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Re: determining inflow of water in to model
I suggest you to use Cauchy bc for variable head because the use of Neuman BC is more related to specific flux like inflow or outflow, this can be applied to rechargue zones.
Using Cauchy you can extent your model domain a little to take a portion of the sea and use it like Cauchy Bc.
However if I have to do that model I would prefer use constant head bc in the coastal because is the simplest way to do, but it depends of the extent of your model.
I hope this can help you