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==================
Glacier flow model
==================

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Modeling glaciers on a digital elevation model (DEM) based on mass balance and
a modified D8 flow algorithm applied to ice.

The modeling is based on a linear relationship between altitude and mass
balance, the so-called mass balance gradient. For alpine glaciers this gradient
is about 0.006 m/m. Continental glaciers tend to be at 0.003 and maritime
glaciers at 0.01 m/m. The alpine gradient is set by default in the model.
To model the glaciers, annual steps are calculated. First the mass balance
(accumulation and ablation) for the area is added to the glacier layer and in a
second step the glacier flow is simulated by using a modified D8 technique
(submodule :code:`fracd8`).
Since the prupose of the D8 flow direction algorithm is modeling surface water
flows over terrain, the D8 algorithm was modified to be able to consider the
fraction of ice that is flowing out of each cell based on the glaciers
velocity. In order to avoid pure convergence of the flow, the surface of the
glaciers is slightly smoothed. The simulation stops when the observed
difference in mass balance for a smoothed curve (default
:code:`MODEL_TREND_SIZE=100`) is less than a given tolerance (default
:code:`MODEL_TOLERANCE=0.0001`).

Getting started
---------------

The **glacier-flow-model** package depends on GDAL, which needs to be installed
on the system.

Get the stable release of the package from pypi:

.. code-block:: shell

    pip install glacier-flow-model

Example data
____________

The package includes an example DEM from `swisstopo <https://www.swisstopo.admin.ch/en/home.html>`_.
It covers a smaller extent around the Aletsch glacial arena in Switzerland with
a raster cell resolution of 200m.

.. code-block:: python

    from glacier_flow_model import PkgDataAccess
    dem = PkgDataAccess.load_dem()

The original DEM can be downloaded `here <https://www.swisstopo.admin.ch/en/geodata/height/dhm25200.html>`_.

Usage
_____

To set up a glacier flow model, a path to a DEM in the GeoTiff file format has
to passed to the model class constructor. By default the mass balance
parameters for alpine glaciers in the year 2000 are set. Keep the input file
size small, otherwise the model may be slowed down remarkably:

.. code-block:: python

    import logging
    from glacier_flow_model import GlacierFlowModel, PkgDataAccess

    LOG_FORMAT = "[%(asctime)s %(levelname)s] %(message)s (%(name)s)"
    logging.basicConfig(format=LOG_FORMAT, level=logging.INFO)

    gfm = GlacierFlowModel(PkgDataAccess.locate_dem())

After initialization, the model needs to accumulate the initial ice mass until
it reaches a steady state, call the :code:`reach_steady_state` method to do so:

.. code-block:: python

    gfm.reach_steady_state()

.. image:: https://raw.githubusercontent.com/munterfi/glacier-flow-model/master/docs/source/_static/steady_state_initial.png
   :alt: https://github.com/munterfi/glacier-flow-model
   :align: center

When the model is in a steady state, a temperature change of the climate can be
simulated. Simply use the :code:`simulate` method with a positive or negative
temperature change in degrees. The model changes the temperature gradually and
simulates years until it reaches a steady state again.

Heating 4.5°C after initial steady state:

.. code-block:: python

    gfm.simulate(4.5)

.. image:: https://raw.githubusercontent.com/munterfi/glacier-flow-model/master/docs/source/_static/steady_state_heating.png
   :alt: https://github.com/munterfi/glacier-flow-model
   :align: center

Cooling -3°C after initial steady state:

.. code-block:: python

    gfm.simulate(-3)

.. image:: https://raw.githubusercontent.com/munterfi/glacier-flow-model/master/docs/source/_static/steady_state_cooling.png
   :alt: https://github.com/munterfi/glacier-flow-model
   :align: center

Export the results of the model into :code:`.csv` and :code:`.tif` files:

.. code-block:: python

    gfm.export()

The GeoTiff contains the following bands, averaged over the last 10 simulation
years (default :code:`MODEL_RECORD_SIZE=10`):

1. Glacier thickness [m].
2. Velocity at medium height [m/a].
3. Mass balance [m].

Check out the `video <https://munterfinger.ch/media/film/gfm.mp4>`_ of the scenario simulation in the Aletsch
glacial arena in Switzerland

Limitations
-----------

The model has some limitations that need to be considered:

- The flow velocity of the ice per year is limited by the resolution of the
  grid cells. Therefore, a too high resolution should not be chosen for the
  simulation.
- The modeling of ice flow is done with D8, a technique for modeling surface
  flow in hydrology. Water behaves fundamentally different from ice, which is
  neglected by the model (e.g. influence of crevasses).
- The flow velocity only considers internal ice deformation (creep). Basal
  sliding, and soft bed deformation are ignored.
- No distinction is made between snow and ice. The density of the snow or ice
  mass is also neglected in the vertical column.

License
-------

This project is licensed under the MIT License - see the LICENSE file for
details
