Jökull - 01.07.2003, Side 52

Achim A. Baylich
Austdalur
The Austfirðir Mountains are characterized by high
annual precipitation, with normally low precipitation
intensities. The mean annual precipitation at the
Dalatangi meteorological station is 1431 mm (1960–
1996), which clearly exceeds the annual sum in Lat-
njavagge. During the 37 years from 1960 to 1996,
the range between the highest sum value (1985 mm
in 1974) and the lowest sum value (1008 mm in the
following year 1975) was 977 mm. The ratio between
the highest and the lowest value is approx. 2:1. Be-
tween 1960 and 1996 there was a gradual increase of
annual precipitation (Beylich 1999a; 2000c). A sim-
ilar increase of the annual sums was also recorded in
other regions of Iceland in the last decades (Jónsson
1991; Lawler and Wright 1996).
In Austdalur, precipitation also occurs quite irreg-
ularly over the year (Figure 11). The month with the
highest precipitation is October, with an average of
245 mm. The lowest precipitation is May with 78 mm.
The ratio between the mean monthly amount of Octo-
ber and that of May is 3.1:1. The precipitation from
December to April, which is also relatively high, is
to a large part temporarily stored as snow, and with
increasing altitude above sea level the share of snow
in total precipitation rises. Of the months accounting
for the major share of the early summer’s snow melt
(May-July), June - with 111 mm - records the highest
average precipitation. The average monthly number
of days with precipitation ranges from 22 in January
and 15 in June (Figure 11). The magnitude-frequency
analyses of daily precipitation of more than 10 mm
(Figure 12) carried out for the months May to Novem-
ber reveal once again high amounts of precipitation in
October and a low level in May and furthermore give
evidence of frequencies and recurrence intervals of
precipitation events of certain magnitudes. Field re-
search in Austdalur proved that daily sums exceeding
20 mm during times of intense snow melt lead to satu-
ration overland flow and to reinforced wash processes
in gullies as well as on lower slope areas which are al-
ready free of snow and vegetation-free due to turf ex-
foliation. These slope wash processes result in a sig-
nificantly increased aquatic slope denudation (Beylich
1999a). Daily precipitation exceeding 20 mm can be
expected once per year in May, 2.2 times per year in
June, 1.8 times per year in July, 4 times per year in
October and 2.6 times per year in November. Without
additional snow melt, rainfall sums of more than 40
mm d result in saturation overland flow and wash
processes, with such daily sums occurring every 3.4
years in May, in June every 1.4 years, in July every
1.9 years, in August every 1.5 years, in September ev-
ery 2.3 years, in October 1.7 times each year and in
November every 1.2 years. Compared with Latnjav-
agge, the debris beds of the tributaries in Austdalur
are less stable and more debris is transported during
higher discharges. Mobility of the channel debris beds
causes also a high supply of fine materials and an in-
crease of the suspended sediment concentrations in
the channels. The higher mobility of the channel de-
bris beds in Austdalur compared with Latnjavagge is
mainly due to the greater steepness of the Austdalur
drainage basin. A transfer of debris on talus cones
and in gullies caused by wash processes was identi-
fied only once during the 2-yr study period after an
extreme rainfall event with 92 mm/d in October 1997.
Furthermore, this extreme event entailed secondary
rockfalls from rock walls, very high runoff, increased
bed- load transport and bank erosion in the main chan-
nels. Daily precipitation of 92 mm can be expected to
occur every 91.1 years in May, every 25.1 years in
June, every 43.8 years in July, every 14.8 years in Au-
gust, every 126.8 years in September, every 5.4 years
in October, and every 20.2 years in November, with
a comparatively high probability being found in Oc-
tober. The geomorphic effect is expected to be espe-
cially high when such rainfalls coincide with intense
snow melt, which happens mainly during the autumn
and–with significantly less probability–in winter and
during snow melt in early summer (V. Þorgrímsson,
pers. comm., 1997). The extreme rainfall event of
October 1997 did not trigger new debris slides and
debris flows on the steep slopes of Austdalur. These
process events require higher amounts of rain than 92
mm d with correspondingly longer recurrence in-
tervals or comparable rainfalls combined with intense
snow melt and/or other favouring factors.
Longer dry spells in May and June have the effect
that runoff during the main snow melt period is rela-
50 JÖKULL No. 52, 2003