[Take two]: Proposed GLIMS definition of "glacier"

[Bruce Raup]

Hello all,

Thanks for the feedback on how to define "glacier" for GLIMS.  Below is the
next draft, after thinking about all the comments.  Originally, the need
for a new definition was driven by the need to solve these two issues:

1) define what to do about connected snowfields above the bergschrund
2) define how to treat merging tributaries

Of course, more issues have come up in the process.  The definition below
generally specifies the minimum of what should be included in a "glacier",
under a single glacier ID.  In some cases, more may be included.  I think
it addresses the two issues above, plus some.  Please let me know what you
think of this version.  Again, please send comments to the list.

Bruce


$Id: glacier_definition.txt,v 1.2 2006/03/01 22:26:52 braup Exp $

Summary of comments posted to the GLIMS mailing list and my notes:

Andrew Fountain:  don't have an artificial lower-bound on
size; let the imaging technology determine what gets included, and
definitely include the small snow patches, since they are important
hydrologically.

Graham Cogley:  What do to about ice shelves?  He proposes to treat them
separately, and define an ice shelf as "An ice shelf consists of the
floating parts of two or more glaciers."  This is consistent with the
historical view of GLIMS to treat ice shelves as separate entities.

My comment on observation D:  Such a snowfield should not be included as
part of the glacier that dominates the basin lower down, but it well
could be included as a "glacier" with its own ID.

Vladimir Konovolov brings in all kinds of other stuff into the discussion,
including "subjective" reasons.  It is these subjective differences that
necessitate a precise definition of glacier for use within GLIMS.  Most of
the differences in GLACE results are from (1) lack of topo info, and (2)
differing ideas about what should be included.

Hugh Kieffer:  Somehow retain old names of feeder glaciers, even if they
are lumped together with the main trunks.  [One solution:  include all
relevant names in the "name" field for that "glacier".  That way it would
be searchable by name.]

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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 and subsequent discussion on the GLIMS
mailing list.

Candidate practical definition of "glacier" for the GLIMS Project:

  A glacier, identified by a single GLIMS glacier ID, consists of a body of
  ice and snow that persists through the end of the melt season, or, in the
  case of tropical glaciers, after transient snow melts.  This includes, at
  a minimum, all tributaries and connected feeders that contribute ice to
  the main glacier, plus all debris-covered ice.  Excluded is all exposed
  ground, including nunataks.  An ice shelf -- floating ice downstream of
  the grounding zone of two or more glaciers -- shall be considered as a
  separate glacier.

Consequences and observations:

A. Snowfields immediately 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.  The name field for
   the glacier should be populated with all relevant names of tributaries.

C. Any steep rock walls that avalanche snow onto a glacier but do not
   retain snow themselves are NOT included as part of the glacier.

D. [deleted]

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, separated at the
   topographic divide.  However, for practical purposes, such an ice mass
   may be analyzed as a unit at the analyst's discretion, 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.

G. Marginal and terminal moraines should be included (as debris-covered
   glacier) if they contain ice.

H. It is possible that an ice body that is detached from another may still
   contribute mass to the latter through ice avalanches, or it may no
   longer do so.  It is practically impossible to tell which is the case
   from a single satellite image.  Therefore, within GLIMS, adjacent but
   detached snow and ice areas should, in general, be considered as
   different "glaciers", regardless of whether they contribute mass to the
   main glacier through snow or ice avalanches.  However, at the analyst's
   discretion, detached ice masses may be included as parts of one glacier.
   This is similar to the situation described in F above.  If the pieces
   are analyzed separately later, each piece should be given a new ID, the
   old one being used as the "parent icemass" ID for all the pieces.

I. What about the lower parts of lateral snowfields, whose extent varies
   from year to year?  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].

-- 
Bruce Raup
National Snow and Ice Data Center
University of Colorado
449 UCB,  Boulder, CO 80309
Phone:  303-492-8814
http://cires.colorado.edu/~braup/