Use minimum area and length in a calendar year for the diagnostics?

[fmaussion]

Thanks @lilianschuster for reporting.

We use the area and length in Dec for the diagnostics - this is much more likely to be influenced by snowfall!

[anoukvlug]

I think it would be very useful to switch to saving the yearly minimum length, area and for consistency also volume in the diagnostics file, instead of the December ones. :)