The Landscape Irrigation Auditor Handbook (3rd Edition) provides the following equation for determining your watering times:
RT = D/PR * 60
- RT: run time (minutes), lower boundary
- D: depth of water to apply (inches)
- PR: Precip Rate (in/hr)
Taking the lower boundary RT, DU is used to calculate the upper boundary RT, which is simply multiplied by the scheduling multiplier – which is a fixed valve based on the DU %.
Assuming DU is 80%, the scheduling multiplier would be 1.14.
- RT: unknown
- D: .50 inches
- PR: .39 in/hr
RT (lower) = 76.9 = 77 minutes
RT (upper) = 77 * 1.14 = 87.78 = 88 minutes
The Irrigation manager should select a run time between the upper and lower boundaries; the Iro takes the average of the two.
From here, the RT would be cross referenced to the Smart Cycle table as outlined in this support article. Please note, these are maximum allowable watering minutes as outlined by the EPA for WaterSense testing.
I hope this helps. In short, Smart Cycle works by taking the RT (either Rachio calculated, or user set) and is cross referenced to the Smart Cycle table. The number of cycles is determined by the maximum number of cycles needed amongst the zones in the watering schedule; i.e. if one zone needs 2 cycles and another needs 4 cycles, all zones will be cycled 4 times.