Why this duration?

Every zone’s detail view includes the full arithmetic behind tonight’s planned minutes, because “trust me” is not a number.

The chain, top to bottom:

1. Bucket deficit (mm): how far the zone’s modeled soil moisture sits below full. Rain and runs fill it; daily crop ET drains it.
2. Crop coefficient (Kc): the species’ seasonal multiplier on reference ET (see the grass species catalog). Hemisphere-aware: south of the equator the curve shifts six months.
3. Heat multiplier: optional extension when the peak heat index crosses the heat-advisory threshold. Each day’s heat index pairs that day’s high temperature with that same day’s humidity (not the current, often night-time, humidity), so a cool morning’s humidity is never combined with a hot afternoon’s peak to inflate the run.
4. Throughput (mm/hr): how fast your sprinklers actually apply water, either measured (catch cups) or the catalog default for the head type.
5. Capture efficiency: how much of the applied water lands in the root zone (wind drift, overspray, runoff losses).

Planned seconds = deficit / (throughput x efficiency), capped by the zone’s max-runtime guard. Every input is shown live with its source, so when a number looks wrong you can see exactly which knob to turn.