Lovely name! (but then, I'm biased, as I have an Arabel). We chose it as Arabel and Arabella are used in DH's family a lot- she does often get called Arabella (sometimes by family members, who have clearly forgotton the exact variation we have gone for!) and I used Bella as a nickname.
I've met a couple of (non-family) Arabellas but only one other Arabel, who was an adult, so I doubt there will be too much of a surge!
Arable never occurred to me and was only mentioned once (by a friend who is a barley farmer so has the word on his mind more than most!).
I think it sounds nice, but I just see Arable. I'd suggest spelling it Arabelle, but I'm against -belle endings - OTT imo. I like Arabella though. Also Amabel, although I'm concerned about the connection with Amiable.
Was reading A&M to DS just yesterday and thinking "I should start a MN thread to see if other people like Arabel too or if it's just too much of an Isabel spin off". But you got there first. Oh well. Maybe I'll have an original thought one day.