Geological Survey Professional Paper 715—B Combined Ice and Water Balances of Maclure Glacier, California, South Cascade Glacier, Washington, and Wolverine and Gulkana Glaciers, Alaska, 1967 Hydrologic Year

DESCRIPTION

South Cascade Glacier (lat 48°22' N., long 121°03' W., fig. 3) is located in the North Cascades of Washington. This valley glacier occupies roughly one-half of a 6.11-km² basin. The mean ELA is 1,900 m. Water from the basin flows into the Cascade River then the Skagit River.

FIGURE 3.—Oblique aerial photograph of South Cascade Glacier and drainage basin, North Cascade Range, Washington, September 20, 1967. An unusual condition is shown here in that much firn and ice are exposed at higher altitudes around Sentinel Peak (upper left). Yet, at a lower altitude, the glacier still retains a considerable amount of surplus snow. The greater snow accumulation on the lower, more level glacier surface is probably due to a more than normal movement of snow by wind drifting and (or) avalanching from higher altitudes to the glacier. The partial nourishment of glaciers hy this means is an important factor but has received little attention in mass balance studies. It has been calculated that 32 percent of the South Cascade Glacier snow accumulation was by redeposition this year.

FIELD PROGRAM

The instrumentation and mass balance measuring program during 1967 was quite similar to that in 1966 (Meier and others, 1971). Most of the recording instruments operated throughout the year except for a short period in the late winter (table 3). Instrument locations together with snowline data during the ablation season are shown on plate 2.

TABLE 3.—Instrumentation at South Cascade Glacier during the 1967 hydrologic year

WEATHER

The weather in the North Cascades during the balance year was characterized by a severe storm in October 1966, heavy midwinter precipitation, a cool, rather dry spring, and an unusually warm and dry summer season. The cloud cover in the North Cascades during the June-September ablation period averaged 15 percent below the long-term mean.

GLACIOLOGIC BALANCES

The measured winter snow balance, _m(s), was determined by field measurements on May 4-5, 1967. Snow pits and density cores were used to determine the snow pack water equivalent at index stations P—1, near the equilibrium line, and P—0, in the ablation area. More than 100 snow depth soundings were made on the glacier and in the nonglacierized area of the basin to expand the index station data to determine values for _m(s). The maximum balance, _x, occurred on approximately May 17. The value of _x is less than _m(s) because it is measured from the beginning of the hydrologic year, t₀. Considerable storage change, ₀, occurred in October 1966 and must be accounted for when determining _x:

_x = _m(s) + ₀(i) + δ(s),

where δ(s) equals the additional snow accumulation between May 5 and May 17. The value for δ(s) for the glacier was estimated to be 0.05 m and 0.03 for the basin on the basis of precipitation and temperature records (pl. 2, table 4, fig. 4).

TABLE 4.—Ice and water balances, South Cascade Glacier basin, 1967 hydrologic and balance years

[Values and errors in metres water equivalent expressed as averages over the glacier and basin except where indicated. Date: Hydrologic year, Oct. 1, 1966 (t₀) through Sept. 30, 1967 (t₁)]

