Let's use an example to illustrate how to add a field in the model. I want to add a field `melting_rate`  to the class `calving` (I want to have `md.calving.melting_rate`) that is defined by the user. 

== Modify the User Interface ==

Go to `src/m/classes` and modify `calving.m` and its python equivalent.

 1. Declare the new field in `properties`
{{{
#!m
   melting_rate = NaN;
}}}
in ISSM, we use `NaN` for vectors and `0` for scalars. The default value is added with some explanation in the `setdefaultparameters` function of the class.

 2. Add a default value in  `setdefaultparameters` (Optionals, for scalars only, Literature reference is encouraged)
 3. Add a consistency check
{{{
#!m
md = checkfield(md,'fieldname','calving.meltingrate','NaN',1,'size',[md.mesh.numberofvertices 1],'>=',0);
}}}
which means that we want no `NaN`, the vecteur should be of size `numberofvertices` with positive values only.
 4. Add description in `disp`
 5. Add field to `marshall`

== Add new Enums ==
Now, you have to [wiki:addnewenum add the Enum] corresponding to your new field whose Enum name will be : ClassenameFieldnameEnum with Classename the name of the class with upper case for the first letter (here it will be Calving and Calvinglevermann for Classename) and Fieldname the name of your field you put in the previous files with upper case for the first letter (I put `meltingrate` in `calving.m` so Fieldname is Meltingrate). So I will add 2 Enums : CalvingMeltingrateEnum and CalvinglevermannMeltingrateEnum. 

NB : don't forget to synchronize the Enums after it.

== Implement the new field in the model ==

First of all, you have to fetch the data in the `UpdateElements` function:

{{{
#!c
iomodel->FetchDataToInput(elements,CalvingMeltingrateEnum);
}}}

After, load the input in the computation function (i.e. wherever you need to use this input):  

{{{
#!c
Input* meltingrate_input = element->GetInput(CalvinglevermannMeltingrateEnum); _assert_(meltingrate_input); 
}}}

You can then use `meltingrate_input` like any other input.

== Add a Nightly run test ==

Now, you have to [wiki:addnewNR add a Nightly run test] to check your new field keeps working as desired. 

NB : don't forget to modify other tests that could be concerned by the addition of this new field. For example in my case, I have to write 

{{{ 
#!c
md.calving.meltingrate=zeros(md.mesh.numberofvertices,1);
}}}

in some other already existing tests.