1.
SLIDE Rules for Estimating Tree
Irrigation Demand …
and Survive Drought
Roger Kjelgren – Utah State University
Dennis Pittenger – University California Extension
Richard Beeson – University of Florida
Thayne Montague – Texas Tech University

2.
SLIDE Rules for Estimating Tree
Irrigation Demand …..
• Background
1. Climate, drought, limited water for landscapes
2. Managing water for your trees: 1) water demand/use rate 2) supply
of water 3) desiccation tolerance 4) Tree size
• Simplified Landscape Irrigation Demand Estimation
1. Limits of using ETo (reference evapotranspiration)
2. Estimating tree water demand from Plant Factors
3. Hydrozones based on water demand
4. Density, isolated-freestanding tree water demand

3.
…. And Survive Drought
• Applying SLIDE Rules to Your Trees
– Water supply in root zone – how much to irrigate
– Using ETo – when to irrigate
• You will be able to make
– 1 measurement (tree size)
– 1 choice (root zone water)
– Then be able to estimate when and how much water to
apply to keep you tree reasonably healthy

4.
Intro – Climate
• West in
drought; CA,
western NV in
long term
drought due to
low snowpack
• Low snowpack
is cyclic; dry
periods loosely
10-14 years in
norther CA, NV,
UT

5.
Intro – Climate
• Normal for west coast U.S.
– Pineapple express – atmospheric river triggered
by sea temperature western Pacific – El Nino

6.
Intro – Climate
• Ridiculously
resilient ridge (high
air pressure)
debut: 2013-2014
• Ridiculously
resilient ridge
encore: 2014-2015
• Drought

7.
Intro – Climate
• “The Blob” – warm water in north Pacific;
contributed warm storms this winter in PNW
• May signal coming wetter winter
• Temperature
in north
Pacific linked
to 30 cycle:
cool temps,
drought;
warm temps
= pineapple
express…….
maybe

8.
Intro – Climate
• Making it al
worse:
absurdly warm
temperatures
in West
January-March
2015
• Climate
change? maybe

9.
Intro – Reno Airport Climate
• 1938-2014
• Reno very, very dry

10.
¿Questions?
• Drought (low winter snow pack) follows cycles;
normal
• High temperatures this year; not historically
normal
• Peek at things come? Maybe
• Next: four factors to estimate when and how
much to irrigate

11.
11
Intro – Plant Water Demand: 4 Irrigation Factors
2. Rooting
Depth =
water
supply
1. Transpiration:
water use
rate
Root Hair
Stomata
2CO
4. Tree size
3. Leaf desic-
cation tolerance

12.
Intro – Tree Water Demand
• Transpiration (water use)—weather factors
– Sun: energy to evaporate water
– Temperature: air space evaporation potential
– Humidity: actual air space
available for evaporation
– Wind: how fast
evaporate water moves
into actual available
air space (less effect on trees)
• Trees more responsive to
humidity, less to wind,
than turfgrass

13.
Intro – Tree Water Demand
13
1. Water use: Transpiration use
that depletes soil water to
threshold of degrading plant
performance
2. Rooting
depth:
water
needed to
refill
root
zone

14.
Water
use
Water
use
Water
use
Irrigate
to refill
root
zone
3. Root depth: irrigate before plant
performance is affected; desiccation
tolerance

15.
Intro – Tree Water Demand;
3. desiccation tolerance
Irrigate
to refill
root
zone

16.
Intro-Tree Water Demand
• Desiccation tolerance (run out of gas) –
varies widely among species
Low - wilting
Medium –
defoliation
Medium low –
Margin burn
High – Not
Visible until
dead

17.
17
4. Tree size - Effectively Transpiring leaf area ≈
crown diameter2
Tree size x water in root zone
= amount of water to apply
X

18.
¿Questions?
Factor Story Irrigation Visual cues
1) Water use
rate
Rate water in root
zone being used
(fuel efficiency)
When to
irrigate
None; need
numbers
2) Root
depth/water
Water supply (gas
tank)
How much to
irrigate (depth)
None direct; infer
from drought
tolerant leaves
3) Desiccation
tolerance
Plant water
stressed (ran out of
gas)
When to
irrigate
Leave: big, thin
low; small, thick
high
4) Tree size 3-dimensional size
of gas tank
How much to
irrigate
(volume)
Yes
NEXT: SLIDE RULES #1 - ETo

19.
SLIDE Rule #1: Reference
ETo-basis for estimating
water use; useful for
uniform plant surfaces, less
useful for non-uniform plant
surfaces

