We don't have data for non-agricultural crops this detailed, and the trees aren't too picky about soil moisture level as long as it's not excessively wet or dry. (After all, nearly all of the state is still in drought status and is many inches behind on rainfall for the year, and while stressed, most of the established trees are pulling through.) The degree of wet- or drought-tolerance also greatly depends on the species of tree, so there can be leeway in either direction based on tree type. Our advice is simply that someone feel the soil about six inches deep and judge from that...it can take practice, but there's wiggle-room for not watering at the exact point in time the moisture content drops below a certain percentage. Moisture-holding capacity also will vary with soil type, as clays retain water longer than sandy soils.

Sufficiently damp soil should feel slightly moist to the touch when probed at that depth; it will be darker than dry soil, will more easily stick to your fingers, and (more prominently in summer) will feel cooler than drier soil. If it were formed into a ball, it will tend to hold its shape when let go better than soil that got too dry. In comparison, soils that are drier than ideal (for most plants) will be dustier and easier to brush off, will be lighter in color, and will not feel appreciably cooler than soil at the surface. One technique might be to stick a wooden stake, dowel, yardstick, wooden skewer, or anything unvarnished down to that six-inch depth; leave it there a while (there's no set time...15 minutes, two hours...shouldn't matter too much) and see how damp it became when you pull it up. If it soaked up some moisture, then the soil is probably wet enough without needing irrigation; if the stick comes out pretty dry, then watering is probably going to benefit the plant.

Our Watering Trees and Shrubs page provides guidance for watering that might be helpful.

Miri