Behaviour and interaction of ponds

The water-level variations and pond area changes characterise a variety of behaviours typical of supraglacial ponds. It is first important to distinguish between water-level changes and pond drainage or filling. A perched pond with no input should remain at the same level unless its bounding geometry changes or connectivity is established with the englacial conduit network. This was the case for pond D in 2013, which exhibited minimal changes in pond area or geometry, and showed a static water level from the May 2013 to May 2014 as observed by the DEMs. For ponds with a subaqueous bare-ice interface, melt induced by pond circulation leads to a significant change in bounding geometry, and as a consequence of the density difference between water and ice, the pond water surface lowers even though pond drainage has not occurred. This was clearly the case for pond I in 2014, where basin expansion and steady water-level lowering were evident, although the pond and surface depression maintained nearly static geometry. Thus, the pond’s drainage would be characterised as a marked deviation from this background water-level lowering. Similarly, small increases in water level could be due to debris deposition within the pond (another geomteric change). Steady water-level increases, on the other hand, can only be attributed to significant water inputs.

Individual ponds

Based on this framework, the observed changes in the four study ponds can be classified into types of behaviour (Table 4.4).

In 2013, pond C initially showed a stable water level (with diurnal fluctuations), then a gradual water-level decline began 25 May, which accelerated around 20 June. These dates are interpreted to bound the changes in the pond’s dynamics: first the transition to significant subaqueous melt (outpacing water inputs) when the pond temperature increases around 25 May, then a slight increase in drainage efficiency around 20 June, leading to a slow, steady water-level decline, and a significant decrease in ponded area by October 2013. In 2014, pond C’s rapid spike in water level is clearly a significant input of water, and interrupts a steady water-level decline. The steady decline is resumed after the temporarily backed-up water leaves the system, and without a change in slope or level, as might be expected given the englacial discharge of water. Thus, in 2013 pond C shows water-level lowering associated with subaqueous melt followed by slow drainage, while in 2014 the pond shows the lowering effect of subaqueous melt, interrupted by a brief filling and drainage event.

In 2013, pond D underwent very few changes, and appeared to have been entirely disconnected from the rest of the glacier’s hydrological network; it fit the definition of a ‘perched’ pond. In 2014, though, pond D exhibited an overall water-level rise followed by a decline, encompassing a period when the pond experienced significant basin expansion and shrinkage. The unsteady water-level rise is interpreted to be due to net water influx until 15 June, when a change in the water balance or increased basin expansion led to a general decline in water level. This decline has a steady underlying slope, suggesting subaqueous melt or basin expansion rather than drainage, but is interrupted by sudden changes in water level. These occasional sudden water-level increases may be due to the capture of very large boulders during basin expansion, which would displace a significant volume for the small pond. Some pond drainage is probable, as the pond audibly established a connection to a flowing englacial conduit and significantly declined in surface area during 2014, but such drainage must have been inefficient.

Pond I experienced moderate changes in basin geometry in 2013, associated with a strong net water-level lowering, ponded area decrease, a change in the subaerial ice cliff’s appearance, and the emergence of a relict conduit segment through roof collapse. The changes are interpreted to indicate pond-filling prior to the May 2013 visit, submerging any thermo-erosional notches, followed by additional water inputs during the monsoon associated with the conduit roof collapse. Eventual drainage during 2013 lowered the water level and decreased the pond’s surface area, exposing vertical segments of the ice cliff and thermo-erosional notches. In 2014, pond I maintained its area but the water level gradually

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Table 4.4 Summary of observed pond changes and inferred behaviour for 2013 and 2014. The column ‘Ice cliff’ indicates whether the pond was observed to be in direct contact with a subaerial ice cliff.

Year Pond Water level changes Area changes Ice cliff Other Interpretation 2013 C Slow water-level decline, change

in slope mid-June 50% area decline Y –

Subaqueous melt, followed by drainage

2013 D No change (DEM) <2% change N – Minimal change

2013 I 6 m lowering (DEM) 25% area decline Y Conduit collapse Subaqueous melt, possible filling, drainage 2013 J Steady slow increase, then

steady decrease 94% area decline Y Conduits revealed Drainage 2014 C Spike in late April, steady slow

lowering

77% area decline, then 36%

increase Y –

Filling, drainage, then subaqueous melt 2014 D Unsteady rise and lowering 24% area decline, then 57%

decline Y – Slight filling, basin expansion 2014 I Continuous lowering 11% increase Y – Subaqueous melt 2014 J Continuous increase to June 60% decrease Apr-Nov Y Conduits revealed Filling and drainage, some

subaqueous melt

declined. This slow rate of water-level lowering is certainly due to subaqeueous melt and the difference in density between water and ice rather than drainage.

Finally, pond J expressed clear filling and drainage in each of the years. The steady water-level increases are certainly pond-filling, and in both years the pond drained during the monsoon. For 2014, the water-level record only indicates a steady rise of 6 m. In 2013, though, pond J’s behaviour is nuanced: a gradual water-level increase until 31 May is followed by stability until 10 June, two sudden increases in water level, then a gradual decrease until 15 June, when a 10-day drainage occurs. This is interpreted as water inputs outpacing subaqueous melt until 31 May, when a balance is reached. The sudden rises in water level on 9 and 10 June represent significant volume inputs given the pond’s size, most likely associated with either boulder capture or calving. The gradual decline in water level (11-15 June) suggests that subaqueous basin expansion is outpacing water inputs. Pond drainage then begins on 15 June when an efficient connection is established with englacial conduits.

