Jökull - 01.01.2013, Page 75

Sensitivity of Icelandic river basins to recent climate variations
Table 2. Hydro-climate variables. – Vatna- og veðurfarsbreytur.
Name Definition
AT Annual mean temperature (◦C)
AP Annual precipitation (mm/year)
AR Annual mean rainfall (mm/day)
AWR Annual mean total water input runoff (mm/day)
AQ Annual mean streamflow (m3/s)
AMS Annual maximum snowpack (mm SWE)
DAMS Timing of annual maximum snowpack (Julian days since 1 Sept.)
ASD Annual snowcover duration (days)
ASR Annual mean snowmelt runoff (mm/day)
AGR Annual mean glacier melt runoff (mm/day)
AGD Annual glacier melt duration (days)
CTS Center of volume date of snowmelt (Julian days since 1 Sept.)
CTW Center of volume date of total water input (Julian days since 1 Sept.)
CTQ Center of volume date of streamflow (Julian days since 1 Sept.)
AMF Annual maximum streamflow magnitude (m3/s)
DAMF Timing of annual maximum streamflow (Julian days since 1 Sept.)
SMF Spring maximum streamflow magnitude (m3/s)
DSMF Timing of spring maximum streamflow (Julian days since 1 Sept.)
FOR Flood occurrence rate (average nb of events / year)
MQ Monthly mean streamflow (m3/s)
MS Monthly mean snowpack (mm SWE)
MSR Monthly mean snowmelt runoff (mm/day)
MGR Monthly mean glacier melt runoff (mm/day)
MR Monthly mean rainfall (mm/day)
used. Temperature-index melt models have been dis-
cussed by e.g. Hock (2003) and Shea et al. (2009).
Some uncertainty can be expected in the estimated
melt rates due to spatio-temporal variation of melt fac-
tors.
For glacier-free catchments, daily rain and
snowmelt runoff were averaged over each watershed,
and total water input runoff estimated as the sum of
these two components. For partly-glacierized catch-
ments, glacier melt runoff and snowmelt outside the
glacier were analysed separately, for comparison of
their respective contribution. The total water input
runoff was then calculated as the sum of daily rain,
snowmelt and glacier melt runoff. Monthly and an-
nual series were calculated from these different com-
ponents and from streamflow measurements, by aver-
aging the daily values (Table 2). The flow of a river
represents the integrated basin response to various cli-
matic inputs, with precipitation and temperature be-
ing particularly important (Hodgkins et al., 2003).
A significant correlation was observed between an-
nual mean streamflow measurements (AQ) and annual
mean total water input runoff (AWR) with a coefficient
of determination (R2) in the range 0.2≤R2≤0.75 and
p<1%, with 6 out of 8 catchments having R2>0.5.
The daily snowpack was averaged over each wa-
tershed and the magnitude (AMS) and timing (DAMS)
of the annual maximum extracted. Monthly mean
snowpack series were also calculated. Annual snow
cover duration (ASD) was defined as the number of
days per hydrological year on which the snowpack ex-
ceeded a threshold, arbitrarily defined as 20% of the
annual maximum mean daily snowpack for the 1971–
2000 period. The glacier melt duration (AGD) was de-
fined as the number of days per hydrological year on
which daily glacier melt runoff exceeded 20% of the
annual maximum mean daily glacier melt runoff for
the 1971–2000 period. The annual flow peak magni-
JÖKULL No. 63, 2013 75