Watchful, a. — Webster enters it under “watchful”
Full of watch; vigilant; attentive; careful to observe closely; observant; cautious; -- with of before the thing to be regulated or guarded; as, to be watchful of one's behavior; and with against before the thing to be avoided; as, to be watchful against the growth of vicious habits. "Many a watchful night." Shak. "Happy watchful shepherds." Milton. 'Twixt prayer and watchful love his heart dividing. Keble.
Syn. -- Vigilant; attentive; cautious; observant; circumspect; wakeful; heedful. -- Watch"ful*ly, adv. -- Watch"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for watchfulness; he lists it under care, oversight.
Where he writes about it:
Watchfulness recognizes the possibility of danger, wariness the probability. — from care