Climate and Plant Water Needs

by Renee Roth, RainScape Designs

With the extended drought,¬† responsible water management includes the idea of water conservation and water efficiency.¬† For residential customers, it requires a better understanding of our water use, our water needs and management of our landscape water needs.¬† For the landscape,¬† 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, climate, type of soil, size of plant, amount of direct shade/sun etc. ¬†Unfortunately, many landscapes were not designed with water efficiency in mind.¬† Grass has typically been used¬†to quickly cover large landscaped areas, without consideration of the recurring cost for both water and maintenance.

To make the most of the water supplies we do have,  we need to understand the amount of water plants need in a changing climate so we can re-prioritize and make the best use of our limited supplies. 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 (ETo) 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 Eto is measured at weather stations across the state. Below is the Eto chart for Soule Park for 2017.


With a little math, we find out that in 2017, it took  54.1 ETo inches of potable water (adding up column 1) and 19.24 inches of rain (adding up column 2 ) in  2017 to maintain the health, appearance and reasonable growth rate of cool season turf in Ojai in 2017.  The ETo baseline is different for every climate region, and is typically used to develop the water demands for each different type of plant in each region.

How does 54.1″ of supplemental water per square foot of turf grass compare with other plants water demands?  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 can be found at 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 (we are hotter and drier in summer, wetter ¬†and cooler in winter). ¬†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.
From this example, you see:
High Water Use Plant: Cool season turf needs about 44-58″ 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 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.

One key strategy is to increase irrigation efficiency by matching water supply to the plant water needs, and using only low water plants.  During low rainfall years,  the climate adapted  native plants have a much better chance to survive in 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.”

Another option is making the most of the rain water we do get by adding shallow basins to capture rainwater.  Capturing rainwater into the soil via a rain garden can  provide supplemental water that helps to reduce your potable water demand.

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. Different plants need different amounts of water. Use a value of .8 for cool season turf. For water loving 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. So for a 30 foot x 50 foot lawn you would use 1500.

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

Example:  Water Needs for Turf Grass
For 1500 square foot grass lawn in zip code 93023:   .23 avg inches per day during the month of June.

Now rewrite the formula inserting your values into it:

  • Gallons of water per day for 1500 sq ft of turf grass in June =
    0.23 (daily Eto value) x .8 (grass plant factor value) x 1500 (sq ft) x 0.62
  • Now do the math, just punch the values into a calculator and get your answer:
    171 gallons/day in June = 0.23 x .8 x 1500 x 0.62

Monthly water demand for the month of June (30 days ) 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. We could figure out the average daily potable water demand in 2017 for each month of the year. Just use the same formula but insert the ETo value  for each month.  (NOTE: ETo from chart above includes rainfall for a total water demand of .)

Remember this calculation just gives you an estimated value. There are¬† other factors that could make this value higher or lower. For example, the efficiency of your irrigation system influences how much irrigation water is available to the plants. Some irrigation water never gets used by the plant,¬† so the irrigation efficiency (IE) value¬† or distribution uniformity (DU) (often mandated to be ‚Č•0.7) can be factored in to compensate for that loss of water. Very well designed sprinkler systems with little run-off that use efficient sprinklers can have efficiencies of 80% (use 0.80).¬† Drip irrigation systems typically have efficiencies of 90% (use 0.90).¬† Generally, your calculations should be reduced by 85 % (5,134 √∑ .85 = 6,040 gallons ) to add in the 15% loss.¬† Also, Stage 3 drought allocations which mandate water restrictions would 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:  Water Needs for Trees/Shrubs
To help determine landscape water requirements for trees, I went to the UC A&NR Landscape Water Conservation and Irrigation management system, then to the calculator and selected the¬† Tree & Shrub Water Demand Calculator.¬† There are two ways to calculate the size of landscaped area for trees:¬† 1) either by diameter of canopy cover or 2) square feet of area covered. Below are two examples (see website calculations below) assuming daily of ETo of .23″¬† and formula for area is based on radius or diameter:

Area = ŌÄ (Pi=3.14) times the Radius squared: A = ŌÄ r2
or, when you know the Diameter:A = (ŌÄ/4) √ó D2

1) Tree/shrub canopy with a radius of 10 feet covers landscape area of 314 feet (canopy  [(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 water/week (3.14/4 x 202=314sf) x (.23x 7=1.61)ETo x .623 x .5 PF= 158 gallons /week or 632 gallons/month.

‚ÄúHow much water pressure will my irrigation system need?‚ÄĚ 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, with reference ETo rates and tree size references.  For more variable and detailed information see

A great reference is also found at UC AG & Natural Resources on how to water trees.  See

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

NEW!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
Coastal California Rain Gardens

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