Understanding Climate and the Water Needs of Plants

by Renee Roth, RainScape Designs and Ojai Valley Green Coalition staff member

With the extended drought, water is in short supply. Responsible water management necessitates that the homeowner gain a keen understanding of the factors affecting water efficiency to lessen the impact of future droughts on our landscape and lifestyle. For residential customers, it requires a better understanding of our water use, our indoor water needs and landscape water needs: applying the right amount of water at the right time, based on the plants water needs is required.

But estimating plant water needs is not easy – there are many variables to consider, such as the plant species, plant size, type of soil and climate. With the costs of water increasing, along with drought restrictions and penalties for water usage above allocations assigned by the water agencies, responsible irrigation management to use water efficiently is now worth an investment of time. Unfortunately, many landscapes were not designed with water efficiency in mind. Grass has typically been used to quickly cover large landscaped areas, but it takes lots of water and maintenance.

To make the most of the water supplies we have and the annual rainfall we do get, we need to re-prioritize where to put the water, and understand the amount of water plants need in the climate we live in (mountains, valleys, coastal areas or desert). Climate data includes the amount of rain, humidity, solar exposure etc. The image below explains how the plants water requirements change with the climate.

Evapotranspiration (ET) is the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues). It is an indicator of how much water your lawn, garden, and trees need for healthy growth and productivity based on your climate.

The baseline for ET calculations is based on the amount of water loss from a large field of 4-to-7-inch-tall, cool season turf that is not water stressed. The ET baseline is different for every climate zone, and is typically used by professionals to develop the water demands for each different type of plant.

The ET is measured at weather stations across the state. Below is the ET chart for Soule Park in 2017.

SOURCE: http://access.weatherreach.com/StationDetail?StationID=68&TableTimeInt=60

ETo is the rate of annual evapotranspiration for a given climate region.  By using the above chart, you can arrive at the ETo for Ojai by adding the monthly ET measurements together.  From this chart, the ETo or total loss of water per square foot to the atmosphere in Ojai in 2017 can be estimated at 54.1 inches.

By looking at the table above, we can also see that in 2017, the 19.24 inches of rain received would not be able to compensate for the loss of water from an annual ETo rate of 54.1 inches.  The difference between ETo and annual rainfall can only be made up by the use of potable water in the form of irrigation to maintain the health, appearance and reasonable growth rate of cool season turf in Ojai in 2017.

How does 54.1″ of supplemental water per square foot of turf grass compare with the water demands of other types of landscape plants? A great reference is found at UC AG & Natural Resources on how to water trees.

Detailed info on plant water needs for the many plant species and climate regions of the State can be found by viewing the Water Use Classifications of Landscape Species IV (WUCOLS IV) website. Ventura and Ojai are both in the South Coastal Region, although Ojai’s ETo is more like the South Inland Valley Region (Ojai area is hotter and drier in summer, wetter and cooler in winter than Ventura). The categories of plant water use are high, medium, low and very low as a percentage related to cool season turf. The image below provides examples of how different plants use different amounts of water.

Screen Shot 2018-03-28 at 3.27.59 PM

Here are some examples:

High Water Use: Cool season turf needs about 45-56″ of water per year, which is 80-100% of ETo;

Medium Water Use: Citrus tree needs about 40-60% as much as cool season turf;

Low Water Use: Meadow sedge uses about 10-30% of cool season turf;

Very Low Water Use: many California native plants and Mediterranean plants along with succulents use less than 10% of cool season turf. Most plants in this range will need extra water to get established, but once established can survive on seasonal rainfall.

A key strategy to increasing Water-Wise landscape efficiency is to replace high water-use plants with climate appropriate plants that have lower water-use requirements. During low rainfall years, the climate appropriate and native plants have a much better chance to survive under variable climate conditions.

“Drought resistant species specifically access greater soil water with deeper and more extensive roots, and often have desiccation resistant leaves able to maintain acceptable appearance at high water stress thresholds.”

from UC ANR Website: SLIDE (Simplified Landscape Irrigation Demand Estimation)

Another option to make the most of the rainwater we get is by adding shallow basins to capture rainwater. Capturing rainwater via rain gardens help to slow, spread and sink the rainwater into the landscape and reduce your potable water demand. This is hard to do with turf.

