Glacier Mass Balance

In Perú, mass-balance measurements were begun in the Cordillera Blanca in 1966 by this author on the Pucahirca Glacier. Following that, mass-balance measurements were made between 1977 and 1983 by the glaciology department of Electroperú on three glaciers chosen for easy access: the Uruashraju, Yanamarey, and Santa Rosa (Dolores and others, 1980). Measurements for the years 1977-78, 1978-79, 1979-80, and 1980-81 are given in table 3. The measurements were made mainly in the ablation areas and only a few scattered measurements were made in the accumulation areas.

Table 3.--Mass-balance measurements of two glaciers in the Cordillera Blanca and one glacier in the Cordillera Raura

¹Equilibrium line altitude (ELA) in meters above mean sea level.

The mass-balance fluctuations were not random; during the 6 years of measurements, they show a similar pattern on all three glaciers (Morales Arnao, B., 1969a; Ames, 1985). Most of the measurements show negative mass balance, but the most dramatic ablation took place in 1979-80 and 1982-83. The accumulation values are estimated from precipitation recorded by rain gages at meteorological stations and from estimates of net annual accumulation determined on Nevado Huascarán by Thompson, Mosley-Thompson, Grootes, and others (1984). The rate of 0.91 m accumulation calculated by Thompson, Mosley-Thompson, Grootes, and others (1984) from an ice core at 5,990 m on Huascarán corresponds to a total accumulation of 1.255x10⁶ m³ a^-1 over the 1.30 km² surface area of the Yanamarey Glacier and a water-equivalent loss of 1.62 m.

The length of Yanamarey Glacier decreased from 1.6 km in 1948 to 1.25 km in 1988; the loss of ice volume was 4x10⁶ m³ during that period. The volume remaining in 1988 was 25x10⁶ m³ (Hastenrath and Ames, 1995).

If the regimen of negative mass balance continues, the Yanamarey Glacier will establish a new equilibrium profile. If the glacier terminus recedes to 4,875 m, the surface area will decrease by a factor of two. The present reduction in glacier size is causing a marked increase in stream discharge in the area (Ames, 1985). The contribution of glacier meltwater to streams in the river basins has been very important (Fliri, 1980). It is probable that the importance of the glacier meltwater will diminish as the glacier decreases in size, as was observed in the French and Swiss Alps from 1940 to 1950. Unfortunately, the estimates of glacier mass balance in the above glaciers are based on limited data. More accurate results would have been possible if the observations had been made over a longer period of time and if more measurement locations had been available, particularly in the accumulation area. Monitoring of the discharge of Río Querococha also would have given a better idea of the contribution of the Yanamarey Glacier to the hydrology of the watershed.

The 6 years of mass-balance measurements have shown strong homogeneity in the variation in mass balance of the three glaciers. The glaciers seem to have had a similar response to the same average climatic conditions, as well as the same interannual variations. It is the first time that South America has had this kind of data, although this kind of regional response has been found in more temperate latitudes, such as in Scandinavia, the Alps, the Ural Mountains, Tien Shan, and the Caucasus Mountains.

If more intensive monitoring confirms that the glaciers in each watershed respond similarly, it will be possible to study more easily new glaciers or groups of glaciers in a watershed by making a few key measurements of ablation, accumulation, and stream discharge and then by comparing them with one well-studied glacier in the area, as in the technique used on Yanamarey Glacier.