When to water the lawn?

Water in the early morning before 8:00 A.M. or when temperature and evaporation rate are low.
Don't water daily. This will produce a shallow root system which makes the grass less drought resistant.

How to water the lawn?

1 to 2 inches of water a week; apply enough water to soak the soil to a depth of 6 to 8 inches remember that water should NOT be allowed to run OFF the surface or to form puddles, as that lead to poor distribution and efficiency of water.
If you notice water running off the surface or forming puddles but you still want to apply more water, turn the water off for 15 minutes, then resume watering until the desired amount has been added.

Deep and infrequent irrigation tends to cause grassroots to grow deeper into the soil, making the plants more drought tolerant.
Shallow and frequent caterings lead to shallow-rooted plants with less drought tolerance.

Watering newly seeded lawn

Keep the soil surface evenly damp (soaking first 1 inch of soil), but not soggy, for the next 6 weeks or until the lawn begins to get established.
As the lawn continues to mature. Water for longer periods at a time but allow more time between watering and allow more time between watering and allow for deeper soaking and penetration into soil (3 to 4times per week). This encourages deeper root growth.
Bare spots: 6 weeks after the initial seeding, spot-seed bare and thin areas. It is important to continue proper watering.
Leaves: In fall, keep the lawn clear of leaves. Leaves can block sunlight and water, and suffocate young seedlings by "matting down".

Watering newly planted shrubs and trees

Newly planted trees have a limited root system and need lots of watering.
Water immediately after planting. Until new roots grow into the soil of the planting site, water the original root ball area and just beyond this area. The root ball area may dry out faster than the surrounding soil, so check the moisture in this area frequently.
Newly planted trees and shrubs need supplemental watering for their entire first year. Water thoroughly, adding 1" to 2" of water, every 4 to 7 days, depending upon how much rain has fallen.
Plants in sandy soil need to be watered more often, while plants in clay soil should be watered less often.