Jökull - 01.12.1970, Síða 62

TABLE 4 - TAFLA 4
The recession of Vatnajökull in the years 1894—1968 according to the average thinning of Breida-
merkurjökull in each 100 m altitude interval. The numbers in the brackets show the recession
according to the average thinning of Breidamerkurjökull.
Rýrnun Vatnajökuls á árunum 1894—1968 samkvœmt þynningarmatlingum á Breiðamerkurjökli.
Decrease
Time intervals in volume
km3
Timabil Rúmmálsrýrnun
kms
1894-1932 .................. 61 (80)
1932-1968 ................. 207 (270)
1894-1968 .................. 268 (350)
1967). The average thinning of the glaciers in
Iceland during the period 1931—1960 can thus
be estimated to have been comparable to that
of Vatnajökull, i.e. about 60—70 cm/year in
water equivalent. The area of those glaciers in
Iceland surpassing 20 km2 in size is 11,700 km2
(Thorarinsson 1958). The annual average in-
crease in the discharge during these 30 years
due to the glacial recession has therefore been
7—8 • 10° m3 which is 4—5% of the total run-
off for the country as a whole as shown on
Rist’s run-off map (1956).
According to the precipitation map compiled
by A. B. Sigfúsdóttir the glaciers receive about
22% of the total precipitation. This part of
the precipitation corresponds to 3.8 • 1010 m3
yielded as discharge. The glacial recession has
according to above calculations increased the
discharge from the glaciers for approximately
20%. As already mentioned Rist’s run-off map
(1956) and Sigfúsdóttir’s precipitation map
(1968) are not quite in accordance. The gap
between these two maps is about 1 • 1010 m3
in credit for the run-off. The glacial recession
factor in the run-off partly explains this dif-
ference but not fully since: the average run-off
seems to be 2 • 10° m3 greater than the average
precipitation. This difference 2—3% in fact is
small, but surely some part of the precipita-
tion goes to the evapotranspiration or escapes
as ground-water seepage direct to the ocean.
Those factors again add to the discrepancy. The
magnitude of the evapotranspiration and
ground water seepage is very difficult to esti-
60 JÖKULL 20. ÁR
Average thinning
per year in cm
water equivalent
Arleg þynning
i cm vatnsgildis
17 (22)
61 (80)
38 (51)
Increase in discharge
kl/sec
Aukning í afrennsli
kl/sek.
47 (60)
164 (215)
103 (135)
rnate, but it hardly exceeds 2—2.5 • 1010 m3/
year. Then the run-off map demands about
15% greater precipitation than the precipita-
tion map accounts for. No opinion will here
be put forward as to the causes of this differ-
ence. There are many possible ones especially
if it is kept in mind how difficult it is to mea-
sure many of the factors influencing the water
balance. The glacial recession, the factor here
under discussion, might e.g. be somewhat under-
valued since those factors to which the mea-
surements do not apply have rather been under-
rated. Although the glacial recession were some-
what underrated it would never suffice to close
this gap.
Despite all, this difference, considerable as
it is, can neither be considered gross nor as
any distrust of the precipitation and run-off
maps, rather it is just an affirmation of the
fact that more measurements and investigations
are needed for determining the quantity of
precipitation and run-off where Iceland is con-
cerned with a fair degree of accuracy.
REFEREN CES
Ahlmann, H. W:son and Thorarinsson, S. 1943:
Vatnajökull Scientific Results of the Swe-
clish-Icelandic Investigations. 1936—1938.
Reprinted frorn Geogr. Ann. 1937—1940,
1943. Stockholm.
Bauer, A. 1955: Contribution á la Connaisance
du Vatnajökull-Islande. Jökull 5: 11—22.