Geoscience Reference
In-Depth Information
2005
for review). Characteristics of glacier discharge include pronounced melt-induced
diurnal fluctuations with daily peaks reaching several fold the daily minimum flows during
precipitation-free days. Glacier runoff shows distinct seasonal variations with very low
winter flows and a larger and seasonally delayed summer peak compared to non-glacier-
ized basins. Hence, glaciers can sustain streamflow during dry summer months and
compensate for otherwise reduced flows. Year-to-year variability is dampened by the
presence of glaciers in a catchment with a minimum reached at 10-40 % of glacierization
(Lang,
1986
). This so-called glacier compensation effect occurs when glacier runoff offsets
precipitation variations. Glaciers may also cause sudden floods, often referred to as Jo-
ekulhlaups, posing a potential hazard for downstream populations. Common causes include
subglacial volcanic eruptions or sudden drainage of moraine- or ice-dammed glacial lakes
(e.g., Lliboutry et al.
1977
; Bjornsson
2002
).
Annual runoff from a glacierized basin is a function of glacier mass balance, with years
of negative balance producing more runoff than years of positive balance. As climate
changes and causes specific glacier mass balances to become progressively more negative,
total glacier runoff will initially increase followed by a reduction in runoff totals as the
glaciers retreat (Janson et al.
2003
). With high percentage of ice cover, the initial increase
in runoff can be substantial, considerably exceeding the runoff changes to be expected
from any other component of the water budget. Adalgeirsdottir et al. (
2006
) modeled an
increase in annual runoff from ice caps in Iceland of up to 60 % until about 2100 followed
by a rapid decrease thereafter. However, in the long term, the loss of ice will lead to lower
watershed yields of water. Observations from gauge records in glacierized basins show
both increases in runoff, for example, along the coast in southern Alaska (Neal et al.
2002
)
or northwestern British Columbia (Fleming and Clarke
2003
) and negative trends in
summer streamflow, for example in the southern Canadian Cordillera (Stahl and Moore
2006
). The replacement of ice by temperate forest and alpine vegetation will further
decrease water yields.
In addition to contributing directly to runoff through ice wastage, glacier coverage
within a watershed decreases direct evaporation and plant transpiration, the combination of
which can result in substantially higher water yields for watersheds with glaciers compared
to unglacierized watersheds (Hood and Scott
2008
). In addition, the proportion of
streamflow derived from glacial runoff has profound effects on physical (Kyle and Brabets
2001
), biogeochemical (Hodson et al.
2008
; Hood and Berner
2009
; Bhatia et al.
2013
),
and biological (Milner et al.
2000
; Robinson et al.
2001
) properties of streams. As a result,
changes in watershed glacial coverage also have the potential to alter riverine material
fluxes. For example, area-weighted watershed fluxes of soluble reactive phosphorus
decrease sharply with decreasing watershed glacial coverage (Hood and Scott
2008
).
Recent evidence also suggests dissolved organic material contained in glacial runoff has a
microbial source and is highly labile to marine heterotrophs (Hodson et al.
2008
; Hood
et al.
2009
).
4.2 What is glacier runoff?
There is substantial ambiguity in the literature with respect to the way the importance of
glacier contribution to total runoff is quantified. Different concepts have been used
(Table
3
), and the importance will depend on how glacier runoff is defined. First, in its
most general sense, glacier runoff is defined as the runoff from the glacierized area, and
hence it includes all runoff exiting a glacier usually in one or several streams at the glacier
terminus (Concept 1 in Table
3
; Fig.
2
). According to this definition, it is the residual in
