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During 1998, the first rapid motion event occurred during sub-period 2, characterised by rapidly increasing catchment discharge following a period o f low and constant catchment runoff (Figure 3.10). Sub-period 2 was characterised by high mean air temperatures and heavy rainfall after low mean air temperatures and snowfall during sub-period 1 (Figure 3.1; Table 3.1). Peak discharge was reached at approximately 00:00 on JD 158 during peak horizontal velocities (Figure 3.10). Peak vertical velocities occurred shortly after the initial rise in discharge and ~ 3 days before the discharge peak (Figure 3.11).

Negative vertical velocities and a rapid decline in horizontal velocities coincided with the rapid fall in discharge on JD 158.

The second rapid motion event during 1998 occurred during sub-period 4, characterised by rising catchment discharge following low and constant runoff towards the end o f sub­

period 3 (Figure 3.10). Horizontal velocities were greatest during the initial discharge rise between JD 170 and 175, as baseflow and diumal amplitude increased rapidly. Peak vertical velocities occurred very shortly after catchment discharge had begun to rise and 2-3 days before peak horizontal velocities (Figure 3.11). Sub-period 4 had rapidly rising mean air temperatures, especially during JD 170-175, after very low mean air temperatures and snowfall towards the end o f sub-period 3 (Figure 3.1; Table 3.1). A significant reduction in velocity occurred as the rapid rise in diumal amplitude ceased at about JD 175, but baseflow continued to climb (Figure 3.10). A further but slower

decline in velocity occurred during sub-period 5 as diumal amplitude increases once more due to a declining baseflow.

The 1999 rapid motion event occurred during sub-period 2, a long period o f rising catchment discharge with a slowly increasing diumal amplitude following low magnitude and low amplitude catchment runoff during sub-period 1 (Figure 3.12). M ean air temperatures also rose following low air temperatures towards the end o f sub-period 1 (Figures 3.1 and Table 3.2). Peak discharge was reached at approximately 00:00 on JD 187 following heavy rainfall (Figures 3.2 and 3.12); peaks in horizontal velocity occurred 2-3 days before the peak in discharge. Vertical velocities were temporally very variable, but typically peaked 2—4 days before the peak in discharge (Figure 3.13).

Horizontal velocities remained high and stable throughout the rise and subsequent fall in discharge during sub-periods 2 and 3, receding gradually during sub-period 4 as catchment discharge and diumal amplitude begins to rise once more (Figure 3.12).

The possible increasing trend in horizontal velocities towards the end of the monitored period during 1998 (Figure 3.10) occurred during periods characterised by high mean air temperatures and high-amplitude diumal discharge variations (Figure 3.1; Table 3.1).

Velocities appear to decline significantly during sub-period 7, where diumal discharge amplitude was significantly reduced (Table 3.1; Figure 3.10).

3.4.2.4 Interpretation a n d discussion

R apid motion events

Periods of rising catchment discharge during the 1998 and 1999 melt seasons are associated with horizontal ice velocities significantly higher than normal summer background levels. During 1998, the glacier was snow covered prior to and during the first rapid motion event: large lags between meteorological variables that control surface melting and proglacial streamflow during sub-periods 1-3 (Table 3.3; Figure 3.3) suggest that the glacier was underlain by a distributed subglacial drainage system. High mean air temperature and heavy rainfall was likely to have initiated rapid melting o f the glacier snow cover and to have delivered significant inputs o f surface melt to the glacier bed. High, spatially persistent horizontal velocities were likely to result from increased

basal motion due to rapidly increasing discharge through a distributed drainage system.

Lower horizontal velocities near the glacier snout may have resulted from the presence o f pre-existing channels (cf. Sharp et al., 1993; Nienow et al., 1998) and the proximity of upglacier areas to surface input points such as moulins (Mair et al., in press). Vertical ice velocities peaked very shortly after the initial rise in discharge; vertical and horizontal velocities rapidly returned to pre-event levels during an equally rapid decline in discharge. This suggests channelisation was able to rapidly release meltwater from temporary storage. It is possible that increased flow through the distributed system mainly occurred near to PDAs beneath the glacier tongue, since: 1) moulins on the glacier tongue tend to be located above PDAs (Sharp et al., 1993); 2) precipitation in the upper catchment may have occurred as snow; and 3) a thick supraglacial snowpack in the accumulation area may have delayed supraglacial runoff.

