Jökull - 01.06.2000, Page 40

Surface and bedrock topography of Mýrdalsjökull
Table 1. Data for five ice catchment basins on Mýrdalsjökull – Gögn um fimm hluta Mýrdalsjökuls.
Name Area Volume Mean thickness Max. elev. Min. elev.
Flatarmál Rúmmál Meðalþykkt Mesta hæð Lægsta hæð
km 0 km 1 m m m
Sólheimajökull 47 12.5 268 1510 120
Entujökull 57 16.2 285 1510 420
Sléttjökull,
Botnjökull, 171 40.0 233 1370 620
og Öldufellsj.
Sandfellsjökull 66 ca. 15 223 1370 240
Kötlujökull 148 ca. 40 274 1497 200
Mýrdalsjökull 598 140 230 1510 120
the glacier. The watershed at the glacier base is loca-
ted where the gradient is zero for the fluid potential
(expressed as pressure, Pa):
23 5476987 3+: ;6 (2)
i. e. the sum of a term expressing the gravitational
potential and the water pressure  6 . The symbol 4 6
= 1000 kgm <
represents the density of water 8 =
9.82 ms < is the acceleration due to gravity and  3
is the elevation of the glacier substratum relative to
sea level. Water flow in an isotropic basal layer would
move perpendicularly to the equipotential lines.
The location of the water divides was predicted by
a theory of water-filled subglacial conduits (Shreve,
1972; Röthlisberger, 1972). The basal water pressure
was assumed to be
 6 >= ;? (3)
where ;? = 4#?@8BA is the ice overburden pressure, and= is a constant, 47? = 916 kgm <
represents the density
of ice and A is the thickness of the glacier. This is
a first-order approximation of the water pressure and
does not describe small-scale variations or fluctuati-
ons with the supply of meltwater.
In places where the water pressure is equal to the
atmospheric pressure (6 = 0) location of the water di-
vide, can be obtained directly from elevation contours
of the glacier bed, as if no glacier were present.
Atmospheric water pressure may occur in steeply-
sloping conduits near the edge of the glacier where
closure due to the ice overburden pressure cannot keep
up with the enlargement due to frictional melting (see
Hooke, 1984); thus, the water divides would entirely
follow the basal topography.
Under thick ice we expect the water pressure to
be close to the ice overburden, at least close to the
water divides where the water is flowing slowly and
melting of the subglacial tunnels by frictional heat is
negligible. On the basis of the predicted potential,
23
,
at the glacier bed, according to equations (2) and (3),
the Mýrdalsjökull ice cap is divided into three main
drainage basins (Figure 11, Tables 2 and 3).
Table 2. Water drainage basins on Mýrdalsjökull.
Delineation of water drainage basins – Vatnasvæði á
Mýrdalsjökli
Outwash plain Area Volume Mean thickness
Jökulsandur Flatarmál Rúmmál Meðalþykkt
km 0 km 1 m
Sólheimasandur 108 20.3 189
Markarfljótsaurar 167 38.5 230
Mýrdalssandur 323 (79) (244)
Total 598 138 230
Table 3. Water drainage basins within the caldera rims –
Vatnasvæði innan Kötluöskjunnar
Outwash plain Area Volume Mean thickness
Jökulsandur Flatarmál Rúmmál Meðalþykkt
km 0 km 1 m
Sólheimasandur 19 7.7 401
Markarfljótsaurar 23 12.2 525
Mýrdalssandur 60 28 (467)
Total 102 48 470
The predicted water drainage basins are larger th-
an they would be if the subglacial water pressure were
atmospheric. The local gradient in the ice overburd-
en pressure drives water out of the caldera through
JÖKULL No. 49 39