Weekly Watering Number

The Weekly Watering Number is the amount of water in inches that your lawn will need that week.  You can also use the Weekly Watering Number for watering other types of plants, by using these general guidelines. 

  • Shrubs: 50% of the Weekly Watering Number
  • Perennials: 50% of the Weekly Watering Number
  • Vegetables: 75% of the Weekly Watering Number (new starts may require more water)
  • Trees: Newly planted trees need regular watering for up to the first couple of years, while established trees may need a deep soak or two in summer.

Be sure to check with your local garden center or landscape professional for more specific information on how much water your plants needs.

Where Does The Weekly Watering Number Come From?

The Consortium contracts with a weather forecasting service to provide a free weather forecast and Weekly Watering Number each Thursday (April - September). The Weekly Watering Number is based on historical data (evapotranspiration, rain fall, and other data points) from the previous week, but it is used to determine how much to water lawns and landscapes during the current week.

Who Should Use The Weekly Watering Number?

Home gardeners and landscape professionals alike can use the Weekly Watering Number to fine tune the amount they water their gardens and landscapes each week.

Why does the Weekly Watering Number change each week?

The Weekly Watering Number changes with local weather conditions.  So, in the cooler, wetter spring it tends to be lower, and in the hotter drier summer it tends to be higher. 

How, do I use the Weekly Watering Number?

The key to watering efficiently is to adjust your watering schedule as the weather changes throughout the irrigation season.

  • Automatic sprinkler systems – First, determine your sprinkler’s output, or how long it takes to water 1 inch of water.  Then, set your base schedule and use the "water budget" or “percent adjust” feature to adjust your weekly watering schedule.
  • Watering with a hose or a non-automated sprinkler system - First, determine your sprinkler’s output, or how long it takes to water 1 inch of water. Then, use the Weekly Watering Number as a guideline to manually adjusting your watering duration or frequency.

Why A Zip Code-specific Weekly Watering Number?

By narrowing the number down to a specific zip code, the Consortium is able to better take into account any variations in weather and rainfall that within your specific area.

For example, the Sellwood neighborhood in Portland might receive significant rainfall from several showers while the nearby Hawthorne area might not receive any rainfall. As a result, their Weekly Watering Numbers would likely vary greatly that week.