The following is from the book “Lawns 1-2-3”, Meredith Books 2007.

There are two rules of watering lawns.  The first: water infrequently. The second: water deep.

If you water deeply and infrequently, you will have healthy grass.  Healthy grass has a dense root system that grows deep into the soil and spreads widely.  The roots are able to reach far into the soil for nutrients.  And even if the soil surface dries out during a drought the roots remain in moist soil.

Water deeply and the roots will follow the water down into the soil.  Frequent light watering on the other hand, concentrate water in a narrow band near the top of the soil.  Roots have no incentive to extend below this band.  They remain short, tap into fewer nutrients, are less healthy, and when moisture disappears during the first drought, so does the grass,

Rule #1: Water infrequently

In most places the conventional wisdom is that you should let your grass begin to wilt before watering it: Don’t ignore the grass, but wait for leaves to just start to roll or fold up and for the lawn to take on a somewhat gray or purplish cast.  You can also test your lawn by walking across it.  Wilting grass is less resilient than grass that has plenty of water, so your footprints remain after walking across the lawn.  When that happens, it’s time to water the lawn.

Rule #2: Water deeply

Ideally, you want roots to grow at least 6 inches deep and closer to 8 inches deep. So you want to wet the soil that deep.  A general rule is to apply 1 inch of water at a time.  In practice, the amount of water you need to apply and the length of time the sprinklers need to run depend on the type of soil you have.  Given the variables, the best way to find out how deep the water and the roots go in your yard is to dig up a bit of lawn somewhere inconspicuous and measure both the roots and the depth of the wet soil after you water.

When deciding if the soil is wet enough, take rain into account as well as irrigation.  Here again, the type of soil plays a role as well as the amount of rain.  As a general rule, a half an inch of rain will soak 6 to 8 inches into sandy soil.  It takes an inch of rain to soak that deeply into loam.  For clay, you need to apply 1 to 1-3/10 inches of rain to soak the soil 6 to 8 inches deep.

Water in the morning

The best time to water is early in the morning.  It’s cool then so little evaporation occurs, and usually there’s less wind so all the water gets to the lawn.  And generally speaking, because there’s less demand for water in the morning, water pressure is higher.  That means more water flows per hour so you’ll be able to water in less time.

In the afternoon, conditions are reversed.  The temperature is higher so water evaporates more quickly.  Lower water pressure results in slower water flow.  And because the day tends to become breezier as the hours pass, the water is easily blown off target when you water in the afternoon.

Watering in the evening leaves the lawn damp overnight, and when dampness lasts for long periods diseases find the conditions they need to move in.