Determining how much water to apply and how often

Water requirements of landscape plants change throughout the year. Daylength, temperature, relative humidity, and wind speed all affect the loss of water from soil and plants. In the absence of rain, these losses must be replenished by irrigating. 

Begin an irrigation plan by focusing on the month with the greatest water needs. In Sonoma County, that is July. Determine the following for July:

• How often?
• How long?
• What time of day?

This information is needed to create a baseline for modifying water needs of plants within a single hydrozone for the rest of the year.  

• Finding the baseline: While irrigation experts are trained to do the calculations for determining the baseline, there are Sonoma County-based online tools that require fewer math calculations. These helpful online tools make a number of assumptions about a drip system, especially emitter rate per square foot. Besides the total amount of water per week, these tools address the “how often” and “how long” questions.
  • Sonoma-Marin Saving Water Partnership Irrigation Scheduling Tool
  • City of Santa Rosa Water Smart Yard, Monthly Lawn and Landscape Watering Schedule

• Choosing an emitter rate: Soil affects the movement of water. Choose an emitter rate and emitter spacing depending on your soil type.
  • Because water moves slowly into clay soil and spreads out use emitters with lower GPH (gallons/hour) and water less often. Because water moves quickly and directly through sandy soil use emitters with higher GPH (gallons/hour) and to water more often. Loam is in between.
    • Clay – 0.5 GPH or less
    • Sand – 0.6 GPH or greater

• Adjusting irrigation for weather changes: Water requirements change throughout the year. There are ways to adjust for this.
  • A Smart controller will do so directly.
  • Other irrigation controllers can be manually programmed with a ‘Seasonal Adjustments’ feature.
  • Even the program on a hose bib timer can be adjusted monthly.

Seasonal (Monthly) Adjustments for Santa Rosa (Petaluma)

Program July at 100% (highest water need month). 

Input the following % under the Seasonal Adjustment function for each month on your controller.

Use the % to adjust the number of minutes for each program on a hose bib timer on a monthly basis.

Main data is for Santa Rosa, Petaluma data is in parentheses.

• March = 48% (49%) of July water need
• April = 66% (68%) of July
• May = 84% (87%) of July
• June = 98% (99%) of July
• July = highest water need month: 6.49 in (6.26 in) (ETo)
• August = 88% (87%) of July
• September = 69% (69%) of July
• October = 48% (49%) of July

Source of data:  CIMIS for Stations 83 and 158

Analysis by Kris Loomis, Sonoma Water

The topics outlined below will help you install a drip system.

• Drip irrigation vs other irrigation methods
• How to get started
• Drip irrigation management
• Key considerations when implementing a drip system
• Drip irrigation basics
• Drip irrigation in different landscape situations
• Determining how much water to apply and how often
• Maintaining an irrigation system
• How to get help