Center for Landscape & Urban Horticulture
University of California
Center for Landscape & Urban Horticulture

Landscape Water Requirement Calculators

Using and Applying the Calculators

There are separate calculators for different types of plant materials and common landscape settings:

  • Lawn and Turfgrass – provides irrigation runtimes and is tailored to California locations.
  • Groupings and Mass Plantings of Trees or Shrubs - situations where plants are spaced closely enough that the canopies of trees, shrubs, or a mix of them cover at least 80% of the ground.
  • Individual Trees and Shrubs - situations where plants are widely-spaced and the canopies of trees, shrubs or a mix of them cover less than 80% of the ground.
  • Non-turf Perennial Groundcovers - situations where a mass planting covers at least 80% of the ground, or where perennial non-turf groundcover is mixed with trees and/or shrubs in a mass planting or bed and the combined plant canopy covers at least 80% of the ground.
  • Mass Plantings or Beds of Herbaceous Perennial Flowers and Similar Plants - situations where at least 80% of the ground is covered by plant canopy.
  • Mass Plantings or Beds of Annual Flowers and Bedding Plants – situations where plant canopy covers at least 80% of the ground, or where annual flowers are planted among trees or shrubs in beds and the combined plant canopy covers at least 80% of the ground.

When trees are located in a planting of lawn, groundcover, herbaceous perennials, or annual flowers, their water requirements will be met by the estimated water requirement of the surrounding planting.

Using the Calculators

To use the calculators, first determine the type of plants and planting involved. Except for the Lawn and Turfgrass calculator, you will need to enter the size of the plants or planted area and the average daily ETo (reference ET) rate, which accounts for the influence of local weather on plant water requirements.  Use the provided CIMIS station link to search for the closest location that reports reliable ETo data to get an accurate estimate of current water requirements.  Enter your own value or an historic average daily ETo value for the period if you are using the calculator to predict water requirements of a planned landscape or to set a base irrigation schedule.  Historic average ETo for California locations can be found at the CIMIS (California Irrigation Management Information System) web site.   

The Lawn and Turfgrass calculator requires you to enter the type of grass and the water application rate of your sprinkler system.  Cool-season grasses include tall fescue, other fescues, ryegrasses, and, Kentucky bluegras, while warm-season grasses include Bermuda, zoysia, St. Augustine, buffalograss, and kikuyugrass.  If you are unsure of the grass you have see the UC Guide to Identifying Your Lawn.  To estimate how much water your sprinkler system applies per hour, follow the procedure in the UC Lawn Watering Guide

Applying Calculator Results

The calculators provide water demand per day and per week, although no established lawn or landscape plants require irrigation on a daily basis.  Use the calculator results to estimate the amount of water a landscape design requires from irrigation or a combination of precipitation and irrigation, or use them as starting points to set irrigation schedules and station runtimes based on the performance characteristics of your irrigation system. 

The Lawn and Turfgrass calculator provides estimated weekly run time minutes.  Divide the total runtimes for an irrigation day into multiple start times to avoid runoff and meet turf needs based on rooting depth, soil texture, and the irrigation system's application rate.  Established cool-season lawns (tall fescue, other fescues, rye, Kentucky bluegrass) require irrigation about 2-3 times per week in summer to perform acceptably, while warm-season lawns perform acceptably when irrigated about every 4-10 days in summer, all depending on the soil water holding capacity and root depth. 

Trees, shrubs, groundcovers, and woody vines perform acceptably when irrigated about every 5-15 days in summer, depending on their root depth and the soil water holding capacity. Herbaceous perennials usually perform acceptably when irrigated 1-2 times per week in summer.  Drip irrigation may need to be applied more frequently to assure plants do not become water stressed.  The estimated gallons of water demand produced by the calculators can be very useful for scheduling drip irrigation.

After setting an initial irrigation schedule, monitor plant performance for a few weeks to a month. If plants appear to be stressed or under-watered, then increase the water amount or apply less water more frequently; if plants appear to be over-watered or if their performance is good but greater water conservation is desired, then decrease the water amount or apply the same amount of water less frequently by extending the interval between irrigations. Many landscape plants, including turf, will survive with some loss in performance (little growth or vigor, wilting, browning and loss of foliage, thinning of turf) if applied water is reduced by up to 30% of the estimated requirement for up to a few months. Water reduction is usually done most effectively by extending the interval between irrigations by up to 30%. Always change irrigation frequency by a day at a time or adjust the water amount (system runtime) gradually in increments of 10%, and observe plants' performance to see if they respond favorably before making any further adjustment.

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