Text S1: Supplemental Background
Each summer, the delivery of supraglacial meltwater to the GrIS bed
causes a rapid initial rise in subglacial water pressure, which reduces
basal traction and enhances ice sliding (e.g. Bartholomew et al.,
2010, Hoffman et al., 2011; Zwally et al., 2002 ). A gradual slowdown in
ice motion then occurs as increasing subglacial efficiency reduces
regional subglacial pressure and increases basal traction (e.g.,Bartholomew et al., 2010; Hoffman et al., 2011; 2016 ).
Superimposed upon this seasonal cycle are short-term accelerations
lasting several hours to several days attributed to variations in
meltwater input (Andrews et al., 2014; Schoof, 2010 ). In the
lower ablation zone, brief increases in ice speed of up to
~300% (with lesser accelerations at higher elevations)
are broadly attributed to the effect of diurnal surface melting on
subglacial hydrology and water pressure (e.g. Andrews et al.,
2014; Cowton et al., 2016; Davison et al., 2019; Hoffman et al., 2011;
Shepherd et al., 2009 ).
At a process level, however, the interaction among subglacial cavity
evolution, subglacial storage, and ice motion remains difficult to
interpret across spatial and temporal scales despite extensive
collection of on-ice surface measurements (e.g., Anderson et al.,
2004; Andrews et al., 2018; Bindschadler, 1983; Cowton et al., 2016;
Flowers et al., 2016; Hoffman et al., 2011; Howat et al., 2008; Iken et
al., 1983; Jansson, 1996; Kamb, 1970; Schweizer & Iken, 1992 ).
GPS-derived ice surface elevations, in particular, are typically noisy
and partitioning the components of uplift is uncertain. This makes
interpretation of melt-induced basal uplift and uplift rates in the
context of ice motion challenging (e.g. Andrews et al., 2018,
Cowton et al., 2016 ). Furthermore, there is a growing appreciation that
meltwater surface routing through Greenland’s large supraglacial river
catchments modulates the magnitude and timing of meltwater runoff
entering moulins (Smith et al. 2017; Yang et al., 2018; 2020 ),
which must surely influence observed variations in basal water pressure
and associated ice velocity (e.g. Banwell et al. 2016; Clason et
al., 2015; Palmer et al., 2011; Pitcher and Smith, 2019; Zwally et
al.,2002 ). Yet, the influence of supraglacial river discharge on
short-term subglacial water storage fluctuations and ice motion has
received little observational study.
The purpose of this study is to examine the influence of moulin input
(i.e. supraglacial river discharge) on localized, short-term
accelerations in ice surface velocity. To achieve this, we explore
temporal correlations between hourly time series of surface energy
balance, ice ablation, supraglacial river discharge, and
horizontal/vertical ice surface motion for Rio Behar, a moderately sized
(~60.2 km2 in July 2016) mid-elevation
(>1200 m a.s.l.) supraglacial river catchment in the
southwest Greenland ablation zone (Smith et al., 2017 ). It
represents a typical catchment of the snow-free, bare ice ablation zone
(Cooper and Smith, 2019; Ryan et al., 2019 ), including intense
melting and development of weathering crust development during the month
of July (Cooper et al., 2018 ).
The novel datasets analyzed here are: (1) 168 high-quality Acoustic
Doppler Current Profiler (ADCP) consecutive hourly measurements of
supraglacial river discharge (i.e. catchment runoff flux,
m3 s-1) acquired 6-13 July 2016
approximately 750 m upstream of the Rio Behar terminal moulin; and (2)
simultaneous GPS measurements of horizontal and vertical ice surface
motion (5-second sampling interval). We also use PROMICE KAN_M AWS data
to estimate surface energy inputs and ablation; and compute proxies for
subglacial storage (S) and its rate-of-change (ΔS) using both GPS and
hydrographic methods. These data are freely available as Additional
Supporting Information (Additional Supporting Information
Datasets S1-S7 ).
Permanent discharge gauging stations are infeasible in the rapidly
melting ablation zone environment. Owing to continuous thermal erosion
of the ice bed, empirical stage-discharge rating curves rapidly
obsolesce, necessitating that discharges be measured in situ rather than
estimated from empirical rating curves relating occasional discharge
measurements to continuously recorded water level changes. This
requirement of hourly around-the-clock ADCP operations (together with
non-trivial logistical challenges of camping and anchoring instruments
in rapidly melting bare ice) explain the relative brevity (1 week) of
our hourly supraglacial river discharge time series.
Simultaneous measurements of air temperature, radiation, and ice surface
ablation were acquired from the nearby PROMICE KAN_M Automated Weather
Station (Fausto and van As, 2019 ). Proglacial river discharges
from two permanent gauging stations (Rennermalm et al., 2013b;
2017; van As et al., 2017; 2019 ) were also incorporated into this
study. The 60.2 km2 July 2016 Rio Behar catchment
boundary was delineated using a fixed-wing drone and WorldView satellite
imagery following the methods of Smith et al. 2017 . This 2016
catchment boundary is presented for illustration purposes inFigure 1 , but is not otherwise used in this study.