I know @azdavidr has already demonstrated a nice understanding the solution space in other posts, so I'll try to provide a slightly different perspective in case I can add any value as a software engineer that works on user interfaces.
Suggested UI Changes
- Custom nozzles: Provide a way to enter a special drip emitter nozzle type that takes GPH
- Zones: When the drip emitter nozzle type is used, add the following inputs to the zone:
1) Average plant canopy size
2) Average number of emitters per plant
Suggested Business Logic Changes
Internal calculations could still arrive at inches per hour via lookup tables. The lookup tables would depend on soil type and plant type (both are already Rachio zone inputs) and would provide the number of gallons per plant as does Table C in the Water Use it Wisely article @azdavidr mentioned. That article's tables are probably specific to Arizona soil which is why the article stipulates, "The amount of water needed will vary depending on soil type and soil conditions." Given the lookup tables and the new UI inputs the software can calculate inches per hour as it does now - no further changes required and backwards compatibility is achieved since the changes are simply extensions of what is already there.
I feel like these changes would provide the simplest way for users to enter a "true" drip nozzle spec that doesn't change as a function of soil type, number of emitters per plant, etc. as precipitation rate can. Turf and everything else can stay the same and thanks for the great UI, Rachio.