Evapotranspiration Rate (ET) is a long word for a simple concept. Simply put, ET is how much water your plants use, or that you need to replace to keep it all green. Watering more than this is a waste of money and water. Weather stations monitor several different climatic factors. They then take this data and calculate ET. In our case they calculate it in inches. This information is then transmitted to our computers and then uploaded here for your use.


The Irrigation Index is a percentage number that coincides with a feature on most lawn sprinkler controllers. Most controllers (or timers) have a key that allows you to increase or decrease your watering time by a percentage.

By coming to this site you can find out at what percentage your controller should be set. This is updated weekly.

It is used as follows:


  1. An index reference Et is identified for the hottest time of the year, for example 1.5"/wk.
  2. The controller is set to water for the hottest time of the year regardless of the time of year.
  3. The Et for the current week is determined from local weather data
  4. A percentage is then calculated for the current (or past) week
  5. The controller percent key is then set at that percentage.

In the case of using this data in the Seattle area, the reference point of the hottest week has already been calculated. If the controller is already programmed for the hottest time of the year, one simply has to input the current week percentage from the website. Some modification to the watering days per week may be needed.


Watering Cycles. You may notice that your watering schedule requires cycles. This is an important water conservation technique. Cycles per day means that you may need to break up your watering in a given day into smaller increments. For example, you may need to water for 20 minutes each day, but after ten minutes the water begins to run-off because of a slope or tight clay soil. This is when watering in cycles would be beneficial. In the example above it is better to water in two 10 minute cycles (each day) instead of 20 minutes all at once. This helps avoid wasteful run-off and water waste. Do not confuse cycling with irrigation frequency. Some people might be tempted to change their twice a week watering schedule to everyday to avoid run-off, this is a mistake. It is better to stay on your twice a week schedule (or whatever it might be) and break it up into cycles on the same day. Too frequent, shallow, watering causes unhealthy plants that are not drought resistant.