I have a mature cottonwood tree on my property.  Adjacent to the tree, the previous owners of my property had a large fire pit feature installed with a boulder retaining wall, likely severing some roots in the process.  This was done in approximately 2016 or 2017.

The tree is surviving, but appears to have a relatively thin canopy, with some branches that have few or no leaves.  When inquiring about removing another tree with advanced Emerald Ash Borer last year, an arborist pointed the Cottonwood out to me, and suggested to start saving for its eventual removal.

I watered the tree last summer with soaker hoses at the base, and am completing some irrigation work elsewhere in the yard.  I am considering putting in some "Deep Drip" watering stakes, connected to a drip system, near the Cottonwood to try and prolong its life.  Is this a reasonable plan?  Would you suggest 24 or 36 inch watering stakes, and how much water per day / week should I try to give the tree?  

Alternatively, is this completely unnecessary, or is the tree destined to eventually die from damage?

Any advice you have is greatly appreciated.