Though lawns do need lots of water, many gardeners are too generous, often providing twice the amount the lawn really needs. Overwatering does more than just waste water. It leaches fertilizer and natural soil nutrients from the root zone; it creates perpetually wet conditions that can encourage disease. And a heavily watered lawn grows faster and requires more mowing.
In general, warm-season grasses require less moisture than cool-season types; the tall fescues are among the least needy of the cool-season sorts. On average, however, most grasses need 1 to 2 inches of water per week (except when rainfall makes up the difference).
To encourage roots to grow deep, it's best to water infrequently, adding the 1 to 2 inches all at one go. If you simply sprinkle on a little water each day, the roots will stay near the surface. If there is then a prolonged dry spell or if you forget to water, the root system won't be able to draw enough water from deeper in the soil to survive.
After watering, wait until the top inch or two of soil has dried before watering again. To check, probe the soil with a thick piece of wire or a long screwdriver: it will move easily through moist soil but stop when it reaches firmer, dry soil. You can also use a soil sampling tube. An even faster way to tell if a lawn needs watering is simply to walk across it. If your footprints remain for several minutes, it's time to water (a well-watered lawn springs right back).
Water early in the day, when there's less moisture loss due to heat evaporation and wind is less likely to blow water away. If you need to water later in the day, do so well before dusk, so the grass will dry before nightfall; grass that stays damp for long periods is typically more susceptible to disease.
Most lawns are watered by sprinklers--either the hose-end sort or those that are part of a fixed system.
Available hose-end sprinklers include oscillating sprinklers, "machine-gun" pulsating types, revolving-arm sprinklers, and "walking" sprinklers that move along a laid-out hose. Delivery patterns and amounts vary from one model to another. The shape and slope of your lawn will affect the type you choose; kinds that deliver water relatively quickly may cause runoff on slopes, for example.
Be aware that all hose-end sprinklers have some uneven distribution, providing more water to some areas than to others. If you end up overwatering most of the lawn just to get enough moisture to those few areas that receive less water, consider changing the type of sprinkler or where you place it. This may be all that's needed to conserve a fair amount of water each week.
Underground sprinkler systems
If lawns in your area require regular watering--or if you'd just rather not bother with manually placing and moving sprinklers--consider installing an underground sprinkler system. If planned carefully, these systems virtually eliminate the problem of uneven water delivery. In fact, the design is the most important part of the job: you'll need to determine the placement of pipes and sprinkler heads, making sure that the water patterns from each head overlap for even coverage. Many gardeners hire a licensed landscape contractor to plan and install such systems. If you want to do the job yourself, consult Sunset's Garden Watering Systems or the booklets provided by the major component manufacturers.