Any way it goes, it appears for now that the ability for the system to get NOAA data is being all but wasted. This ability was one of the leading factors in my purchase of the BlueSpray system.
My suggestion below was exactly that, please don't take it as an affront to your product. No solution is perfect considering the extraordinary number of individual situations; climates, soil types etc...
However, given that there are two major contributors to water usage, Percolation and evapotranspiration, it would follow that an ideal solution (no such thing... I know) would start with those factors, or at least take them into consideration.
Percolation can generally be taken care of by simply adjusting the watering time for the given zone and its soil type. Possibly using multiple programs/schedules so that the water has time to be absorbed.
Evapotranspiration... not as easy. But, the major contributing factors of evapotranspiration are temperature, humidity, and wind ... all forecast as a part of the NOAA data. It would ensue that using this data to adjust watering time would make sense. Not perfect, but better than nothing.
Providing just a time and a seasonal adjustment factor is a good start, but a factor based on weather data would help to save that much more water.
As for the accuracy of weather data, it appears from my experience to be fairly accurate for the coming 24 hours. Sure its +/-10%, but that is more than accurate enough for a water factor. And, since the soil provides a buffering factor this shouldn't make a big difference.
Please take my suggestion above, and the alternative suggestion below, digest these, think about them, mull them over with your team. They are not intended to be a final solution. Only food for thought. If you decide to implement some form of a weather forecast based watering time factor... I for one will be an enthusiastic user.
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Another thought for weather based adjustments:
Instead of a single set of conservation parameters, provide a per-schedule set of conservation parameters. That way, if a day is going to be hot, dry and windy, I can have a second schedule add a bit of water to replenish that lost to evapotranspiration.