20.
SLIDE Rule #1: Evapo-transpiration
Rate: ETo
• Calculated water used by hypothetical 12 cm/4 inch high cool
season, clipped/mowed turf<=sun, heat, wind, humidity
• Range 0 – 6 mm/day
(0-0.25”/day)
• ETo tells you nothing about
what plants are actually doing Plant Transpiration
Soil
Evaporation
+ET=

21.
SLIDE Rule #1: Reference ETo-basis
for estimating water use; useful
for uniform plant surfaces, less
useful for non-uniform plant
surfaces
Turf: ETo useful for seasonal
estimates, daily irrigation
scheduling
Trees: ETo useful seasonal,
somewhat for daily
Desert plants: ETo useful only for
Seasonal estimates

22.
Landscape Plant Water Demand
• ETo x adjustment factor = estimated plant water
use

23.
SLIDE Rule #1 – The limits of ETO
• Reno Airport growing season ETo, 1938 to
2014

24.
¿Questions?
• ETo is useful for comparison between times and
among places; managing turf
• Useful on seasonal basis for woody plants
• NEXT: SLIDE Rule #2 – estimating plant water use
with Plant Factors applied to ETo

25.
SLIDE Rule #2: Plant factors (PF)--simple
downward adjustments to ETo to estimate water
use of turf, non-turf, and desert plants
• SLIDE Rule #2 is the American Society of
Agriculture and Biological Engineers imminent
national standard
• Defines Plant Factors (PF): adjust ETo
downward represent water loss (transpiration)
major plant types: turf, non-turf, desert plants

26.
ASABE Standard S623,
SLIDE Rule #2
Fraction of ETo (Plant Factor) to estimate water use yet
maintain acceptable appearance of established landscape
plants
Recommended Plant Factor
Turf-Cool Season 0.8
Turf-Warm Season 0.6
Woody plants-Humid 0.7
Woody plants-Arid 0.5
Desert plants 0.3

27.
27
High temps, high
VPD: – low humdity
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
- H
+
H
+
O
-
H
+
H
+
O
-
H
+
H
+
O
-
Cool temperatures,
low vapor pressure
deficit: high
humidity
SLIDE Rule #2: Tree stomate
response to dry air (low humidity,
high vapor
deficit)

28.
SLIDE Rule #2: Plant factors (PF)--simple downward
adjustments to ETo to estimate water use of turf, non-
turf, and desert plants
• Woody plant response to dry air (low humidity, high
vapor deficit: close stomates, moderate water use
different from ETo
• Study in Utah and
Florida on Sweetgum
cultivar ‘Moraine’
• Sweetgum in Utah
moderated water use
at ETo levels above 4
mm (~0.1 inch) per
day

29.
• Where woody
plant PF = 0.5
applies
• Map of July
average daily
high vapor
deficit (low
humidity)

30.
SLIDE Rule #2: Plant factors (PF)--simple downward
adjustments to ETo to estimate water use of turf, non-
turf, and desert plants
• Desert plants survive because BY NOT INCREASING
WATER USE WITH ETo; Performance, water use mostly
untethered from ETo
• Fewer, cooler leaves = less water loss
• Wider spacing
= more water per
plant
• PF=0.3; coarse
approximation
• Mostly herbaceous
perennials, shrubs

31.
Plant Size, Density Decreases with Elevation
31

32.
Leaf Traits That Aid in Avoiding,
Tolerating Drought
• Leaf temperature
– Smaller, less leaf area
– Vertical leaf orientation , curled
– Leaf color (blue reflects light)
• Transpiration
– Trichomes increasing boundary,
reducing water loss
– Sunken stomata
• Dense, thick, evergreen
(desiccation tolerant)
• Visual cues tells story of
plant water demand
Shepherdia rotundifolia
Ephedra viridis

33.
¿Questions?
• Plant Factors are simple numbers to estimate
water demand of landscape plant types
– All trees PF = 0.5
– Desert plants (shrubs, wildflowers) PF = 0.3
• NEXT: SLIDE Rule #3 – hydrozones

34.
SLIDE Rule #3: Hydrozone—within a zone
controlled by an irrigation valve the species with
highest water use dictates irrigation schedule
• Highest water use plants within a zone
dictate when to irrigate, how long to irrigate
– Higher PF species, such as turfgrass with
imbedded trees; turf dictates schedule
– Zone with sun and shade; sunlit area uses more
water, dictates schedule

35.
• Abandoned landscape;
tree survived, but turf and
burning bush did not
• Tree in turf
parking lot;
turf
dictates
irrigation

