Geological Survey Professional Paper 715—A Combined Ice and Water Balances of Gulkana and Wolverine Glaciers, Alaska, and South Cascade Glacier, Washington, 1965 and 1966 Hydrologic Years

SOUTH CASCADE GLACIER

Field Program

Instrumentation used in 1966 was virtually the same as in 1965, and the same sites were used (pl. 4A). Air temperature at two locations, wind speed, precipitation, and streamflow records are virtually complete for the whole hydrologic year (table 3). The distributions of snow and ice for late winter and for the end of the balance year are shown on plate 4B and C.

TABLE 3.—Instrumentation at South Cascade Glacier during the 1966 hydrologic year

Climate

The climate in the North Cascades during the 1966 hydrologic year was characterized by below-average amounts of precipitation, particularly during the winter season, and by above-average temperatures (pl. 4D, 1st and 2d graphs). In June and July, however, temperatures were below normal and precipitation was slightly above average. Snow fell at altitudes above 1,800 m on several occasions in June. With the exception of March and June, high pressure dominated the weather pattern in the Pacific Northwest during the hydrologic year. The mean winter air temperature was -3.5°C, the mean summer temperature was +6.87deg;C, and the annual mean temperature was +0.8°C.

Ice Balance

The map of measured late-winter balance (pl. 4A) as of May 12, 1966, shows a typical pattern of snow accumulation but less than normal amounts; _m(s) = 2.52 on the glacier and 1.82 on the whole drainage basin. At this time the snowpack density, in megagrams per cubic meter, was 0.498 on the glacier (P-1) and 0.615 off the glacier (site 1). The maximum balance, _x, was attained on May 22; the value for _x is less than that for _m(s) because of considerable melting at the beginning of the hydrologic year (table 4). The annual accumulation, _a, was 2.59 m on the glacier or 1.99 m averaged over the drainage basin.

TABLE 4.—Snow, ice, and water balances, South Cascade Glacier, 1966 hydrologic and balance years

[Parameter values and errors in meters except where indicated, Date: Hydrologic year, Oct. 1, 1965 (t₀), to Sept. 30, 1966 (t₁)]

	Glacier		Basin		Date	Term	Explanation
	Value	Error	Value	Error
Parameters for annual mass balance
a	-.094	0.10	-0.45	0.12	Hydrologic year	Annual balance	Net change in glacier mass from t0 to t1; approximately equal to the difference between precipitation as snow and melt-water runoff for one hydrologic year.
n	-1.03	.10	-.50	.12	Nov. 3, 1965 (t0') to Oct. 16, 1966 (t1').	Total mass net balance	Net change in mass from (t0') to (t1'); change in mass during one balance year.
n(fi)	-1.07	.10	-.52	.12	Nov. 24, 1965 (t0'') to Nov. 5, 1966 (t1'').	Firn and ice net balance	Change in mass of old firn and ice during a single melt season; the mass between two consecutive summer surfaces.
Parameters relating annual and net mass balance
0	0.08	0.03	0.04	0.04	Oct. 1 to Nov. 2, 1965	Initial balance increment	Balance change between first time of minimum balance (t0') and t0; relates balance-year quantities to hydrologic year quantities.
0(s)	0		0			Initial snow balance	New snow accumulated on summer surface (ss0) at t0.
0(i)	.20	.05	.10	.05	Oct. 1 to Nov. 23, 1965	Initial ice balance	Ice melt in the ablation area after t0 and before ice melt begins the following spring; measured by ablation stakes.
1(s)	0		0			Final snow balance	New snow accumulated on summer surface (ss1) at t1.
a(i)	-1.03	.08	-.49	.10	Hydrologic year	Annual ice balance	Ice and firn melt in the ablation area for the hydrologic year.
n(f)	.09	.05	.04	.02	Nov. 5, 1966	Net firnification	The increment of new firn in the accumulation area at t1''; measured after ablation ceases in the autumn.
Parameters for snow accumulation and ablation
x	2.39	0.16	1.77	0.22	May 22, 1966	Maximum balance	Maximum value of the balance (in relation to balance at t0) for the hydrologic year, similar to the "winter balance" or the "apparent accumulation."
m(s)	2.52	.15	1.82	.20	May 12, 1966	Later-winter snow balance	Balance measured to the summer surface (ss0) in late winter or spring; measured in pits or by probing.
a	2.59	.18	1.99	.20	Hydrologic year	Annual accumulation	Accumulation of snow between t0 and t1.
a	-3.53	.20	2.44	.20	do	Annual ablation	Ablation of snow, ice, and firn between t0 and t1.
Parameters for glacier dimensions
S	12.84	0.04	6.11	0.02		Area
AAR	2.30	2.05	2.14	2.03	Sept. 30, 1966	Accumulation-area ratio	A rough index of annual balance, measured at time t1, neglecting new snow overlying ss1.
ELA	2140	20			do	Equilibrium-line altitude	Do.
δL	-16	3			Hydrologic year	Advance or retreat	Average horizontal-distance change of terminus in direction of glacier flow.
Parameters for water balances
pa			1.77	0.35	Hydrologic year	Measured annual precipitation	Value measured with a gage at one point.
pa(r)			.46	.06	do	Measured annual precipitation as rain	Do.
pa*	3.09	0.33	2.80	.22	do	Calculated annual precipitation	Calculated average for glacier or basin, using annual basin runoff and balance.
a	4.02	.30	3.25	.15	do	Measured annual runoff
1Square kilometers. 2Dimensionless.

