Simplified Landscape Irrigation Demand

Estimation

Roger Kjelgren and Larry RuppCenter for Water Efficient Landscaping

Utah State University

A New Method of Estimating Landscape Water Demand

Why do we use water in

landscapes?“I was nearly twelve before I saw either a bathtub or a water closet; and when I walked past my first lawn,… I stooped down and touchedits cool nap in awe and unbelief. I think I held my breath - I had not known that people anywhere lived with such grace.”

Wallace Stegner

Why should you understand landscape water demand?

1. Water allocationo Water agencies increasingly are allocating fixed

amounts to end usero Landscape designs need to estimate expected

demand within water allocation; agencies can track through billing data analysis

2. Landscape architect/contractor/maintenanceo Guidelines for irrigation system designo Aid in irrigation scheduling – when and how much?

o Droughto Minimum water for survivalo Especially important for woody plants

Defining Landscape Water Demand

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J F M A M J J A S O N D

Rain

or E

T, m

m

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ater use, MGDET, mm Rain, mm Water Use, MGD

Irrigation is used to bridge the difference between water demand and water availability

How is water demand typically quantified?

• Inches of water lost to evapotranspiration (ET)• Amounts to about 0-0.25 inch/day

𝑬𝑻𝒐=𝑷𝒍𝒂𝒏𝒕𝑻𝒓𝒂𝒏𝒔𝒑𝒊𝒓𝒂𝒕𝒊𝒐𝒏+𝑺𝒐𝒊𝒍 𝑬𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒊𝒐𝒏

Evapo-transpiration Rate: ETo• Calculated water lost from hypothetical 12 cm/4

inch high cool season, clipped/mowed turf<=sun, heat, wind, humidity

• Range 0 – 6 mm/day (0-0.25”/day)

Plant Transpiration

Soil Evaporation

+ET=

ETo is based on field research with agricultural crops

Uniform crop surface similar to turfgrass surface

Estimating Water Demand Based on ETo

• Determine the ETo (historical or real time)• Determine the percentage of ETo that provides

optimum yield for a specific crop (Plant Factor-Kp)

• Determine ETa (actual) by multiplying ETo by Kp• Replace ET by irrigation as needed based on

soil type, etc.Plant Water Use (Eta) = ETo X Kp

ETo-Kp Approach: Turf OK

ETo Approach: Problems• Assumes continuous uniform

surface, water use 2-D; urban landscapes often non-uniform, non-continuous, water use 3-D

• Assumes fixed Kp to avoid water stress for optimum yield; landscapes do not have economic yield-how much is enough?

• Assumes small # food/fiber species; >1,000 landscape species differing in water use traits

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ETo Background: Problems; #2• In landscapes, $ value water input ≠ ≈ value of landscape

• Instead, minimum water demand for acceptable appearance

Water applied

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Prescriptive Landscape Water Demand Estimation: ETo-Kp

• California-Water Use Coefficients of Landscape Species (WUCOLS)• Landscape Water Use = ETo X Kp X Density

Factor X Microclimate Factor• Species factor=3 water demand

categorizations based on committee decision

• EPA Water Sense• ETo x Kp, but Kp variable based on plant

type, size• Too comple: complicated, difficult,

false precision

• The very complexity of excellent landscapes makes it difficult to prescribe a water management plan

• A descriptive approach is more practical

• Law complexity-simplicity: Complexity , precision Precision , simplifying Simplifying , knowledge

Law supply-demand:Water supply ,Knowledge

• Prescriptive=complex• Descriptive=simplifyin

g with knowledge

Descriptive Approach: Simplified Landscape Irrigation Demand

Irrigation= SLIDE Rules• Kp = minimum water demand

=>acceptable appearance may include water stress

• Assume all species Kp = 50% of ETo unless evidence otherwise

• Number of transpiring leaves more important than Kp

Minimum water demand; many

landscape species can be water

stressed but look OK

SLIDE Rules • If water stress OK, how much

water needed to avoid not OK• Assume Kp = replace 50% of ETo

will work for almost all landscape plants

• When you know a species can tolerate less, irrigate at lower rate; greatest risk with trees that are greatest investment

Minimum % of ETo required for trees varies with climate and species, but 50% safely overlaps most non turf plants

Reference evapotranspirationETo for Salt Lake City: water to

Month May June July Aug. Sept.Ave Monthly ETo, in. 5.4 6.5 7.7 6.6 4.5

Rainfall, inches 2.0 0.9 0.8 0.9 1.5Net monthly waterdemand turf, 80% 2.3 4.3 5.4 4.4 2.1ETo minus rain Net monthly water demand trees 50% 0.7 2.4 3.0 2.4 0.8ETo minus

Reference ET and Leaf Area• More leaves = more water loss• Larger trees or increased numbers of trees

takes more water

Desert is characterized by large distances between plants

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Native Species: Evidence of Water Use

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Native Species: Response to Microclimate

Eriogonm corymbosum

Medium/small leaves, evergreen

More broadleaf species

Small, thick, blue leaves

Very small to no leaves

Aspen

Oak

Buffaloberry

Mormon tea

USU Botanical Center Landscape Lysimeter Study

Simplified Landscape Irrigation Demand Estimation: SLIDE

Rules• Minimum demand is based on acceptable

appearance, therefore a mild deficit is okay• Basic assumption is woody plants have a

coefficient of 0.5 unless there is concrete evidence for a higher or lower value

• Actual leaf area is best indicator of total water use

Total Plant Water Use = ETo X Kp X Leaf Area

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Transpiring leaf area ≈ crown diameter2

Crown diameter2 may underestimate true leaf area

How do you estimate leaf area?

6’

6’

A tree with a 6 ft crown diameter has roughly 36 ft2 of leaf areaSimplifying assumption: crown diameter in ft2 ≈ gallons needed to apply 2 inches of water

Simplest SLIDE Approach for Drip Irrigated Non Turf Plants

• How much: apply 2 inches water every irrigation = crown diameter2

• How often: seasonal ETo (after rain stops) x 0.5 (Kp) ÷ 2 inches/irrigation

What is the risk of plant damage if estimate to high or

low ?• Drought can also harm or kill landscape plants

Reducing risk? Knowledge• Understand plant types and adaptations• Use good plant selection for situation• Be aware of soil conditions• Be observant• Be flexible

Summary SLIDE Rules• Non turf urban landscapes are

complex; simplify water demand• Knowing volume of transpiration (from leaf

area) more important than rate of water use

• Assume 0.5 Kp as rate of water use (% ETo) for all plants unless evidence otherwise

• Irrigation amount: same each irrigation• Drip irrigation, 2” water ≈ gallons needed

crown diameter2

• Irrigation frequency ≈ ETo x 0.5 ÷ inches per irrigation

Questions?