Behavioural themes

A few common themes are evident from the inferred pond behaviours. First, it is apparent that slow water-level lowering occurs as a background signal at nearly all locations due to basin expansion, commonly associated with subaqueous bare-ice melt. The pond drainage events also appear to occur slowly, spanning several days or weeks, in contrast to a sudden pronounced drainage. On the other hand, moderate water-level increases can occur both rapidly or over the course of several weeks. Diurnal variations of several centimetres occur for all ponds, which were associated with the timing of water supply and evaporation, while

irregular increases in water level due to sudden mass input can occur for both small and large ponds.

Seasonal variability is observed for the whole glacier’s ponded area from the orthoimages and from Landsat (Chapter 3), but it is notable that some individual ponds may experience repeated seasonal filling and drainage. In both years, ponds C and J exhibited higher pond area in the premonsoon, followed by a drainage prior to the post-monsoon observations. Pond D also fits this characterisation in 2014, and pond I in 2013. Seasonal fluctuations of individual ponds could be driven by variations in ablation and water delivery, but a structural mechanism is required to enable water to back up. It is possible that conduits return to inefficient configurations after a drainage event due to englacial structural collapse, snowfall, or debris deposition and freeze-on. Alternatively, distinct drainage paths may be utilised in subsequent years, as the water-level lowering renders the previous efficient drainage path inaccessible.

The four pond systems observed in detail persisted over both monsoon seasons, while the small perched ponds on the lower glacier tongue (A,B,E,F,G,H) were only observed to persist for one monsoon season. The striking difference is pond area: ponds that persisted all had

mean surface areas > 200 m2, while those that did not all had mean surface areas < 200 m2.

Other common attributes of the persisting ponds is that 1) they were adjacent to ice cliffs or developed them, and 2) the water level experienced an overall decline year to year. These attributes may be closely linked, as a continuous water-level decline is only possible with rapid basin expansion and lowering, while a persisting pond is likely to absorb excess energy and warm, leading to eventual subaqueous subdebris ablation and surface lowering.

Pond interaction

The longer records of water level for ponds C and J in 2014 captured an interesting set of events in the early pre-monsoon of 2014. The peak of pond C’s water level in the afternoon of 21 April is closely tied to the sudden rise in water level at pond J (estimated volume increase

of 6000 m3 over the following day). Subsequently, the pond C water level drops over ten

days (estimated pond volume change of 1600 m3) before switching into a gradual decline,

while pond J sustains an increase in water level until at least June. I interpret this behaviour as a reconfiguration in the englacial hydrological network on 21 April, which initiated the backup of water in pond J. This change must have simultaneously cut off the supply of water to pond C, allowing it to discharge until its return to a local hydraulic table. Unfortunately, no water-level records were available for other ponds at this time to determine if this event had any other effects on the glacier.

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Fig. 4.20 Decline in pond diurnal temperature fluctuations as Tadeclines leading into winter.

Winter behaviour

As pond C was equipped with a thermistor string (measuring temperature at 50 cm depth, 2 m depth, and the pond-bottom) during the 2013-2014 winter, observations encompassed the freeze-over and thaw-out process ponds undergo annually. The initial period of observation in the late post-monsoon shows a strong diurnal pattern in each of the records (Figure 4.20). During this period, all sensors record a damped response to the diurnal forcing by the air temperature, which is above zero during the day and below zero at night (not shown). The pond-bottom temperature is slower to respond to decreasing air temperatures and the pond-bottom retains heat during the night, resulting in a decreased diurnal amplitude.

Following the post-monsoon decline in diurnal temperature peaks, a distinct regime emerges for the period December-March, characterised by sub-zero temperatures for the 50 cm and 200 cm temperature sensors and freezing-point temperatures for the pond-bottom sensor (Figure 4.21). This period corresponds to ice formation and growth on the pond surface, allowing temperatures to decline below 0°C. The two profile sensors record similar patterns, while the pond-bottom temperature remains constant for the entire season. In early April, the pond’s surface thaws and the 50 cm and 200 cm temperature sensors both resume a diurnal cycle, with peak temperatures to 2°C and daily minima above freezing (Figure 4.22). Soon thereafter, the pond-bottom temperatures also rise above 0°C. The diurnal pattern in all three temperature sensors becomes more prominent into late April.

The freezing-in period of the sensors at the surface takes about a month, with the surface frozen about 1 December. The ice-cover temperatures significantly deviate from zero over winter, suggesting development of a moderately-thick insulating layer. This conveniently aligns with an increase in observed water pressure of 0.25 m.w.e., which I interpret as snow

Fig. 4.21 Sub-freezing winter temperatures recorded at depth as a layer of ice forms on the pond’s surface. The pond is liquid beneath, as pond-bottom temperature is held constant at the freezing point.

Fig. 4.22 Emergence of diurnal temperature fluctuations in early April, coupled with increas- ing pond levels, then drainage and subaerial temperature records.

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snow accumulation on the iced surface of the pond for much of January 2014, and the lowest portion of this snow surface would have formed a thicker ice crust, such as the remains of ice layers observed at many ponds in the study area (e.g. Figure 4.23). These over-winter observations clearly indicate that the ponded water has little effect as a heat reservoir in winter, and that the pond surface does not directly interact with the atmosphere for this period. Therefore, the pond does not act as an energy source for melt until the surface ice and snow layer diminishes.

Fig. 4.23 (a) Floating ice observed in pond C in May 2013 was the remains of a pond-surface ice layer of moderate thickness, rather than due to supraglacial calving. (b) The collapsed remains of a pond’s winter ice surface from Langtang Glacier in May 2014, where the water level lowered sometime after freeze-over. In both cases, the ice fragments have a flat, plate-like appearance.

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