Proposed GLIMS definition of "glacier"

[Andrew G. Fountain]

Dear All,

The database for the American West (exclusive of Alaska) includes all 
permanent snow or ice bodies of which glaciers are a subset (perennial 
snow or ice that moves).  Our map and satellite investigations will not 
be able to determine movement on the small features.  Whereas an area 
threshold could be one part of a definition it also depends on slope.

On the other hand, from a hydrologic perspective, it doesn't matter if 
the snow or ice body is a glacier or not.  Although small, there are 
many small perennial snow or ice bodies of which the aggregate sum is 
substantial.  To ignore these is to ignore, what I argue, is a major 
water resource in high alpine zones.  Furthermore, why define a 
threshold?  Let the technology define that and let the user of the data 
set the threshold for their particular purpose.

The question for me is what defines "perennial"?  Again we cannot 
determine that from a single image.  We do have a legacy of historic 
photographs that shows some small snow patches composed of ice and have 
existed for decades.


Andrew

-- 
Andrew G. Fountain
Departments of Geology and Geography

Address:                            FedEx/Shipping Address:
------------------------            --------------------------
Department of Geology               Department of Geology
Portland State University           17 Cramer Hall
Portland, OR  97207-0751            Portland State University
USA                                 1721 SW Broadway
                                     Portland, OR 97201
Email:  andrew at pdx.edu              USA
http://www.glaciers.pdx.edu
Phone:  503-725-3386
Fax:    503-725-3025

> Through experience with data submitted to the GLIMS Glacier Database, as
> well as the GLACE experiments, it has become apparent that a more precise
> practical definition of "glacier" is needed within the GLIMS project.  Such
> a definition will help ensure consistency of analysis results in the
> database, facilitating more meaningful multi-temporal comparisons and
> change-detection results in the future.
> 
> The following candidate definition is the result of discussion at the GLIMS
> Workshop in Twizel, New Zealand.  Trevor Chinn began the discussion by
> reciting several definitions of "glacier", and the ensuing discussion
> touched on various practicalities of observing glaciers from space.  The
> resulting definition is heavily influenced by the original glacier mapping
> instructions for the World Glacier Inventory (WGI).
> 
> Candidate definition of "glacier" for the GLIMS Project:
> 
>   A glacier consists of a body of ice that persists for a number of years
>   and is greater in area than approximately 0.01 km2 (approximately 40
>   ASTER VNIR pixels), measured at end of summer, or, in the case of
>   tropical glaciers, after transient snow melts.  This includes all
>   tributaries, connected feeders,  disconnected or icefall-interrupted
>   feeders that contribute ice to the main glacier, plus all debris-covered
>   ice.  Excluded is all exposed ground, including nunataks, together with
>   any adjacent snow and ice areas that do not contribute ice, even though
>   they may be the source of snow avalanches and wind redistributed snow,
>   both of which are considered to be part of seasonal accumulation.
>   Joined glaciers should generally be separated at their topographic
>   divides, though in some cases, an analyst may treat several glaciers that
>   are connected but that have divergent flow (i.e., they don't contribute
>   mass to each other) as a single unit.
> 
> Consequences and observations:
> 
> A. Snowfields above the accumulation zone of a glacier shall be considered
>    part of the glacier, because they contribute snow (through avalanches)
>    and ice (through creep flow) to the glacier.
> 
> B. A tributary in a glacier system that has historically been treated (and
>    named) as a separate glacier should, within the GLIMS framework, be
>    included as part of the glacier into which it flows.
> 
> C. Any steep rock walls that avalanche snow on to a glacier but do not
>    retain snow themselves are NOT included as part of the glacier.
> 
> D. Separate snowfields above the accumulation zone of a glacier shall NOT
>    be considered part of the glacier, because they contribute only snow
>    (through avalanches) in the same manner as avalanches off rock.
> 
> E. A stagnant ice mass still in contact with a glacier is part of the
>    glacier, even if it supports an old-growth forest.
> 
> F. If no flow takes place between separate parts of a continuous ice mass
>    they should, in general, be treated as separate units.  However, for
>    practical purposes, such an ice mass may be analyzed as a unit at the
>    analyst's discretion.  See question 1 below.
> 
> G. Marginal and terminal moraines should be included if they contain ice.
> 
> Questions:
> 
> 1 Does this mandate the delineation of flow divides?
> 
>   ANSWER: No, an ice mass containing flow divides can still be analyzed as
>   a unit, if delineation of the flow divides is impossible or impractical.
>   If the same system is analyzed in the same way later, it will have the
>   same glacier ID, and can therefore be compared.  If the system is
>   analyzed in more detail later by breaking it into its component glaciers,
>   those pieces will get new IDs (ID of system will be "parent icemass" ID),
>   and future analyses of those pieces, if done in the same way, will be
>   comparable.
> 
> 2 What about the lower parts of lateral snowfields, whose extent varies
>   from year to year?
> 
>   ANSWER:  Map only at the end of summer to exclude seasonal snow.  Then
>   map everything that is connected to the glacier.
> 
> For details on how to produce outlines consistent with this definition,
> see the GLIMS Glacier Analysis Tutorial at
> http://www.glims.org/MapsAndDocs/assets/GLIMS_Analysis_Tutorial.pdf
> [to be updated soon].
>