Distribution of Flow Velocities

The longitudinal distribution of flow velocities along the centre line of the ablation area is shown in Figure 2a. These velocities indicate mean values over 5–16 d in October-November 1985. After a sharp drop in the velocity below the icefall, it decreases gradually down-glacier. Also shown in Figure 2b are the longitudinal strain rates obtained from the surface-velocity gradients. A mean strain rate, about –3 × 10⁻⁴d⁻¹ (–0.1 a⁻¹), indicates a much larger compression than that normally observed in mountain glaciers (e.g. Reference PatersonPaterson, 1981). Since the width of the glacier is almost constant over the ablation area, the transverse strain rate can be regarded as negligibly small. Then, from a continuity condition of incompressible ice, a vertical strain rate can be estimated at about +0.la⁻¹. Assuming a mean thickness of the glacier ice as 300 m (Fig. 2), the strain rate would correspond to 30 m a⁻¹ of the emergence component of ice flow. The annual amount of ablation is then taken as this order of magnitude, if we assume a steady-state condition.

Short-term Variations in Glacier Flow

Variation in ice-flow velocity was obtained from the repetitive measurements at point S1, as shown in Figure 3a. The mean velocity was relatively low because of the proximity of SI to the glacier terminus; yet, the velocity varied considerably within a short time. Velocity variations were also obtained at points PI, S2 and S3 by angular measurements with intervals of a few days or a half day in the summers of 1983 and 1985. The velocity at S3 changed greatly from day to day, with the maximum being six times larger than the minimum, whereas the velocities at P1 and S2 fluctuated even less, but greatly exceeding the range of the measurement errors.

Fig. 3. (a) Variations in the surface-flow velocity at point SI near the glacier terminus, and the amount of global radiation measured at BC; (b) water discharge measured at station II and air temperature at S3; (c) ablation rate (in water equivalent) averaged over six points in the ablation area and the amount of precipitation at BC in November 1985.

The intrinsic error in the distance meter used for the measurement of point S1 is about 2 mm. Since the tripod of the reflector was buried firmly in the surface ice at S1, the error due to tilting of the reflector is almost negligible. The direction of the ice flow at S1 deviates only 13° from the direction of S1 to γ, so that a displacement of SI can be obtained from the measured distances corrected by a small factor. Considering these error components, the total error involved in the displacement of the point can be assumed to be 1 cm at a maximum. Then an error in the short-term (3h) velocity would be about 3 mm h⁻¹. Hence, velocity variations greater than 3 mm h⁻¹ in Figure 3a should be regarded as valid.

Meteorological and Hydrological Data

Some meteorological and hydrological data are also shown in Figure 3. Each ablation rate was derived by averaging six data sets, three from the debris-free ice (surface albedo 0.42–0.44) and three from the ice covered with small pebbles and/or fine sand 0.5–3 cm in thickness (albedo 0.09–0.11). The components of the heat source causing melting of the debris-free ice were calculated to be about 65% for the radiative balance and 35% for the sensible heat during the observation period.

Relation Between Ice-Flow Velocity and Meteorological-Hydrological Parameters

In Figure 3, it appears that there is a high correlation between peaks of the ice-flow velocity at point S1 and of global radiation or air temperature. However, at point S3 such a tendency cannot be recognized. From a close examination of the various parameters measured in the field, the best correlation was found between the ice-flow velocity at S1 and the amount of water discharge at station H. Figure 3 suggests a time lag between the peaks of the velocity and the water discharge. A time lag of 7.5 h was obtained from calculation of the auto-correlation between them every 0.5 h, with a correlation coefficient of 0.85. From this analysis, we conclude that the flow velocity is strongly related to the amount of water discharged from the glacier.

Predominance of Basal Sliding

We can point out in Figure 3 that the largest flow rate (c. 16 mm h⁻¹) is about four times the smallest (c. 4 mm h⁻¹). The flow velocity measured at the glacier surface is the sum of velocities caused by plastic deformation, basal sliding and fracture of the ice body. The first mechanism cannot be considered to vary within a short period of time. The third one may be possible at Glaciar Soler which has many crevasses and seracs (Reference JacobelJacobel, 1982). However, if this is a dominant factor causing the velocity variation, the directions of ice flow should vary with time according to the widening or narrowing of crevasses and cracks. In Glaciar Soler, any significant change in the flow direction was not observed. Consequently, it appears best to attribute the observed variations in velocity to variations in basal sliding.

