Text S6: Proglacial river discharge data and processing
Hourly proglacial discharges for Qinnguata Kuussua/Watson River
(van As et al., 2017; 2019 ) were downloaded fromhttps://doi.org/10.22008/promice/data/watson_river_discharge.
Proglacial discharges have been recorded at this location since 2006,
using in situ pressure transducer measurements of stage (water level)
and an empirical stage-discharge rating curve calibrated with
intermittent in situ discharge measurements acquired from different
techniques including current meters, the float method, and Acoustic
Doppler Current Profiler (ADCP) transects. The discharge estimates have
an estimated 15% uncertainty due to rating curve fit and errors in
cross-sectional area and velocity measurements (van As et al.
2017 ).
Hourly proglacial river discharges in Akuliarusiarsuup Kuua, a major
headwater tributary of Qinnguata Kuussua ~33 km upstream
of the Kangerlussuaq bridge and ~2 km downstream of the
ice edge, were downloaded from
https://doi.org/10.1594/PANGAEA.876357. Proglacial discharges at
Akuliarusiarsuup Kuua have been recorded since 2008 at a road bridge
crossing (AK4 station, Rennermalm et al., 2013b; 2017 ). Stage and
water temperature data are collected sub-hourly using a Solinst®
Levelogger pressure transducer and atmospheric barometric pressure
logger (accuracies 0.003 m and 0.05°C for the Levelogger, and 0.001 m
for the barologger, respectively). Discharges are estimated from the
continuously recorded stage data using an empirical stage-discharge
rating curve calibrated by periodic in-situ discharge measurements
collected from the bridge, using either USGS-style Price AA current
meters or a SonTek River Surveyor® ADCP.
The discharge monitoring station at Kangerlussuaq was chosen as the
preferred proglacial discharge dataset due its capture of proglacial
outflow from a much larger area of the ice sheet, thus maximizing the
likelihood of subglacial linkage to Rio Behar moulin, despite uncertain
basal routing in the region (e.g. Lindbäck et al ., 2015). To
convert the Kangerlussuaq time series into a measure of estimated peak
daily discharge at the ice edge, a cross correlation analysis between
hourly AK4 and GEUS station measurements was used to determine to
estimate the mean timing difference between peak daily flows observed at
Kangerlussuaq and peak daily flows at the ice edge.
During the study period, the mean time difference between daily flow
peaks at Kangerlussuaq and at AK4 station (33 km distance) averaged 7.6
hours (0.83 s/m). A small additional timing correction between AK4 and
the ice edge (2 km distance) is estimated as 0.46 h, by multiplying the
mean Kangerlussuaq-AK4 timing correction (0.83 s/m) by 2 km. The total
timing correction of 8.1 + 0.9 h was rounded to 8-h and
temporally subtracted from daily peak timings observed at Kangerlussuaq
bridge. Note that this 8-h proglacial correction is not the same thing
as a proglacial flow routing delay because it does not distinguish
between wave celerity and Lagrangian flow, but is sufficient for our
purpose here, which is simply to estimate the daily timing of peak
proglacial outflow at the ice edge using measurements from
Kangerlussuaq. Furthermore, note that this 8-h proglacial timing
correction does not represent the time difference between peak daily
discharge entering Rio Behar moulin and peak daily discharge at the ice
edge. Cross-correlation analysis between daily peaks in supraglacial and
estimated ice-edge proglacial indicate that the moulin-edge timing
difference (again, timing difference only, not subglacial routing) is
~20-h (Figure 3).
Though we generally use this corrected proglacial discharge during
analysis (e.g. Figures 2, 3 & 5), we do remove the immediate linear
trend from proglacial discharge when examining the lagged correlation
between proglacial discharge and ice speed in order to eliminate the
potential for autocorrelation.