Jökull - 01.01.2013, Side 83

Sensitivity of Icelandic river basins to recent climate variations
the elevated catchment VHM-144 where annual flow
peak shifted later in the year in wet years and for the
southern catchment VHM-96 where the spring flow
peak occurred earlier in wet years. Increased annual
precipitation caused a significant increase of flood oc-
currence rate (FOR) in a majority of the catchments,
by a factor of 2–4, especially above the moderate
threshold (POT-3). The flood occurrence rate above
the higher threshold (POT-2) significantly increased
in the south and in central northern Iceland, but re-
mained unchanged for the three northernmost catch-
ments.
The annual cycle
The hydrological response to climatic variations was
analysed at a monthly resolution in order to help in-
terpreting the changes in annual patterns mentioned
above. Monthly mean streamflow discharge (MQ),
rain (MR), snowpack (MS), snowmelt runoff (MSR)
and glacier melt runoff (MGR) were calculated and
their median values in warm, cold, wet and dry years
compared with the Mann-Whitney test. Results for
all watersheds are summarized below and examples
for three catchments i.e. one non-glacierized and two
partly-glacierized, are given in Figures 5–7.
Snowpack
The monthly snowpack usually peaked in April. Tem-
perature and precipitation variations significantly af-
fected snowpack seasonality. A thicker snowpack
was accumulated in cold years compared with warm
years in winter, spring and early summer for all catch-
ments, except VHM-66 in the west where no signif-
icant differences were found. For the southern wa-
tershed VHM-64, the period during which the snow-
pack was significantly different between warm and
cold years was shorter and started later than for the
other catchments. Wet years accumulated a signifi-
cantly thicker snowpack throughout winter and spring
than dry years, for all catchments, which then sur-
vived longer in summer.
Snowmelt
Winter snowmelt runoff was usually not significantly
different between warm and cold years, except in Jan-
uary where it was commonly larger in warm years.
For most catchments, spring snowmelt runoff was af-
fected by temperature increase and was often larger in
warm years than in cold years, usually in March and
April, seldom in May, depending on catchments. In
June and July, snowmelt runoff was generally lower
in warm years than in cold years, because the smaller
initial spring snowpack was depleted earlier. The an-
nual peak of monthly snowmelt runoff was often esti-
mated to occur one month earlier in warm years than
in cold years. Snowmelt runoff remained mainly un-
changed between wet and dry years at most catch-
ments throughout most of the winter and spring, ex-
cept in the south where spring snowmelt was larger in
wet years than in dry years. A significant snowmelt
increase was seen almost everywhere in June and/or
July in wet years compared to dry years, especially in
June, because of a larger snowpack to melt. No signif-
icant difference was found during these months for the
southern catchment VHM-96, probably because the
snowpack increase in wet years was not large enough
to produce significantly larger snowmelt.
Glacier melt
The glacier melt occurs in May to October with a
peak in July. As expected, temperature variations
had a strong influence on monthly glacier melt runoff,
which was estimated to be significantly larger in warm
years than in cold years. A comparison between wet
and dry years did not reveal significant differences for
most of the melting period.
Rain
The amount of rain is highest between July and Oc-
tober and lowest in winter. Warm years brought more
rain than cold years to most catchments in Septem-
ber, but not to the elevated VHM-144, where it was
unchanged, nor to the northeastern catchment VHM-
26 where a decrease was found. No difference was
usually noted in winter between warm and cold years
except in the south (VHM-64 and VHM-96) and in the
northwest (VHM-19) where rain increased in some
months in warm years. A significant increase of rain
was found at most catchments in warm years com-
pared to cold years, in spring. Rain did not change
much between warm and cold years in summer. A
significant increase was found in wet years compared
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