Before we go into the correct method to watering the lawn let’s make one thing perfectly clear. If a lawn does not get enough water it will turn brown. When it turns brown it means that the grass has gone dormant in order to survive periods of stress. It does not mean that the grass is dead. (Unless of course it is attacked by chinch bugs because it is in a weekend state.) Many times customers will panic when the grass turns brown because they fear permanent damage. It is your job to let them know that is perfectly acceptable for grass to go dormant during times of stress.
Much to the dismay of every irrigation system installer there is no universal predetermined schedule to watering a lawn. A lawn should only be watered when it needs to be watered. Gratefully for us in the lawn care business water falls freely from the sky on a pretty regular basis. Generally there is more of it in the springtime then there is in the summer time. It’s even a little bit difficult to put an actual quantity on the number of inches of water a lawn needs to stay healthy because different grass types require different amounts of water. The temperature also plays a huge role in the amount of water that is necessary because if it is hot the water will dry up. If it is cool the water will be used more effectively.
If I were pressed into giving an actual number of inches needed to maintain healthy turf I would have to say one and half to two inches of water per week. This is very difficult to say because of all the factors involved. If we put two inches of water onto a bluegrass sod lawn growing out of six inches of high compost loam in 70 degree weather we will get much different results than if we put two inches of water on a fine fescue lawn/rye grass lawn in sandy soil during a heat wave. In other words there really is no universal answer to the question, “how much water does a lawn need?”