Irrigation Scheduling

Reference evapotranspiration ET₀ describes how much water a short, uniformly wet grass surface would lose. An orchard on drip is neither short nor uniformly wet, so that reference figure has to be adapted before it means anything for scheduling.

The dual crop coefficient approach splits the adaptation in two. Kₜ covers transpiration through the canopy; Kₑ covers evaporation from the soil the drippers actually wet. Their sum K₊ scales ET₀ into ETc, the orchard's real demand.

Orchard geometry enters on the transpiration side. Widely spaced trees intercept less radiation than a closed canopy, so Kₜ is reduced by a density term built from canopy cover and the solar elevation at noon — which is why the same weather gives different demand at different spacings.

Evaporation is bounded by how little floor a drip system wets. The wetted fraction f_w here is only — of the ground each tree occupies, so Kₑ stays small and nearly all demand is transpiration.

Converting demand into a run time is the last step, and the only one that depends on hardware: hours = ETc / rate. At the current rate, each millimetre of demand costs about — hours at the valve.

Weather comes from the Open-Meteo forecast API, fetched server-side and cached. Vapour pressure deficit is averaged from the hourly series rather than taken from the daily maximum, matching the notebook, because leaf resistance responds to the deficit the canopy sits in through the day.