This is a pretty interesting calculator that gives me the total Evapotranspiration that is lost daily from a specific plant, in our case, grass. There are more variable at plays such as humidity, sun/shade, cloud cover etc., but basically this says that we should irrigate our turf with a total of 3.8 mm of water each night, or 0.15" of water. Our irrigation system delivers approximately on average 0.1" of water at the base of the head in a 10 minute period. This is one of those numbers that works on if we lived in a vacuum. With undulating ground, there will always be runoff that creates a source of error in this model. However, the goal with irrigation is to replace what was lost the prior day. Irrigation systems are meant to supplement rainfall, no completely replace it. Lets hope the 70% chance of thunderstorms produces results today!