Most lawns, garden plants, flowers and shrubs are the happiest when they get about an inch of water per week (but they may require more based on sun exposure and heat during dry spells). *Pro Tip: Use a rain gauge and place it in the garden to measure the amount of weekly rainfall.
Use drip irrigation to water at the soil level and soak the root ball thoroughly. The root ball is usually as wide as the plant and could be 1-2 feet deep depending on the type of plant and how established the plant is.
This ensures that the plant is hydrated and ready for the heat of the day while also saving water by preventing moisture loss due to evaporation. In addition, it will also make it much more difficult for plant disease to get a foothold when all plant material had the entire day to dry out. Second best time is early evening.
Typically, the emitters on your drip system release .6 GPH. This allows the soil to saturate slowly and evenly limiting run off (saving water and maintaining fertilizer and plant nutrients.) Lawns and annuals concentrate their roots in the top 6" of soil; for perennials, shrubs and trees, it's the top 12". In heavy soil, it may take hours for the water to percolate down to a depth of 6-12". *Pro Tip: Use your finger or a shovel to check the progress.
Plants need oxygen as much as they need water. It is actually beneficial for plant material to dry out a bit in between watering (especially true for container plants). Best practice is watering slowly, deeply, and less frequently.
Apply a thin layer (~1 inch) of organic mulch such as compost, shredded leaves, shredded bark or pine needles to help reduce evaporation even further and reduce runoff. Any more than 1 inch and it can actually start working against you by preventing moisture from reaching the roots.