Simplified Landscape Irrigation Demand Estimation Roger Kjelgren and Larry Rupp Center for Water...
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Transcript of Simplified Landscape Irrigation Demand Estimation Roger Kjelgren and Larry Rupp Center for Water...
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
0
50
100
150
200
250
J F M A M J J A S O N D
Rain
or
ET, m
m
0
100
200
300
400
500
600
700SLC W
ater use, MGD
ET, 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
Eco
nom
ic
Yield
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
23
Native Species: Evidence of Water Use
24
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
27
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 area
Simplifying 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?