Add snow threshold for glaciermelt as a parameter

[PeterNelemans]

Feature type

Adding new functionality

Improvement Description

Currently, glacier melt in Wflow is only enabled if the snow cover < 10 mm. This is not always realistic, as glacier melt can still occur even if there is still a significant snowpack on top. Therefore, it would be nice if you can adjust this threshold.

Implementation Description

Add a new parameter to the staticmaps, which sets the threshold snow thickness for which glacier melt can occur. Check if we want to take the uneven snow distribution on a glacier into account (comment @jaapschellekens).

Additional Context

Together with @jaapschellekens, @cvanstrien and @tanerumit I was working on a wflow_sbm model of the Vakhsh catchment in Tajikistan, which is very glacier dominated. We had quite some trouble getting a realistic glacier and snowpack representation, as well as a decent fit of the modelled discharge with observations.