificant Thus individual ablation stakes will be used in future years. If a stake melts out. it is not utilized in assessing ablation. Stake melt-out has been infrequent; two stacks melted out in 1987 on Lower Curtis Glacier and in 1992 on Rainbow Glacier.

The error in annual ablation measurements is estimated at ±0.25 m, due to ice-density variations, low sampling density and stack settling, This estimate is based on the standard deviation in ablation along 50 m long transects with ablation stakes placed 5 m apart. Two of these transects were emplaced on Columbia Glacier 1987 and on Rainbow Glacier in 1991. There are at least three ablation-measurement sites on each glacier. The sampling density is low at 6-20 points km². but comparable to the mean density of 3-17 points km² used by the USGS and NVE (Table 1).

A mass-balance map for the entire glacier is then compiled tor each glacier. The mass balance Iiir the entire glacier is calculated by summation of the product of glacier area within each 0.10 m mass-balance contour and the net balance of thai interval. The error in mass-balance calculation for the entire glacier is ±0.17 0.22 m. The annual balance from 1984 to 1993 for the nine North Cascade glaciers and in 1994 for eight glaciers is shown in Table 2. Spider Glacier will no longer be monitored: it could not be reached in 1994 because of forest fires in the watershed. The mean annual balance of 0.38 m1 for the eight glaciers during the 1984-94 period is a mean loss of 3.5-5.0 m of glacier thickness during the last 11 years. This is a significant amount given the thin nature of North Cascade glaciers estimated to range from 30-60 m (Reference Post, Richardson, Tangborn and RosselotPost and others 1971).

Cross-Correlation of Net Annual Balance

Table 3 contains the cross-correlation of annual balance for the eight North Cascade glaciers. The high cross-correlations (0.83-0.97) indicate the similarity of each glacier’s annual balance response to the annual climate Conditions. Figure 3 displays the annual balance of the eight North Cascade glaciers observed bv NGGP from 19814lo 1994. The trend from year to year is quite consistent, illustrating the high cross-correlations. The actual range in annual balance between the glaciers Ibr any given year is significiant (Fig. 3).

Fig. 3. The annual balance of eight North Cascade glaciers. Note that there is a Significant range in annual balance between the glaciers and that the pattern of change from year to year is similar for of the glaciers.

The individual glaciers were selected to represent a range ol geographic and topographic characteristics. Geographic characteristics of each glacier are given ill Table 4.

The moderate range of variation in annual balance makes distinguishing which geographic characteristics are most important in determining climate sensitivity difficult. This tendency for small alpine glac iers in the Pacific Northwest to have different in mass-balance histories, yet high cross-correlation coefficients, was previously noted by Reference Letréguilly and ReynaudLetrréguilly and Reynaud (1989). Reference Letréguilly and ReynaudLetrréguilly and Reynaud (1989) compared the mean annual balance histories from 1956 to 1985 of Blue Glacier, Olympic Mountains (+0.3.¹) and South Cascade Glacier. North Cascades (0.45 ma¹). which were very different. However, their sensivity to specific climate conditions, as indicated by a cross-correlation coeificient of 0.69, was quite high For two glaciers in different, though adjacent, mountatain ranges.

Reference PeltoPelto (1988). in examining the first several years mass-balance data. postulated that the variation in annual balance between glaciers is due tu their different geographic characteristics. The high cross-correlation of annual balance of each glacier suggests that lhe geographic characteristics are of secondary importance to acutal climate conditions but does not suggest that geographic characteristics are unimportant. The mean annual balance differences between glaciers do reflect important differences that are probably the result of changing geographic characteristics. The annual balance record is insufficient to assess fully this hypothesis yet.

Net Annual Balance Climate Correlation

A comparison of the long-term and short-term mean for monthly precipitation and temperature Irotu die eight NOAA Slate of Washington Division .5 Weather Stations (Cascade Mountains) illuslrates three important climate changes in the North Cascades Ior the 1984-944 period. 1 Mean ablation-season temperalnrr has been 1.1°C above the long-term mean (1950-80). (2) Winter precipitation has been 11% below the long-term mean. (3) Mean April-June temperature has been 1.3°C above the long-term mean. All three of these changes lead to more negative balances and have been the cause ol the rapid glacial relreat that has occurred in the North Cascades during the last 5-7 years (Reference PeltoPelto, 1993).

