retrofitting water harvesting for established trees and bushes

Does anyone have any advice for how to make a mulch basin around trees and bushes that you want to keep when you begin your permaculture project?  Or how to use swales when you already have trees that may or may not be in the right spot?  All of the examples I have seen are starting from scratch.

The best resource I've found is Rainwater Harvesting for Drylands Volume 2 by Brad Lancaster.  There's also a lot of information on his website:  https://www.harvestingrainwater.com/

Basins and swales can be dug around existing trees and shrubs - just be careful to dig outside of the root zone, which may extend beyond the dripline of the tree or shrub. You can also make swales or basins by piling up dirt excavated from other areas.