Answer: I wish there were a magic formula that everyone could use to figure out when they've applied one inch of water to their lawn, but there are just too many variables. And too many soil types. The easiest way to calculate how much water your sprinkler puts out is to place tuna or cat food cans in several places on your lawn and turn the sprinkler on. Then check the amount of time it takes to collect one inch of water in the cans. If water begins to run off and into the street before one inch has collected in the cans, you may have to turn the water off, wait 15-20 minutes for it to soak into the ground and then turn the water on again, until you get the full one inch in the cans. This will give you a fairly good idea of how long it takes your water pressure to deliver (through whatever size hose you have) one inch of water. Once you have this calculation you can leave your sprinkler running for the prescribed length of time and feel confident that you're applying just the right amount of water.
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