The map of total mass net balance, _n (pl. 4C), shows the snow and ice cover on the basin at t₁', the date of minimum balance (Oct. 16, 1966). The change in ice storage of the glacier during the 4-month period since May 22 is very large, amounting to over 3.5 m for the glacier and 2.3 m for the total basin (pl. 4D, 3d graph). The basin-storage change during this period represented about 70 percent of the total annual basin run off, _a (3.25 m). The remaining runoff during the hydrologic year was due to precipitation as rain (p_a(r), 14 percent) and snow and ice melt between October 1, 1965, and May 22, 1966 (16 percent).

The annual balance, _a (-0.94 m on the glacier), was a slightly greater loss in mass than the average since 1958. The annual loss in ice storage in the basin contributed 13 percent of the annual runoff from the basin and about 23 percent of the runoff from the total glacierized area. The greater-than-average loss in mass can be attributed to both the deficient winter snowpack, which resulted in an early exposure of glacier ice, and the warmer-than-average ablation season. This loss of mass eliminated most of the winter snow on the glacier except at the very highest altitudes (pl. 4E and fig. 7).

FIGURE 7.—Vertical air photograph of South Cascade Glacier and basin on September 22, 1966, The thin veneer of snow remaining on the glacier at this time is rapidly disappearing. The transient accumulation-area ratio is 0.40, which can he compared with the values of approximately 0.30 on September 30, 1966, and 0.15 on October 16, 1966, the end of the balance year. Although little snow remains on the glacier, accumulation patterns are still obvious in this photograph. The strong influence of the prevailing western storm wind is indicated by drift patterns and by the predominance of snow remaining on the western side of the glacier (north to top of photograph).

Precipitation

Precipitation remains one of the most difficult parameters to measure accurately. Standard gages in high mountain environments serve, at best, only as indices of the total precipitation. For example, the well-shielded recording gage at site 1 (alt 1,610 m) measured just 63 percent of the estimated basin precipitation for this hydrologic year. This was due, in part, to this gage's location at the lowest point in the basin. But the gage is inaccurate mainly because about 80 percent of the precipitation occurred as wind-driven snow which is only partially intercepted by the gage.

The actual basin precipitation, _a^*, was estimated as the sum of the annual runoff, a (3.25 m), and the annual balance for the basin, _a (-0.45 m). The ratio of this calculated precipitation (2.80 m) to the measured precipitation, p_a (1.77 m), is 1.58 as compared with 1.67 in the 1965 hydrologic year. The cumulative basin precipitation, σ^* (pl. 4D, 3d graph), was determined by multiplying the cumulative measured precipitation, sigmapa, by this fixed ratio. The hydrologic-balance curve (pl. 4D, 3d graph) is the difference between cumulative precipitation and cumulative runoff.

Contribution of Glacier to Runoff

The runoff from small glacierized basins, such as this one, contributed a significant volume of water to the Cascade River during the months of August and September. During these two months most of the discharge from the South Cascade Glacier basin originated from the melting of glacier ice. The negative balance (-0.94 m) resulting from this high degree of ice melt thus increased the runoff of the low altitude streams to a value far above that produced by precipitation alone. For the South Cascade Glacier basin (6.1 km²), ice melt contributed 85 percent of the total runoff in August and September; for the Cascade River basin (435 km²) during the same period melting of ice in South Cascade Glacier alone contributed 7 percent of the total runoff, and ice melt from all glaciers (16 km²) in the basin contributed approximately 35 percent of the flow. The total runoff from South Cascade Glacier for the 1966 hydrologic year was fairly high (4.02 m).