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

Winter balance, _w, can be considered to be an index of annual accumulation, _a, and annual ablation, _a, can be considered an index of annual energy received. The winter balance at Maclure Glacier is almost equal to an annual accumulation. At progressively higher latitudes, however, the winter balance becomes a poorer index of annual accumulation. The northern glaciers receive much more snowfall during the summer season than the southern glaciers. In the same manner the annual ablation is a better indicator of energy received on the southern glaciers than on the northern glaciers. The annual ablation is greater at both Wolverine and South Cascade Glaciers than at Maclure Glacier, probably because Maclure is at a very much higher altitude than the other glaciers. The amount of summer rainfall, _a—_a, in 1967 was greatest at Wolverine Glacier, which is adjacent to the Gulf of Alaska. The large amount of rain combined with the very negative balance produced the greatest glacier runoff at Wolverine Glacier.

The dates and lengths of the summer and winter seasons followed an expectable pattern. The winter season is shortest and the summer season is longest at the southern glacier. The time of maximum balance was less variable than the time of minimum balances. The minimum balances occur considerably earlier in the north than the south. Since the summer season ended after the end of the 1967 hydrologic year at Maclure and South Cascade the annual accumulation of firn, _a(f) could not be defined and neither glacier had an initial snow balance, (s), nor a final late snow balance, ₁(s).

Long-term average balances, precipitation, and runoff cannot be calculated at these glacier basins by the usual means because the period of record is too short. However, by using the available balance measurements, the known long-term equilibrium altitude of the glacier, nearby precipitation records, and estimating the mass change ~ _a, for each glacier over the past half century, fairly reliable predictions for mean annual precipitation, _a, accumulation, _a, glacier balance, _a, _a, and runoff, _a, can be obtained.

All United States IHD glaciers have had a negative balance averaging 0.2 to 0.5 m per year during this century. Over the period South Cascade Glacier received the greatest mass and had the highest runoff. The greatest glacier activity along the North Pacific Coast can be inferred to be at approximately 54° N. latitude. Glaciers in the Brooks Range are the least active.

The rapid decrease in glacier activity from Wolverine Glacier to McCall Glacier is produced by the combined effects of latitude and remoteness from the primary precipitation source, the Pacific Ocean. (McCall Glacier information was provided by D. Trabant, oral commun., 1975.) A similar but less pronounced decrease in glacier activity occurs from west to east across the cordillera because precipitation is blocked by the coastal mountains. Therefore, local variations from the glacier hydrology shown here (fig. 9) are to be expected.

FIGURE 9.—Annual glacier hydrology as a function of latitude. Average calculated for 1910-60, dashed; measured values in 1967, solid. McCall Glacier information provided hy D. Trabant (oral commun., 1975).

A comparison between the 1967 hydrologic year data and the 50-year means indicates that the 1967 annual ablation was close to that predicted for the mean at all glaciers. The annual accumulation of snow was, however, much more variable. The differences in 1967 accumulation from the average of each glacier correlated directly with the departures of the 1967 annual balance of each glacier from the long-term mean balance.

The 1967 precipitation total did not depart from the mean in a way that could be used to predict the balance. Maclure Glacier had a greater than normal precipitation and a higher than normal balance. On the other hand, both South Cascade and Wolverine received normal amounts of precipitation but only Wolverine had a strongly negative balance.

Annual precipitation in 1967 also could not be used as a predictor of runoff. Maclure Glacier received abnormally large amounts of precipitation yet had near normal runoff. Wolverine Glacier had a normal amount of precipitation and unusually high runoff.

In 1967 the variations from normal annual accumulation controlled the glacier balances. However, from these preliminary results it cannot be determined whether winter or summer seasons are, on the average, more important to variations in glacier health. Further more, simple reliance on values such as _x, _m(S), and _w, as indicators of mass and energy exchange can be misleading.

Part of the snow accumulation at each glacier is by redeposition from areas surrounding the glacier. Assuming that _m(S) (basin) represents the original deposition value of snow on both the glacier and non-glacier areas, the much larger value for _m(S) (glacier) is then due to the transport of snow from the slopes. The original deposition value includes direct precipitation, rime icing plus snow blown into the basin, and minus snow blown out of the basin. Fraction, F, of glacier accumulation derived from slopes by drifting and avalanching is:

F=[_m(s)glacier-_m(S)basin]/_m(s)glacier.

The percent contribution by avalanching and wind drift to the measured snow balance on each glacier during 1967 was 43 percent of the total glacier snowpack for Maclure, 32 percent for South Cascade, 24 percent for Wolverine, and 15 percent for Gulkana. That a small cirque glacier like Maclure received nearly half of its nourishment by redeposition shows that redeposition on smaller glaciers is a significant factor.