	Glacier		Basin		Date	Term	Explanation
	Value	Error	Value	Error
Yearly mass balances
Yearly mass balances
a	-0.58	0.15	-0.27	0.08	Hydrologic year	Annual balance	Total change in snow, firn, and ice storage during the 1967 hydrologic year, from t0 to t1; approximately equal to difference between precipitation as snow and melt-water runoff for the hydrologic year.
n	-.63	.15	-.31	.08	Oct. 17, 1966 to Oct. 14, 1967	Total mass net balance	Change in storage from the minimum balance in 1966 autumn (t0') to the minimum in the 1967 autumn (t1').
a(fi)	-.58	.15	-.27	.08	Hydrologic year	Annual firn and ice balance	Change in firn and ice storage during 1967 hydrologic year; excludes late summer snow on the basin.
Accumulation and ablation
m(s)	3.28	0.12	2.22	0.23	May 4-5, 1967	Measured winter snow balance	Snowpack on the 1966 summer melt surface (ss0); measured in late winter or spring in pits and by probing.
x	3.18	.15	2.18	.25	May 17, 1967	Maximum balance	Storage change from the beginning of the hydrologic year (t0) to the maximum in the spring. Winter balance (bw) equals maximum balance (x) plus the absolute value of the initial balance increment (0).
a(f)	.50	.10	.23	.05	Hydrologic year	Annual firnification	The increment of new firn in the accumulation area at t1, measured after melting from this residual snowpack (firn) ceases in the autumn..
a(i)	-1.08	.10	-.50	.05	do	Annual ice balance	Ice and old firn melt in the ablation area during the hydrologic year.
a	3.40	.20	2.27	.25	do	Annual accumulation	Total accumulation of snow between t0 and t1.
a	3.98	.20	2.54	.20	do	Annual ablation	Total ablation of snow, ice, and old firn between t0 and t1.
Values relating annual and net ice balances
0	-0.10	0.05	-0.05	0.05	Oct. 117, 1966	Initial balance increment	Storage change between time of minimum in 1966 autumn (t0') and beginning of hydrologic year (t0).
0(s)	0	0	0	0		Initial snow balance	Snow accumulated on the 1966 summer surface (ss0) at beginning of hydrologic year (t0); measured in pits and by probing.
i	-.15	.02	-.07	.02	Oct. 1 to Nov. 5, 1966	Initial ice balance	Initial ice balance Ice and old firn melt after t0 and before the winter snowpack covers the glacier; measured by ablation stakes.
1	-.05	.02	-.02	.01	Oct. 1-14, 1967	Final balance increment	Storage change between time of minimum in 1967 autumn and the end of the hydrologic year.
1(ls)	0	0	0	0		Final late snow balance	Snow accumulated on the 1967 summer surface (ss1) at t1.
1i	-.10	.05	-.04	.02	do	Final ice balance	Ice and old firn melt after t1 and before the next year's snowpack covers the glacier.
Glacier and basin dimensions
S(fi)	12.80	0.04	3.15	0.05	Sept. 30, 1967	Glacierized area	Glacier value includes firn and ice areas which normally are attached to the main trunk glacier. Basin value is the main glacier plus all other small glaciers and perennial snowfields in the drainage basin. Ice-cored moraine and other permafrost areas are not included as glacierized areas.
S	12.80	.04	6.11	.02	do	Total area	Glacier and water drainage basin above the stream gaging station.
AAR	2.58	.05	.29	.09	do	Accumulation area ratio	Area of new firn, accumulation area, divided by the total area. An index of annual balance.
ELA	1,870	20			do	Equilibrium line altitude	Average altitude where snow ablation equals snow accumulation. An index of annual balance.
δL	-17	3			Hydrologic year	Advance or retreat	Average horizontal distance change of terminus of the glacier in direction of flow.
Precipitation and runoff
pa			2.17	0.30	Hydrologic year	Gaged annual precipitation	Total snow and rain caught during the hydrologic year by a shielded gage at 1,610-m altitude.
pa(r)			.42	.08	do	Gaged annual precipitation as rain	Precipitation occurring as rain caught during the hydrologic year by the same gage at 1,610-m altitude.
a	4.00	0.35	2.87	.20	do	Annual basin precipitation	Area averaged snow and rainfall measured during the hydrologic year by precipitation gages and snow balance measurements.
a(r)	.60	.05	.60	.05	do	Annual basin precipitation as rain	Area averaged rainfall measured by gages during the hydrologic year. Rain distinguished by air temperature records, visual observations, and photographic record.
pa*			3.56	.16	do	Calculated annual precipitation	Area average annual snow and rain precipitation; the sum of annual stream runoff and annual storage change (a).
a			3.83	.15	do	Annual runoff	Stream discharge for the year divided by glacier or basin area, expressed as average depth of water over the area; measured by stream stage recorder and stream discharge measurements.
1Square kilometres. 2Dimensionless.

FIGURE 4.—The balance of South Cascade Glacier, 1967 hydrologic year. (Refer to table 4 for additional values.)

During the early ablation season, over 30 ablation stakes were installed on the glacier and other areas of snow in the basin. These stakes were periodically measured and serviced. The last readings of the season were made in late October by probing through 1-2 m of snow to the firn or ice for the minimum 1967 balance. The ablation stake data, periodic density cores and pits at index stations, and fall photography resulted in a map of net balance, _n. Because the station was not occupied October 1, 1967, estimates for values of _i, _i(ls), and _i(i) were made based on precipitation and temperature records.

Ablation during the summer was intense and resulted in a proportionally greater amount of mass loss from the higher levels of the glacier than in previous year. As a result, the equilibrium line altitude for 1967 was 1,870 m, slightly below the average equilibrium line altitude of 1,900 m. Despite the large spring snowpack the annual balance on September 30, 1966, was -0.58 m. The values for glacier accumulation and ablation (c_a and _a) are somewhat higher than _x-_a as they account for snow that fell and melted before May 17 and snow that fell and melted after May 17.

HYDROLOGIC BALANCE

The hydrologic balance throughout the year for the basin is shown on plate 2D. The hydrologic balance is the difference between precipitation and runoff. This may appear to be a simple calculation; however, there are several complications. Precipitation is difficult to measure directly and accurately. The recording gage catch of 2.17 m, P_a, is probably much below the actual precipitation. This is because most of the precipitation occurs as wind-driven snow, which the gage catches inefficiently, and because the gage is located at a nonrepresentative place in the basin—the lowest area. A better measurement is to determine a value for basin rainfall, by use of several simple gages scattered over the basin in the summer, and a value for basin snowfall, by determining the water equivalent of the maximum snow accumulation at many points in the basin. The sum of the two values for 1967 was 2.87 m, designated _a. This method does not account for snow that fell and ablated before the maximum snowpack was measured.

The second method, _a^*, is the sum of the annual runoff and the measured storage change or basin balance. The main problem with this method is that liquid storage within the glacier is unaccounted for so that runoff does not equal input. The difference between _a and _a^* (0.69 m) can be due to either or both errors in measurements and (or) delayed runoff. In the cumulative precipitation curve (pl. 2D), _a^* was used as basin precipitation and distributed according to the gage precipitation at site 1. Neither method accounts for evaporation, condensation or sublimation.

Total runoff for the year was 3.83 m—15 percent derived from glacier ice melt, 49 percent from glacier snow melt, 20 percent from nonglacier snow melt, and 16 percent precipitation as rain.