I have found a few sources of ET data. Obviously this is extremely useful as it eliminates the need to gather all the data needed by the ET equations.
This map shows 100 or so stations where ET and other irrigation data is available:
http://access.weatherreach.com/map_stations
Click on a station and in the Irrigation Requirements box the 24 Hours value indicates how much water your lawn should need for the next day.
Knowing how much water is needed, the next issue is determining how long to water to deliver that irrigation. I see two ways to approach this:
1. Determine the amount of water deposited by the sprinkling system per minute. This can be determined by placing several gauges (or simply empty cans) on the lawn and watering for, say, 10 minutes. Once the inches of water per minute rate is know it could be entered into the watering program for each zone.
2. Determine the amount of water needed for an average day and assume that the current watering schedule delivers it. Historical weather data can be used to get Spring, Summer and Fall averages to be used in an ET calculator (like the one in the post above) to get a baseline or 100% ET which could be input to the watering program to be compared with the weather-based needed ET which will be used to adjust the current schedule of water times up or down.
OK, that’s all I have for now, take it away.