What exactly is it you are trying to accomplish? Are you hoping the trees
will
suck up excess water? They will take up only what they need - no more, no
less. How deep the roots penetrate is not really material. If you have
soggy
ground, then yes, it does make sense to plant trees which are moisture
lovers,
but don't expect them to achieve good drainage for you. They are simply
not
programmed in that fashion.

I wouldn't really call it drainage per se -- that's part of the goal here,
but there's much more to it than that.

The area gets heavy runoff during the spring courtesy of the church that
sits on the hill behind us -- the whole property is a hill behind a hill
behind a hill. The topmost hill is every bit of a few hundred feet up from
where our house sits, and about a mile away as the crow flies. Trees that
are moisture lovers will enjoy sucking that water out of the hill -- if they
have an agressive root system, they'll do even better. I've researched the
use of trees for these purposes and it's actually quite common in industrial
applications.

Deep roots will ensure a couple of things:

1) The area is already surrounded by trees however these species do not have
very deep root systems. Competition for resources during drier periods
could prove difficult for the new trees, so if they have deep roots they'll
have access to far more nutrients.

2) When there is a heavy rain and/or heavy runoff, the area back there
becomes saturated and water of course seeps further into the ground. A tree
bank with deep roots will aid in absorbing some of that water up. It will
also help in anchoring soil.

3) Sometimes we some pretty gusty winds back there -- I'm surprised that a
few trees have managed to survive (I've seen them bend pretty far over).
Deeper roots typically means better anchored.

4) Things on the surface with shallow root systems can survive under things
with deep root systems. That means I can get a decent groundcover to grow
or possibly even some grass.

So it's a multi-problem solution.