Saving Water Partnership

Calculate how much to irrigate your landscape

Use the Water Budget Calculator to determine how much water a lawn needs, based on the size of your irrigated landscape and yearly weather averages for the Seattle area.
Use the Sprinkler Scheduler to create watering schedules for automatic in-ground irrigation systems, based on the types of your plantings and watering system.

ET (Evapotranspiration Rate) is how much water your plants use—thus, how much water you need to replace to keep your landscape green. Watering more than this is a waste of money and water.

Raw Data

Users who need access to weekly raw ET/rainfall data in CSV format can download the data by clicking here.

Average ET, Actual and Effective Rainfall and Irrigation Index (for the irrigation season)
Month ET Actual Rainfall Eff. Rainfall (33%) Irrigation Index
April 2.38 2.87 0.89 53%
May 3.14 2.60 0.86 70%
June 3.50 2.00 0.66 78%
July 4.46 0.52 0.17 100%
August 3.71 1.11 0.37 83%
September 2.54 1.29 0.43 57%
Totals 19.73 10.24 3.38  

Please note:
irrigation is not needed in the Seattle area during the months of October through March.


Past Year's Data Comparisons

* Note
As with most technology, weather stations and the technology that ties it all together, are sometimes victims of malfunctions. Occasionally the weather data produced and posted on this site might be temporarily inaccurate. Please check the data being used for obvious errors. (for example: if the ET one day is .13" and the next it is 300" then caution is advised)