After building and ronovating many landscapes in the desert I have started using a product call "Netafim" a drip line that can be arranged in a grid like pattern over the roots of established trees.

I believe this would be very useful in the case of your Ash tree since a bubbler here or there or even numerous drip emitters placed at the base of the tree will not as easily give you the uniform coverage over the entire existing root system that I have seen so far with the "Netafim" systems.

Although an established Ash tree is very tolerable of drought it is not a xerophyte and although the Netafim can help it would also be a good idea to incorporate an organic material over the roots. Even integrating lets say your flagstone.. some boulders and some larger sized rock with and organic surface would make soil conditions more tolerable for your Ash tree.

Landscape fabric is alomost always if not always a waste of time and money and I have not used it in over 15 years but I manage xeriscapes that have acres of drought landscapes. Weeds will still grow .. and the fabric will work it's way to the surface and haunt you .. plus "good fabrics" .. there are not any ...cost as much as sod.

Take out the grass carefully without undo damage to the tree roots don't simply cover it.

Roses for example are another plant that would do best with an organic base and not surrounded by rock mulch. If you are going to add a rock mulch to your landscape know thy plants and have a thought out reason why you are using rock and do not mindlessly add rock as some have covered the desert with grass.

A preemergent .. maybe .. What for ?? What weeds ?

If you are redoing the landscape get rid of as much old irrigation as you can .. old pipes and old valves should best be replaced. Less aggravation in the long run...

http://www.netafim-usa.com/

Good Day ....