Jökull - 01.12.1981, Page 99

RIVER MORILLA, KALDALON . JULY 1979
6] AIR TEMPERATURE i
•t4; aA_ rv r\[^y\ (
2- v \J A / V V/A /
o-l—.. ______________________________
Fig. 2. Variations in meterological, ablation
and meltwater characteristics at Kaldalóns-
jökull and the River Mórilla, July 1979. —
^íynd 2. Línurit af veður- og rennslisþáttun við
Ka/dalónsjökul.
the other hand, showed two roughly coin-
cidental peaks on 9— lOth July and 16—17th
July. Meltwater discharge displayed marked
diurnai patterns with low flows in the early
morning and high flows in late afternoon.
Peaks occurred on 9th July and 17th July. A
similar twin peak emerged in the suspended
sediment concentration curve, whilst the
solute concentration curves showed high
solute concentration at low discharges. The
twm peak of suspended sediment emerged
clearly when converted to total daily transport
rates, whilst the solute load remained
relatively constant in spite of variations in
discharge.
DISCUSSION
Diurnal variation in discharge appears to
relate to the air temperature variation, low
flows lagging behind low temperatures by
roughly 6 hours. More significantly, the over-
all trend in temperature does not display the
two distinct peaks of the discharge curve. This
indicates that the daily discharge rhythm,
related to solar radiation and temperature was
obliterated by fluctuations in run-off related
to precipitation and wind velocity variation.
As no direct sunlight reached the ice surface
over the study period, the relatively good
agreement between discharge, precipitation
and ice ablation leads to the suggestion that
during periods of high precipitation, ablation
increases since condensation and convection
increase. Such heat exchange processes are
greatly enchanced by high wind velocities and
this may have occurred on the 16th July when
the effects of generally lower air temperatures
may have been largely offset by higher wind
velocities. The crude sampling interval used to
measure wind velocity, ablation and
precipitationjon the ice surface (24 hrs) clearly
masks much of the detail, yet the precipitation
and ablation peaks appear to precede the dis-
charge peak by approximately 12 — 24 hrs.
Peak suspended sediment concentration
leads peak discharge by 2—4 hours in agree-
ment with numerous studies elsewhere (0strem
et al, 1967; Church and Gilbert, 1976). The prin-
cipal reasons for such hysteresis are generally
accepted to be the greater availability of
freshly slumped channel banks and the
greater velocities and water surface slopes
during the rising limb of the hydrograph.
However, the overall concentrations of sus-
pended sediment samples in River Mórilla are
low in comparison with glacial streams elsew-
here. Peak concentrations from Skeidarár-
sandur reach 39 g. 1 _1 (Church and Gilbert, 1975)
whilst 0strem (1975) records peaks of 2.5 g.l _1
and mean concentrations of 0.3 g. 1_1 from
Erdalsbre in Norway. The Mórilla peak con-
centrations were only 0.35 g.l-1 with mean
concentrations of 0.18 g. 1 However, the low
JÖKULL 31. ÁR 97