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It is important to note that this definition is not equivalent to the glacier's mass balance
(budget) since it does not include the accumulation term but only the fraction of meltwater
that does not refreeze, and exits the glacier.
Third, sometimes glacier runoff is understood as the meltwater runoff originating solely
from ice/firn melt, i.e., melt of snow on the glacier is accounted for separately (Ko-
boltschnik et al.
2007
; Weber et al.
2010
). This definition is consistent with the view that
all other components (snow melt, rain etc.) would exist for the glacierized area even if the
glacier did not exist. Hence, only the excess water due to the presence of the glacier is
considered (Concept 3). This component is difficult to measure directly since it requires
detailed measurements of melt at the surface in concert with the observations of snow line
retreat and therefore is better quantified by mass-balance modeling which can separate the
components of mass change.
Glacier runoff following Concepts 1-3 will affect river runoff in a glacierized catch-
ment no matter whether or not the glacier over the time span considered had a positive, a
negative, or a balanced mass budget. In contrast, glacier runoff is sometimes defined as the
runoff component that is due to glacier net mass loss, hence referring only to the water
originating from the glacier volume (storage) change (Concept 4, Huss
2011
). Lambrecht
and Mayer (
2009
) refer to this component as ''excess discharge'' since it constitutes
additional water due to the reduced storage volume of glaciers that is not available in
unglacierized catchments. Accordingly, in contrast to Concepts 1-3, glacier runoff is zero
when the glacier's mass budget is balanced or positive, no matter how much meltwater is
leaving the glacier. Hence, a glacier only affects runoff if there is a net mass loss during the
considered time period. In this case, the glacier runoff is equivalent to the glacier's
(negative) mass budget, which can be measured directly using the methods described in
Sect.
2
. Some studies have extended this definition to include the balance of liquid pre-
cipitation and evaporation (P
l
-E; Concept 5; Dyurgerov
2010
). Finally, Kaser et al. (
2010
)
consider glacier mass loss assuming a balanced annual mass budget, i.e., water from net
mass loss is not considered. In this case, annual glacier runoff effectively corresponds to
annual snow accumulation (Concept 6).
In summary, definitions vary with respect to the inclusion of water not generated from
melt and whether snow accumulation is included. Snowmelt runoff from the glacier can be
substantial (Fig.
2
) and is included in some, but excluded in other studies. It is obvious that
the absolute amounts of glacier runoff and the degree to which glacier runoff affects total
runoff of a glacierized catchment depend on the concept used in defining glacier runoff. It
is paramount that any investigations aimed at assessing the importance of glacier runoff in
total runoff clearly define the quantity used.
5 Assessing global-scale impacts of glaciers on the hydrological cycle
Analyses based on the observations or modeling in individual glacierized river basins have
highlighted the role of glaciers in the hydrological cycle and indicated significant hydro-
logical changes in response to climate change, including changes in total water amounts
and seasonality (e.g., Braun et al.
2000
; Casassa et al.
2006
; Rees and Collins
2006
; Hagg
et al.
2006
; Horton et al.
2006
; Yao et al.
2007
; Huss et al.
2008
; Immerzeel et al.
2013
;
Koboltschnik et al.
2008
; Stahl and Moore
2006
; Kobierska et al.
2013
). However, few
studies have investigated the hydrological effects of glaciers on regional or global scales.
Dyurgerov (
2010
) updated an earlier study by Dyurgerov and Carter (
2004
) and
investigated the role of glaciers in freshwater inflow to the Arctic Ocean by comparing the