Such phenomena have recently been observed and analysed in various glacial areas. The first extensive work was by Reference IkenIken (1978), who measured surface movements during intervals of 4–12 h at four glaciers in the Alps and showed a time lag between velocity maxima and water-discharge maxima in the river downstream. Reference IkenIken (1981) then proposed the effect of subglacial water pressure on sliding velocity based on an idealized numerical model. At Findelengletscher, the basal water pressure was measured in 11 boreholes and an empirical relationship was obtained between the water pressure and flow velocity (Reference Iken and BindschadlerIken and Bindschadler, 1986). During the mini-surges of Variegated Glacier, Alaska, strong variations in velocity were obtained within short periods, and their relation to the basal water systems was discussed (Reference KambKamb and others, 1985; Reference Kamb and EngelhardtKamb and Engelhardt, 1987). Short-term variations in velocity have also been reported from Columbia Glacier, Alaska (Reference VaughnVaughn and others, 1985), and in strain and surface tilt from a northern Swedish glacier (Reference Jansson and HookeJansson and Hooke, 1989). Today, the importance of basal water pressure is fully recognized in the study of basal sliding and surge phenomena of glaciers.

Englacial and Subglacial Water System

The electrical conductivity measured at station H was almost inversely proportional to the amount of water discharge, varying from 5 to 9μS cm⁻¹ (at 0°C) with a correlation coefficient of –0.93. Since the conductivity of the supraglacial water was 0.00μS cm⁻¹, this inverse relation must be caused by an increase in meltwater to the discharge at station H. We now roughly evaluate the contribution of the meltwater from glacier ice to the total discharge based on the data shown in Figure 3. The mean ablation rate during this period was about 40 mm d⁻¹. Multiplying this amount by the ablation area 15 km², we obtain the daily total amount of water, 6 × 10⁵ m³ d⁻¹, which corresponds to 7m³s⁻¹ of water discharge. At station H, the mean water discharge during that period was about 11 m³s⁻¹ The rest of the water, about 30%, should be supplied by melting of snow in the accumulation area, by precipitation in the drainage basin, and also by five incoming streams on the right bank.

Numerous supraglacial water streams were formed in the ablation area during fine and warm weather conditions, and most of them terminated at moulins, crevasses or ponds. Since the water level in such crevasses and ponds always remained below the glacier surface, we can suppose that these water intakes are connected with the englacial and subglacial water channels. Abundance of rounded gravel and sand in the end moraines (Reference AniyaAniya, 1987) also suggests very active and well-developed subglacial streams. The component ratio of dissolved substances measured at station H differed from those at other points in the drainage system. For example, the composition at station H was characterized by enrichment in K⁺ (0.77mgl⁻¹), while that at an incoming stream near station β was 0.24 mg l⁻¹. The compositions of other cations were almost the same at both sites. Since supraglacial and englacial streams have virtually no dissolved substances, the water at station Η cannot be considered to be a simple mixture of incoming stream water and supraglacial and englacial ones. The property of enrichment in K⁺probably originates from the subglacial bed.

Based on this evidence, we can conclude that a major part of the total amount of water produced at the glacier surface should be drained through the subglacial water system. Hence, the amount of discharge at station H can be considered as strongly reflecting the amount of subglacial water. Although we have no information about the suglacial water network, it is reasonable to suppose that the high amount of subglacial water discharge is accompanied by an increase in the thickness of the water layer at the bed and/or by an increase in the pressure of basal water channels. We therefore conclude that the high surface velocity can be attributed to basal sliding accelerated by the existence of abundant water at the glacier bed. The water is supplied mostly by melting of surface ice and also by the heavy precipitation. The time lag, 7.5 h, can be interpreted as the apparent travel time of the water mass along a distance of about 2 km from station SI to H. If we regard the minimum observed velocity as due to the plastic deformation of the ice body, basal sliding accounts for approximately three-quarters of the surface velocity near the terminus during the spring season.