The software can be configured to use online weather data to make daily adjustments to the station water time automatically. The data is retrieved from Weather Underground and is available for most locations in the world. The current adjustment method makes use of the following parameters:

  • Average humidity for the previous day.
  • Average temperature for the previous day.
  • Total precipitation for the previous day.
  • Total precipitation for the current day.

The formula assumes a baseline of 70 °F (21 °C), 30% Humidity and 0 precipitation. Under the baseline condition, the formula returns 100% water time (i.e. no change to the water time). The percentage increases or decreases as the weather conditions deviate from the baseline. See below for details.

NOTE: the weather adjustment feature is only supported on OpenSprinkler 2.x with firmware 2.1.0 or newer.


Click on "Edit Options" from the homepage and open the weather control tab, shown below.

Update: as of May 2018, Wunderground has stopped issuing free weather API keys. We have switched the weather service to use OpenWeatherMap. Our current weather algorithm no longer requires API key.

In the Weather Adjustment Method list, select the method you prefer. If you select Manual method, you can manually set a watering percentage in the % Watering input box. The acceptable range is from 0% to 250%. If you select any of the automatic adjustment methods, the % Watering input box will be disabled indicating that it is set automatically by the weather formula. 

Set a Program to Use Weather Adjustment

Each program can be individually set to use or not use weather adjustment. The option is available in the program editing page:

Weather Adjustment Methods

1. Zimmerman Method

The Zimmerman method calculates the watering percentage using the following formula:

  • Humidity: Average humidity for the previous day above 30% is subtracted from the weather adjustment, below 30% is added to it. Example: 77% Max Humidity + 26% Min Humidity / 2 = 51% Avg Humidity, 30% - 51% Avg Humidity = -21% Humidity Adjustment
  • Temperature: +4% for each degree Fahrenheit above 70, and -4% for each degree Fahrenheit below 70.
  • Precipitation: -2% for each hundredth of an inch precipitation from today and yesterday. Example: 0.12" rain today + 0.05" rain yesterday * 100 = 17 hundredths of an inch of rain * -2 = -34% Precipitation Adjustment

The result is then clamped to between 0% to 200%.

Currently the Zimmerman method is the only provided method for weather adjustment. Additional methods, such as based on Evapotranspiration (ET), will be added in the future.