ISY-99i/ISY-26 INSTEON:Evapotranspiration and Irrigation

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What is Evapotranspiration

  1. ET is an approximation of the amount of water leaving the soil via a combination of evaporation and transpiration (a fancy word for plants drinking it up).
  2. The goal of an ET-aware irrigation system is to approximately (despite all the appearance of precision, we have to remember this) replace this water.
  3. It does this by tracking the water used/lost from the soil until a threshold is reached (where the plants would starve for water if much more left without replacement).
  4. In most climates, this will result in watering every few days.
  5. Rain may offset some of the water use/loss, resulting in less frequent watering.
  6. Calculations performed once every 24 hours are sufficient for this tracking.


The 3 key concepts here are:

  1. There's some amount of allowed depletion (based on the plant types, root lengths, etc.).
  2. This depletion will not typically happen in a single day.
  3. What is varied is when to water, not how much.

Irrigation with Evapotranspiration


  1. A running balance is used to decide when to irrigate.
  2. It's not necessary to exactly replace all the water which has left the soil (i.e. zero-out the balance).

ISY-26 INSTEON / ISY-99i Series INSTEON : How-To Guide