Funny how different things are from region to region :-)) Your "major drought" is our normal state of affairs from June thru September - we typically average just a scosh over 3" during this period but haven't hit it yet. In fact, if it weren't for a pretty decent rainstorm on the 1st, we'd be well under "normal" rainfall totals.

All of that is to say is that drought is relative. While your larger established trees may have come to 'expect' more rainfall than you have received this season, they can probably get by without it just fine - for the most part, ours do :-) Yes, we see some early drying and dropping of foliage in some cases but what is lacking in rainfall in summer here is offset by adequate rainfall the rest of the year. (And before you think that we are inundated with rain here in the soggy Pacific Northwest, the Puget Sound area, including Seattle, averages only 36" of rainfall annually, compared to Boston's 44")

But to answer your question, dumping the rain barrel at the base of the tree - the area immediately surrounding the trunk - was very probably more of a waste of water than a boon to the tree. The fine feeder roots - those that seek out and absorb the bulk of the required moisture - ARE located at the dripline and extend quite far beyond as well. And because these large, well-established trees have such a far reaching root system, it is often difficult to supply water in times of perceived need. And generally is unnecessary :-) Watering trees here in summer pretty much only applies to those that are newly planted and not yet well established.

Have you watered your lawn at all? Other flower or planting beds anywhere in the tree's vicinity? If so, you may have supplied your tree with more water than you realize.