Your park with its mesquite bosque is quite a treasure and critical open space along Tanque Verde Creek. Below are several next step suggestions to consider to help the HOA save money on irrigation and maintenance, improve resident's appreciation and experience in the park, and enhance the wildlife habitat and pollinator value along the Tanque Verde as an important riparian area.
1. Irrigation Scheduling - starting immediately I would suggest shifting the park's irrigation schedule for the established trees to once per month with a duration of 3-5 hours. The irrigation for this zone can be turned off from November 1 to March 1 completely as all the trees are native trees (save 1 or 2) and are adapted for our climate and rainfall patterns. For next year I would suggest watering the trees 2x per month (every other week) between March through Oct. Then the following year it could be reduced to 1x per month.
2. Drip emitters - adjust the drip emitter placement to promote extension of the tree's roots out towards the canopy edge of each tree. This will promote healthy root growth which also leads to tree stability during high winds.
3. Design, plan, and implement pocket rain basins (rain gardens) among the trees. Be opportunistic and use the open, bare areas for creating these pocket basins. The basins should be scraped down to a depth of 4-6" and the excavated soil can be mounded (4:1 side slopes at a maximum) across the downstream edge to create a berm that enables even more water to be captured in the basin. Seed pocket basins with wildflower seeds (see seed mixes from Native Seed Search store) and some mid-story floodplain, habitat shrubs like desert hackberry, wolfberry, greythorn, four-wing saltbush, etc. Be sure to cage young plants to protect from ground squirrels, rabbits, and javelina.
4. Define and plan park zones that includes A) high use clean zone for the playground area, B) transition zone that expands out from the high use zone and along heavy use corridors, and C) a habitat zone along the perimeter and the northwest extension of the park.
5. Maintenance practice regime - based on the defined zones (see #4) shift maintenance practices in the habitat zone to allow for understory growth, filling out of mesquites, accumulation of leaf litter and mulch on the soil surface while maintaining a clear 1-2' buffer along trails. In the transition zone allow accumulation of organic mulch and understory pollinators but keep the mid-story clear for visible sight lines, and in the high use clean zone continue as is to keep a safe, clear environment.
6. Follow up and evaluate - be sure to stay on top of maintenance and irrigation regimes and regular communication feedback to contracted landscapers. Host educational sessions with wildlife experts to highlight benefits of habitat, roles of flora and fauna, etc to build interest in the shifting park habitat dynamic.
Here are a few additional resources that may be helpful:
1) WMG's Field guide for rain garden care
2) Agua Dulce Homeowners Association case study of using passive water harvesting (rain gardens) to save irrigation costs (Youtube video)
3) WMG's rainwater harvesting classes - see our calendar of upcoming events
4) Tucson Audubon has a "Habitat at Home" program that can be helpful to residents to understand habitat structure and needs for birds.