Watering outdoor plants
If a plant is being watered by a sprinkler system and the foliage and a portion of the soil (but not all of the surrounding soil is getting wet), is the plant getting enough water if the sprinkler is running for twenty to thirty minutes?
Certified GKH Gardening Expert
It is typically not recommended to water foliage only because this can lead to fungal infections when the plants are not or unable to dry off due to poor air circulation. Therefore, bottom watering or drip irrigation is generally preferred. This does not mean you cannot use overhead watering, however. Simply do it at a time when you can ensure that the plants will have plenty of time to 'air out' so the foliage will dry.
For watering amounts, the rule of thumb is 2"per week. Normally with drip irrigation you can measure this by placing a small cup or jar in the area being watered and seeing how long it takes to fill it to 2 inches, usually 20-30 minutes. Overhead sprinklers would likely take much longer as the water is not going directly into the soil.