Jökull - 01.07.2003, Side 49

Morphoclimates and morphodynamics of the northern Swedish Lapland and east Iceland
fect, which, however, is considered to be rather small.
Frost creep recorded on talus cones at higher altitudes
of the Austfirðir Mountains requires seasonal freeze-
thaw cycles with frozen ground of several months and
frost penetration depths of more than 50 cm (Beylich
1999a). High rates of geomorphic activity in the Aust-
firðir Mountains occur when air temperatures rise sig-
nificantly above freezing point during winter (Figure
10). High maximum temperatures in autumn and win-
ter cause high channel discharges due to intensive
thaws. In the lowest parts of the study area, there is
no complete snow cover over several months during
autumn and winter.
GEOMORPHOLOGICALLY RELEVANT
ASPECTS OF THE PRECIPITATION
REGIMES
Latnjavagge
Precipitation totals in this environment are mainly
connected with cyclonic activity. At the Latnjajaure
Field Station, the mean annual precipitation is 818
mm (1990–2001). The highest recorded annual sum
value is 990 mm (1993), the lowest 605 mm (1996).
The range between the highest and the lowest annual
sum thus is 385 mm. The ratio between the highest
and the lowest annual sum is approx. 1.6:1.
Precipitation is quite irregularly distributed over
the year, with October, recording 111 mm on average,
being the month with the highest precipitation (Figure
11). The lowest precipitation is in May with an aver-
age of 34 mm. The ratio between the mean for Octo-
ber and that for May is 3.3:1. Most of the precipita-
tion which occurs between October and May, and to-
gether accounting for 66% of the mean annual precip-
itation, is temporarily stored as snow. The thickness
of the snow layer in Latnjavagge normally reaches its
maximum in April. During the summer months June–
August, August records the highest mean precipita-
tion (82 mm). Altogether, precipitation from June to
August accounts for 24% of the mean annual precipi-
tation. The average number of precipitation days per
month ranges from 23 in October to 16 in July.
The magnitude-frequency analyses carried out
with daily precipitation values above 5 mm for the
months of May to October (Figure 12) provide infor-
mation on frequencies and recurrence intervals of pre-
cipitation events of certain magnitudes. Field research
in Latnjavagge showed that even daily amounts of
31.2 mm (rainfall event on August 8
, 2000) and 31.5
mm (rainfall event on July 12
, 2002) do not trigger
debris flows or slides on the slope systems. Due to
the stability of the slope systems and the almost com-
plete and very stable vegetation cover, there is also
no significant increase of suspended sediment concen-
trations (suspended sediment concentrations are nor-
mally 0–4 mg L ) in the creeks and channels of the
catchment area. Daily precipitation of 31 mm, caus-
ing saturation overland flow on the slopes and high
channel discharges are most frequent in August and
have a 4-year recurrence interval in this month. In
July such rainfall events can be expected every 16
years. It should be noted that the rainfall events of
August 8
, 2000 and July 12
2002 had durations
of several hours. The rainfall intensities during these
events were not extremely high. It is known that short
summer rainstorms with very high rainfall intensities
can cause debris flows and slides on the slope systems
and increased suspended sediment concentrations and
bedload transportation in the channels in a number of
other valleys in the Abisko mountain area (Jonasson
and Nyberg 1999). In the more stable Latnjavagge
drainage basin, higher concentrations of suspended
sediments (up to 50 mg L ) were only observed dur-
ing intense snow melt and were mainly caused by the
ice patches in the valley, mobile channel debris beds
exposing fines and material mobilized by slush flows.
The channel beds are characterized by stable step-
pool systems developed in debris. These fluvial step-
pool systems have been stable over the entire inves-
tigation period and only movements of single stones
over smaller distances (<15 m) occurred.
Longer periods without precipitation are highly
probable in June and above all in July. Longer dry
spells during the snow melt period have the effect
that the runoffs are to a large extent thermally de-
termined. Summer dry spells after snow melt lead
to low runoffs, with smaller creeks drying up com-
pletely. The drying up of vegetation-free regolith can
lead to increased deflation.
JÖKULL No. 52, 2003 47