The four primary climatic variables affecting North Cascade glacier are ablation-season temperature, accumulation-season precipitation. summer cloud cover and May and October freezing levels (Reference TangbornTangborn, 1980) Since summer cloud cover is not monitored in the North Cascade region, this parameter cannot be examined. Reference PorterPorter (1977) and Reference TangbornTangborn (1980) demonstrated that summer cloud cover is highly correlated With summer temperature. is inherently included in the temperature, record and is not an independent variable. Freezing-level elevations arc incorporated by including onlv Mav and October precipitation occurring when Stevens Pass temperature is below 7 C as accunuilation-season precipitation

Comparison between net annual balance lor each glacier and accumulation-season and ablation-season eondilions at NOAA Washington Stale Division 5 weather stations is presented in Table 5. Four different measurement methods accumulation-season precipitation (ppt). are used: (1) October March ppt.. (2) October April ppt, (3) November March ppt,, (4) all precipitation from October to May that falls when the temperature at Stevens Pass is below 7°C Precipitation data used is the monthly mean for Division 5 weather stations. Weather records from 11 individual weather stations were also correlated with annual balance but each yielded lower correlalion coefficients than the Cascade Mountain Division record, probably due to the significant local changes in precipitation for many storm events.

The highest correlation coefficients were for measurement method 4 (all precipitation from October to May that falls when the temperature- al Stevens Pass is below 7°C) ranging from 0.36 lo 0.59. Only on Yawning Glacier was another method as accurate: method I (October March ppt.) with an identical correlation coefficient. This demonstrates the robustness of method 4.

During the ablation season, four elimate variables were used: (1) May August mean lemperalure. (2) Mav September mean temperature, 3 June August mean temperature. (4) June September mean temperature. Ieniperaiiires used were monthly means from Division 5 Weather stations. Method 1 (May August mean temperature) proved to be the most accurate with correlation coefficients ranging from 0.63 to 0.84. This was true on all but Lynch Glacier and Rainbow Glacier. have the least negative mean annual balance and highest mean accumulation-Zone altitude of the eight glaciers. These two glaciers were more closely related to June-September temperature, Except for Lynch and Rainbow Glaciers. method 1 yielded correlation Coefficients between 0.7.5 and 0.84.

The correlation between annual net balance is higher for ablation-season temperature than for a accumpulation-sesson temprature. This does not demonstrate that the glaciers are more sensitive to ablaliou-sesson conditions. It is mote- likely a result of temperature being a better measure of ablation than preripilalion of actual accumulation.

Conclusion

The annual balance of North Cascade glaciers between 1984 and 1994 has been moderately negative at −0.38 ma¹. Crevassing on all nine glaciers where annual balance measurements have been made has also diminished Significantly. A Loss of 4.2 m of ice thickness un glaciers with an estimated mean thickness of 30 50 m (Reference Post, Richardson, Tangborn and RosselotPost and others, 1971) is significant.

The result of negative annual balance has Keen glacier retreat In 1985. of the 47 glaciers we observed 38 were retreating. In 1994. 46 of 47 glaciers observed were retreating (Reference PeltoPelto 1993). Lewis Glacier with an area of 0,09 km ² initially selected for annual mass-balance measurements. Alter completing measurements From 1984 to 1988. Lewis Glacier melted away in 1989 and 1990. leaving reliet ice with an area of 0.03 km³ by the end ol 1990. In 1992. David Glacier near Glacier Peak eeased to exist. In 1993. Milk Lake Glaceier disappeared. Each of these three glaciers has low winter accumulation. Glaciers with geographic factors increasing mean net annual accumulation have had less negative annual balances and slower glacier-retreat rates (Reference PeltoPelto 1993). During the 1984-94 period, glacier-retreat rates have increased substantially due to negative annual balances (Reference PeltoPelto, 1993). The high negative annual balance of recent years Combined with the small size of the glaciers will ensure a contiued retreat Iiir die next several years.