Peak horizontal velocities during the second event were generally o f lower magnitude but show a more pronounced difference between upglacier and downglacier areas. Prior to the event, high lags between meteorological variables and streamflow suggest large areas of the glacier were still underlain by distributed drainage (Table 3.3; Figure 3.3). Rapidly increasing air temperatures during the event are likely to have resulted in significant inputs of surface meltwater from snowmelt that also exposed areas of low-albedo ice on the glacier tongue. The downglacier propagation and decrease in magnitude o f horizontal ice velocities are likely to reflect the pattern of surface meltwater production and the spatial extent of an immature channelised system formed during the first event (cf. Mair et al., in press). Increasing baseflow after the event was probably due to increasing volumes o f melt from upglacier areas routed through the supraglacial snowpack and distributed subglacial drainage system; however, horizontal velocities declined due to the extension and increase in efficiency o f the channelised system releasing meltwater from

‘storage’ in the distributed system. A slow decline in horizontal velocities and baseflow during sub-period 5 suggests a decline in the proportion o f melt routed through distributed supraglacial and subglacial drainage systems and an increasingly extensive and hydraulically efficient channelised system. Channel formation during the event is likely to have occurred under closed flow conditions during consistently high discharges; however, predominantly open flow conditions will occur during sub-period 5 as supraglacial runoff becomes increasingly peaked.

During 1999, peak horizontal ice velocities occurred 3 days before the peak in discharge.

The glacier was snow covered prior to the event, and large lags between meteorological variables and proglacial streamflow again indicate the existence o f a hydraulically inefficient, distributed subglacial drainage system (Table 3.3; Figure 3.4). Rising mean air temperatures and a strong fohn during sub-period 2 (Table 3.2) are likely to have resulted in the increasing contribution o f supraglacial meltwater to a predominantly distributed subglacial drainage system. A similar downglacier decrease and propagation o f horizontal ice velocities occurred during 1998. The relatively small amplitude variation in vertical velocities probably reflects a slow rise in discharge through the distributed system due to a slow increase in surface melting. Vertical and horizontal velocities did not return rapidly to pre-event levels, suggesting the slow extension and increase in efficiency o f the channelised system following the event. However, velocities did gradually return to pre-event levels during sub-periods 3 and 4.

Vertical ice velocities during all events peak very shortly after the initial rise in discharge and before peak horizontal velocities, suggesting either: 1) the increasing efficiency of the drainage system during the events; or 2) declining water pressures due to increasingly widespread basal separation (cf. Mair et al., in press).

Other events

The slow trend o f increasing horizontal velocities towards the end of 1998 is not related to increasing catchment discharge but occurs during periods o f high diumal discharge amplitude. Short lags between meteorological variables and proglacial streamflow during these periods (Table 3.3) suggest very efficient routing o f meltwater through a channelised subglacial drainage system. Flow in the channels following the first event is likely to have been open for a large proportion of the diumal cycle due to the increasing peakedness o f supraglacial runoff; however, increasing peak discharge will have resulted in the increasing overpressurisation of channels for longer periods o f the diumal cycle.

This effect may have been strengthened by a slight adjustment in channel size due to deformation o f the overlying ice to reflect predominantly open conditions during the diumal cycle. Meltwater was likely to have been forced from channels into the distributed system (Hubbard et al., 1995), reducing basal friction near to PDAs and

causing in a net daily increase in horizontal velocity (cf. Iken, 1974; Iken and Bindschadler, 1986; Willis, 1995).

3.4.2.5 Conclusions

Rapid motion events are precipitated by significant increases in surface melting early in the melt season when the subglacial drainage system is almost entirely or predominantly distributed. During 1998, the first event demonstrates a rapid response to increased supraglacial runoff, although the response is attenuated near to the snout due to pre­

existing channelised drainage. Some channelisation appears to have occurred towards the end o f the event, causing horizontal velocities to return rapidly to pre-event levels. The second event was attenuated with respect to the first and with increasing distance downglacier, probably due to channel formation along PDAs beneath the glacier tongue.

A rapid, higher magnitude and temporally more persistent response occurred upglacier, probably due to increased flow through ‘new ’ areas o f the distributed system where channels were absent or immature. Declining horizontal velocities, despite increasing baseflow suggestive o f increasing flow through a distributed drainage system, suggests the initially rapid expansion o f the channelised system towards the end o f sub-period 4.

Gradually declining baseflow and horizontal velocities during sub-period 5 suggest the continued expansion or increase in efficiency o f the channelised system up to approximately JD 190. During 1999, a single event shows a gradual response reflecting a steady rise in surface melting and an attenuated response with distance downglacier probably due to the presence o f pre-existing channels. The gradual decrease in horizontal velocities to pre-event levels suggests a slow expansion and increase in the efficiency of the channelised system following the event.

The slow increase in horizontal velocities towards the end o f the monitored period during 1998 suggests increased flow through the distributed system due to overpressurisation from increasingly peaked supraglacial runoff through efficient channelised drainage. Channels may also adjust in size to reflect predominantly open flow conditions, further increasing the magnitude o f overpressurisation at peak discharge. The hydraulic gradient between the high-pressure distributed system and the low-pressure channel is reversed, resulting in basal separation near to channels and along preferential drainage axes that may influence velocity patterns across the glacier. The

response is likely to be dependent upon the magnitude and rapidity o f surface runoff and will therefore increase throughout the season.