Maybe we are overthinking this. For most of us using this to control sprinklers in our yards, a simple Water Index value as such would be most appropriate:
http://www.bewaterwise.com/wat_index_02.html
Perhaps I can email them to find out what parameters they use to come up with their Water Index (WI) value.
Edit… Okay, the Watering Index is simply: (ETw / ETwm) * 100, where ETw is the total of the previous 7 day ETo values, ETm is the highest weekly ETw in the last year. This is pretty straightforward, but the real issue is how to obtain those ETo values. This is not particularly difficult in California due to the CIMIS system (http://wwwcimis.water.ca.gov). I’m not certain how one would automatically obtain those values for a specific location using the web. If that were doable, it would be great for those of us in California. But, it would be preferable to have a way to use web-based weather data to estimate ETo for a specific location, in the spirit of providing this functionality to everyone. After looking through the calculations, it doesn’t look all that simple. The simplest calculations only consider temperature. Also, the Penman–Monteith equation appears to have several variables that are unlikely to be readily available from weather sites. However, some of those variables may be less likely to change over a yearly basis, and thus could be removed from the calculation as the WI gives a ratio, not an absolute value. We really don’t need to know the predicted volume of water lost, just a daily fraction of water lost to evapotranspiration vs. the maximum daily water lost for a specific location.