Get a flat bladed screwdriver with a 4" blade and push it into the soil. If it will go in up to the handle, you don't need to water. If it stops or is super hard to push in, then water. This is a very general test of soil moisture.

You only water when the grass needs it. So if Mother Nature is supplying you with enough, don't waist your money as it only creates problems.

Try to identify the earliest sign of water stress on your lawn. Flat grass blades will roll up into a spindle. You can often see this during the hottest part of the day on the driest patch of your lawn. The plant is struggling to conserve whatever water it already has. As the day gets cooler, the stress is reduced and the leaves unfurl again. When water is scarce in the root zone, the same symptom(s) will show up but they will not go away until water is applied to the root zone. Observing this cycle will give you a pretty accurate irrigation interval for your lawn. In practice we do not want to stress the lawn more than necessary so we irrigate just a little earlier than the stress point. Grow a plug of your lawn in a pot in the open and after it is growing well, stop watering it. Observe how it reacts and watch what happens when you water it. Believe me, the PhD's are doing the same thing with a more 'scientific' approach but their final answer is likely to be similar. If you care to, search (Google) 'irrigation interval zone 5' and scroll down to 'Chapter 1: Introduction'. It is explained there very well.

Posted by texas-weed:
You only water when the grass needs it. So if Mother Nature is supplying you with enough, don't waist your money as it only creates problems.

If I read the post correctly the OP was asking how to know when you have enough. I have the same question, only I usually get half an inch of rain at a time. Do I water another half inch right away, or wait three days and water an inch?

Here where I am we haven't had much rain in the last couple weeks or so. Last wednesday I broke out the sprinkler for the 1st time and helped out the front a little bit by giving the yard some water, some of the yards around here are starting to brown a little in spots. Yesturday evening we had about a 1/2 inch of rain from a storm but it wasn't really a good soaking rain. Tomorrow I might put down some fungus control as a preventer and water that in.

Get a flat bladed screwdriver with a 4" blade and push it into the soil. If it will go in up to the handle, you don't need to water. If it stops or is super hard to push in, then water. This is a very general test of soil moisture.

Shoot, I can do this in several areas of my lawn right after watering, hitting nothing but rocks or maybe building materials. =(

I certainly can't use sprinklers on spikes. =( =(