Calculate Gallons of Water Demand

The formula to calculate the gallons of irrigation water needed per day:

Gallons of Water per day = (ETo x PF x SF x 0.62 )

ETo: Get this from the chart above, taking the month of June ETo of 6.85″ ÷ 30 day for the daily ETo of .23″.

PF: This is the plant factor. Use a value of .8 for cool season turf. For water loving or large shrubs use .8, for average water use shrubs use 0.5, for low water use shrubs use 0.3. Citrus trees have a PF of .65. Detailed information on plant water requirements for each plant species is available at Water Use Classification for Landscape Species (WUCOLS).

SF: This is the area to be irrigated in square feet. For a 30 foot x 50 foot lawn you would use 1500.

0.62: A constant value used to convert inches to gallons.

Example #1: Water Needs for Turf Grass

For the month of June, 1,500 square feet of grass lawn in zip code 93023, we calculate water demand is estimated to be 5,134 gallons (171 g/d x 30 days = 5,134) or 6.86 units (5,134 g ÷ 748 gallons/unit= 6.86) units of water, based on the formula below:

0.23 (daily Eto value) x .8 (grass plant factor value) x 1500 (sq ft) x 0.62 =

171 gallons/day in June

Remember this calculation gives you an estimate. Many factors that could make this value higher or lower. For example, the efficiency of your irrigation system influences how much water is available. Some irrigation water never gets used by the plant, so the irrigation efficiency (IE) or distribution uniformity (DU) may need to be factored in. A well designed sprinkler systems with little run-off that uses efficient sprinklers have efficiencies of ~80% (use 0.80). Drip irrigation systems typically have efficiencies of 90% (use 0.90). Also, Stage 3 drought allocations, which mandate water restrictions will reduce the water to apply by 30% to 3,594 gallons (multiply 5,134 x .70 = 3,594) to live within your drought allocation for the month of June.

As you can see this can get complicated.

Example #2: Water Needs for Trees/Shrubs

To help determine landscape water requirements for trees, selected the Tree & Shrub Water Demand Calculator from Water Use of Turfgrass and Landscape Plant Materials/Water Demand Calculators website. This site is helpful to calculate water needs based on the size of the tree (diameter/radius of canopy cover or square feet of area covered*). Below are two examples for a tree that covers an area of 314 feet needs, with water needs of 632 gallons/month.

1) Tree/shrub canopy with a radius of 10 feet covers landscape area of 314 feet (canopy area [(3.14 x r2) with r=10]) requires 22.52 gallons/ day or 158 gallons /week or 632 gallons/month.

2) Tree/shrub has canopy diameter of 20 feet, the gallons of water/week (3.14/4 x 202 = 314 sf) x (.23 x 7= 1.61) ETo x .623 x .5 PF = 158 gallons /week or 632 gallons/month.

Below are the data input and results from the Demand Calculators Website.

“How much water pressure will my irrigation system need?” The answer depends on a lot of factors, but as a rule of thumb, 50-60 PSI of water pressure is a good starting point for sprinklers, 30-45 PSI for drip systems. If your water pressure is higher, consider putting in a pressure reducer.

To estimate the amount of water a fruit tree needs, see the [PDF] Water Management Guide for Temperate Fruit Trees, based on ETo rates and tree size. For more variable and detailed information see http://homeorchard.ucanr.edu/The_Big_Picture/Irrigation/

A great reference is also found at UC AG & Natural Resources on how to water trees. See http://ucanr.edu/sites/UrbanHort/Water_Use_of_Turfgrass_and_Landscape_Plant_Materials/Estimating_Water_Requirements_of_Landscape_Trees/

See links below for more irrigation and water management information from the UC AG & Natural Resources and Master Gardeners program.


Keeping Landscape Plantings Alive under Drought or Water Restrictions

Gardening Basics: How do I practice sustainable gardening?

Lawn Watering Guide for California

Managing Turfgrass During Drought

Managing Water, Sustainably

Questions & Answers About Water Conservation and Drought in the Landscape

Irrigating Citrus with Limited Water

Water Conservation Tips for Home Lawn and Garden

NEW! Coastal California Rain Gardens

City of San Jose Green Gardens Healthy Creeks Sustainable Landscaping Guidelines for Residents PDF


*Formula for Area of a circle = π (Pi=3.14) times the Radius squared: A = π r2

or, when you know the Diameter of a circle :A = (π/4) × D2

were D = diameter and R = radius