Simplified Landscape
Irrigation Demand Estimation
A New Method of Estimating Landscape Water Demand
Roger Kjelgren and Larry Rupp
Center for Water Efficient Landscaping
Utah State University
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 touched
its 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 allocation
o
o
Water agencies increasingly are allocating fixed amounts to
end user
Landscape designs need to estimate expected demand
within water allocation; agencies can track through billing
data analysis
2. Landscape architect/contractor/maintenance
o
o
Guidelines for irrigation system design
Aid in irrigation scheduling – when and how much?
o Drought
o
o
Minimum water for survival
Especially important for woody plants
Defining Landscape Water Demand
ET, mm
Water Use, MGD
700
600
200
500
150
400
100
300
200
50
100
0
0
J
F M A M J
J
A S O N D
Irrigation is used to bridge the difference between
water demand and water availability
SLC Water use, MGD
Rain or ET, mm
250
Rain, mm
How is water demand typically
quantified?
•
•
Inches of water lost to evapotranspiration (ET)
Amounts to about 0-0.25 inch/day
𝑬𝑻𝒐 = 𝑷𝒍𝒂𝒏𝒕 𝑻𝒓𝒂𝒏𝒔𝒑𝒊𝒓𝒂𝒕𝒊𝒐𝒏 + 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)
ET=
+
Soil
Plant Transpiration Evaporation
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
ETo Background: Problems; #2
•
Economic Yield
•
Water applied
In landscapes, $ value
water input ≠ ≈ value of
landscape
Instead, minimum water
demand for acceptable
appearance
13
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 complexitysimplicity:
Complexity , precision
Precision
, simplifying
Simplifying
, knowledge
Law supply-demand:
Water supply
,
Knowledge
• Prescriptive=complex
• Descriptive=simplifying
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 evapotranspiration
ETo 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
1.5
Net monthly water
demand turf, 80% 2.3
4.3 5.4
4.4
2.1
ETo minus rain
Net monthly water
demand trees 50% 0.7
2.4 3.0
2.4 0.8
ETo 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
Native Species: Evidence of Water Use
23
Native Species: Response to Microclimate
Medium/small leaves,
evergreen
Aspen
More broadleaf
species
Oak
Small, thick, blue
leaves
Buffaloberry
Very small to no
leaves
24
Mormon tea
Eriogonm
corymbosum
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
Transpiring leaf area ≈ crown diameter2
Crown diameter2 may underestimate true
leaf area
27
How do you estimate leaf area?
6’
A tree with a 6 ft
crown diameter has
roughly 36 ft2 of leaf
area
6’
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?