In order to know how much water you are using on your lawn, you will need to measure your watering systems output. The best way to do this is by using containers similar to a tuna can or cat food can.  Place several cans around on your lawn. (Using several will show if there are spots that are getting too much or too little water.) Run your sprinklers for a predetermined length of time (ie: 20 minutes).  Measure how much water was collected in each of the cans. Add the measured amounts together and then divide by the total of cans to come up with an average. Once you have an average you will know how much you are watering for that amount of time.  Example:  You use 4 cans and run the sprinkler for 20 minutes. When you measure you see that can 1 has 1/2 inch (.50), can 2 has 3/8 inch (.375), can 3 has 5/8 inch (.625), and can 4 has 1/2 inch (.50). Add these together (.50 + .375 +.625 + .50) and you get 2.  Take this number divide by 4 (number of cans used). The result is .50 or 1/2. This means that 20 minutes of watering, your sprinkle will put out 1/2 inch of water.