Maclure Glacier (lat 37°45' N., long 119°16' W., fig. 1) is a small cirque glacier near the crest of the Sierra Nevada in Yosemite National Park, California. This glacier is 0.5 km long and covers an area of 0.2 km2; the mean ELA (equilibrium line altitude) is 3,650 m. Runoff from the 0.97 km2 basin flows into the Tuolumne River, which flows west into the San Joaquin Valley of California.
The study at Maclure Glacier began in October 1966 with the construction of a combination gaging stationshelter hut, air temperature and precipitation instrumentation, and the commencement of mass balance measurements. No streamflow record was obtained prior to May 24, 1967; however, it is estimated less than 5 percent of the total runoff occurred from October 1, 1966, to May 24, 1967. Air temperature was recorded at the gaging station shelter by a thermograph attached to the water-stage recorder. The thermograph sensor was located on the north side of the access chimney about 5 m above the ground. Precipitation was recorded near the basin outlet by a water-stage recorder over a storage tank. A nitrogen gas bubbler was used to mix the antifreeze (ethylene glycol) and precipitation to keep it in a liquid state. Many problems were encountered in the first year's operation of the precipitation gage. These problems included recorder insensitivity, clock stoppage, snow blowing into the shelter, and the possibility of the collection orifice bridging over with snow. Because of the poor quality of the record only the total precipitation between visits was determined (table 1). The total precipitation was broken into daily values of rain and snow using the precipitation record from the weather station at Yosemite National Park Headquarters (1,210-m altitude) and the air temperature record at site 1 in Maclure Glacier basin (p. 1A) as guides (pl. 1D). Instrument locations are shown on plate 1A.
TABLE 1.Instrumentation at Maclure Glacier during the 1967 hydrologic year
Most precipitation in this area comes in the form of snow during the winter and spring months (November-April). This precipitation results from the orographic and frontal lifting associated with storms that move in from the Pacific Ocean. The major storms are usually of short duration and of high intensity. A small quantity of precipitation comes from local thunderstorms in the summer months.
Major frontal storms in early December 1966 and late January 1967 were responsible for most of the early winter precipitation. Below normal temperatures in March and April in conjunction with above normal precipitation contributed to a near record snowpack in the central and southern Sierra Nevada. Thunderstorm activity was present in the Maclure Glacier area July, August, and September. The weather station at Yosemite National Park Headquarters (1,210-m altitude) recorded 1.44 m of precipitation during the 1967 hydrologic year. This was 155 percent of the 61-yr average of 0.93 m.
Although air temperatures varied markedly from normal in most winter and spring months, the average temperature for the 1967 balance year (hydrologic year) was near normal at most Sierra Nevada stations. The average air temperature for the station at Yosemite National Park Headquarters was about 1°C (Celsius) above the long-term (61-yr) normal of 12°C The average temperature for the balance year at Maclure Glacier measured at 3,520-m altitude was approximately 0°C.
The measured winter snow balance, ms was determined by measuring the depth of snow and its density in the latter part of May 1967. The depth of the snow was measured at many locations over the entire basin; a snow pit was dug and samples taken to determine the density. These measurements, together with photographs, were used to draw a contour map (p. 1B) showing lines of equal water equivalent. The measured winter snow balance was 1.98 m averaged over the basin and 3.46 m on the glacier. The estimated error (table 2) in these values is high for several reasons. The extremely deep snowpack in 1967 made depth sounding difficult, some areas in the basin were inaccessible because of the steep slopes, and inconsistent depth soundings were obtained in some non-glacier covered areas because of the loose rock debris under the snow. The high values (greater than 5 m) for accumulation on the upper portion of the glacier are believed to be caused in part by snow blowing into the basin from the southa normal mode of accumulation on small cirque glaciers.
TABLE 2.Ice and water balances, Maclure 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)]
The annual balance, a, was determined by combining measurements of the depth and density of the snow remaining at the end of the summer season, n(f), and by measurement of the amount of ice and firn ablation during the summer season, n(i), and using the adjustment terms 0 and 1 (a = 0 + n(f) + n(i) - 1). A positive annual balance was found for both the basin (0.37 m) and the glacier (1.05 m). It is interesting to notice that for the 1967 hydrologic year almost one-half of the old firn and ice ablation occurred in the first 3 weeks of October 1966 (fig. 2). Due to the above normal winter and spring precipitation, 86 percent of the glacier was still covered by snow at the end of the summer season. These annual balance figures are considered to be more accurate than the winter balance figures (table 2) because the annual balance can be more easily measured. Photographs are helpful in determining the extent of residual snow in accessible areas.
Because of the small size of this glacier and cessation of melting soon after the end of the hydrologic year, the stratigraphic and annual balance changes are nearly synchronous. Values relating annual and net balances (table 2) were small, and only a few correction factors are necessary (table 2). The slight difference between the annual and net balance is due to the difference between 0 and 1, a reflection of the higher melt rates in October 1966 than in October 1967. Figure 2 illustrates the mass balance values that were measured and some important calculated values.
Basin runoff during the hydrologic year totaled 1.60 m (1.51 m measured, plus 0.02 m estimated during the winter months, plus 0.07 m between Oct. 1 and Oct. 20, 1966). The source of this runoff was approximately 1 percent ice melt, 22 percent glacier snowmelt, 69 percent nonglacier snowmelt, and 8 percent rain. The sum of the components (0 + x = a + a(r)) serves as an independent check on runoff; they gave 1.75 m, or 0.15 m more runoff than indicated by streamflow. This difference is within the limits of estimated errors for all the parameters entering into the calculation.
The recording precipitation gage located near the mean altitude of the drainage basin gave a value of 2.03 m for the year. This is slightly larger than the calculated precipitation, a*, as determined by runoff and ice storage changes.
Last Updated: 28-Mar-2006