From Iowa DNR –
DES MOINES ― As drought conditions continue, Iowans are concerned about the amount of water their yard trees need. Drought conditions can lead to tree decline, pest problems and, in extreme cases, tree mortality. Supplemental watering for mature trees will help maintain tree health during drought.
The best way to provide supplemental water to trees during a drought is manually. A bucket to measure how many gallons is best, but a garden hose, moved often, can provide a good soil soaking. Sprinklers are less efficient.
Water the soil surface within and just beyond the canopy, one gallon of water per square foot. For newly planted trees, apply water directly over the root ball.
Water trees at night or in the early morning. This is when trees will be able to receive the most water with less evaporation.
Water one to three times a week, based on temperature and soil type, if there is no rainfall in that week. One or two deep and heavy watering times are much better than many light, shallow watering times.
Trees that have been in the ground for less than five years should have priority over older trees. Smaller trees need about one inch of rain a week. That is equivalent to about 10 gallons of water.