Jökull - 01.01.2013, Side 87

Sensitivity of Icelandic river basins to recent climate variations
During positive NAO phases, warm and moist mar-
itime air is advected in winter by enhanced west-
erly flow over much of Europe while cold air is car-
ried by stronger northerly winds over Greenland and
northeastern Canada, decreasing temperature over the
northwest Atlantic (Hurrell et al., 2003). Cold season
cyclones tend then to be more numerous and slightly
more intense in the climatological Icelandic Low (IL)
region (Serreze et al., 1997). The NAO can exhibit
decadal variability and anomalous circulation patterns
associated with high NAO index have been observed
to persist over many winters (Hurrell, 1995). In Ice-
land, temperature and precipitation have been found
to be moderately but significantly correlated with the
NAO (Hurrell, 1995; Hanna et al., 2004; Crochet,
2007; Jónsdóttir and Uvo, 2009). This weak corre-
lation is probably related to the fact that Iceland lies
near one dipole of the NAO (Hanna et al., 2004).
CONCLUSIONS
Historical temperature, precipitation and streamflow
datasets were examined in order to study the sensitiv-
ity of various types of river basins in Iceland to past
climate variations. For this purpose, a set of 22 hy-
drological variables were chosen. The series were di-
vided into four subsets corresponding to warm, cold,
wet and dry years, and shifts in the hydrological vari-
ables between these subsets were analysed. Snow
storage and glaciers turned out to have a dominant
role in the streamflow generating mechanisms of the
catchments. Snow provides seasonal storage which
contributes a large fraction of the runoff in spring and
early summer while glacier melt is released in summer
adding a substantial contribution to streamflow until
early autumn for partly glacierized catchments. For
some catchments, large groundwater aquifers provide
a buffering effect on streamflow and regulate seasonal
variations. The nature of the hydrological response
of the watersheds in question to observed tempera-
ture and precipitation variations differed depending
on catchment type, geographical location and altitude,
revealing regional patterns.
A median annual temperature difference of 1.1–
1.4 ◦C between the warm and cold datasets was found
to alter streamflow seasonality and timing of hydro-
logical events substantially. Rain increased in the
south and west, snow storage was depleted almost ev-
erywhere and snow cover duration was reduced by
several weeks, causing the spring peak timing to oc-
cur several weeks earlier in the majority of the catch-
ments. Glacier melt increased of 20–40%. All these
changes had an impact on streamflow seasonality. For
non-glacierized catchments, an increase of winter and
early spring flow and a decrease of summer flow were
found, leading to longer summer recession whereas
for partly-glacierized catchments, summer flow was
maintained by increased glacier melt. As a conse-
quence of this temporal flow redistribution within the
year, the date on which half or more of the annual vol-
ume of water had flowed, occurred several weeks ear-
lier. In contrast to these changes which are associated
with streamflow timing and seasonality, annual flow
volumes remained similar between cold and warm
years in the north and northeast, whereas a signifi-
cant increase (10–20%) was observed in warm years
in southern and western Iceland, because temperature
increase was associated with a rainfall increase, rein-
forced by an increase in glacier melt. An increase in
flood occurrence rate by a factor of 2–4 was also noted
in southern Iceland in warm years which could partly
be related to rainfall increase.
A median annual precipitation difference of 40–
58% between the wet and dry datasets yielded signif-
icant increase of seasonal and annual rain, snow stor-
age and snowmelt, leading to an annual streamflow
increase of 10–35%. Glacier melt however did not
change significantly. Monthly streamflow increased
in a variable manner within the year, depending on
catchment type and location, but the flow seasonal-
ity and the timing of hydrological events, such as the
spring flow peak and center of volume date, were
not much altered because the temperature was usually
not significantly different between wet and dry years.
Precipitation increase was also associated with an in-
crease of flood occurrence rate almost everywhere, es-
pecially in southern Iceland, by a factor of 2–4.
In conclusion, the results of this work provide a
quantitative estimate of the sensitivity of Icelandic
rivers to temperature and precipitation variations that
are valuable for climate impact assessment studies.
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