36.
Irrigation Plan• How to ensure that plants are watered
• Irrigation plan=water delivery system layout

37.
Irrigation Plan
• Low water/sustainable
landscape more
complex, requires more
planning and information
transmitted to end user
• Use hydrozones to define
water needs and type of
irrigation

38.
Not hydrozoned:
trees imbedded in
turf
Hydrozoned:
trees, wildflowers
separated from
turf

39.
SLIDE Rule #3: Hydrozone
• Within plant type (the same Plant
Factor ), there may be differences in
rooting depth
• Irrigate for shallowest root zone
• Deeper rooted species will access
irrigation and soil water
• More knowledge,
hydrozones
can be separated
based on leaf
desiccation (drought)
tolerance

40.
¿Questions?
• Hydrozones common practice in landscapes
• Here defined as plants with same Plant Factor
• Even in hydrozone, may be differences in rooting
depth, or even leaf desiccation tolerance
• NEXT: SLIDE Rule #4 – Plant density

41.
SLIDE Rule #4: Density—within a zone, plant
density >80% ‘big leaf’ water use ;
<80%, of individual plant water use
• Above 80% plant cover within a hydrozone, ‘big leaf’
water use
– Water use estimated as ETo x Plant Factor (highest
water use plant)
– Root zones intermingled, irrigate entire surface
• Below 80% plant cover within a hydrozone, individual
plant
– Water use estimated ETo x Plant Factor x leaf area
– Leaf area approximately crown drip line (projected
crown) area
– Irrigate within drip line area

42.
>80% plant cover, trees in
turf: turfs get water from turf
irrigation; if turf stressed,
trees may be stressed
If trees isolated, like in
parking strip, they can be
watered individually

43.
>80% plant cover,
leaf area, water
use intermingles
80% of ETo
50% of ETo

44.
Incomplete plant cover;
key tool to reduce
water demand

45.
Oasis incomplete
plant cover; high
density (>80%) areas
imbedded in
hardscape

46.
• Individual plant crowns, several layers; non
sunlit layers transpire little
• Shaded area approximates projected crown
area

47.
Estimating water demand of single,
isolated tree
GALLONS OF WATER BY CROWN DIAMETER AND DEPTH OF WATER
Crown diam.,
feet
0.05 0.1 0.15 0.2 0.3 0.5 1 2
depth of water to apply, inches
1 0.0 0.0 0.1 0.1 0.1 0.2 0.5 1.0
2 0.1 0.2 0.3 0.4 0.6 1.0 2.0 3.9
4 0.4 0.8 1.2 1.6 2.3 3.9 7.8 15.6
8 2 3 5 6 9 16 31 63
12 4 7 11 14 21 35 70 141
20 10 20 29 39 59 98 196 391
Range, daily turf water demand Extended Extended trees
Range, daily tree water
demand turf
Sandy
soil Loam soil
Irrigating isolated tree; water application does not need
to be perfectly uniform; just need to close

48.
¿Questions?
• >80% plant cover, treat as 100% plant cover
• <80%, calculate water use of individual plant
• NEXT: Applying to isolated trees – combining
depth of root zone water and estimated water
demand

49.
Applying Water: Root Depth
• Rooting depth, soil type determines
the amount of water to apply per
irrigations
• Generalized assumptions about
plant type rooting depth
– Rooting depth proportional to
plant size: turf least, trees most
– Desert species across plant types
(shrub, herbaceous perennials)
have deep roots

50.
Rooting Depth and Irrigating
• Difficult to know rooting depth
• Simplest
to assume
a depth
of water
to be
applied
at each
irrigation

51.
Rooting depth • Cool season turf
generally shallow
rooted
• Poor soil, frequent
irrigation = more
shallow
• When turf
sodded over
subsoil, turf
rooting depth
visible

52.
Rooting Depth
• Genetic
Rooting
depth
deeper
as
rainfall
is less
Cercocarpus ledifolius
Foothills, dry
rocky soils
Ribies aureum
Very wet
riparian
Mahonia
fremontii
Dry, desert/
steppe
52

53.
Rooting Depth
 Rooting depth is genetic; turfgrass shallow, woody
plants deeper, drought adapted plants deepest
 Rooting depth x available soil water = water
available to plant
Shallow: most, but not
all, common non-turf
species Deep: drought adapted non-
turf species
Turfgrass: shallowest

54.
54
Soil Texture and Water
• Sand holds less water, so applied water moves
deeper but not sideways
• Sand irrigate less water, more frequently
1 HOUR
2 HOURs
3 HOURs
4 HOURS
Sandy soil Loam soil

55.
Sand at field capacity
Soil Texture and Water
55
Sand at wilting point
Loam at field capacity
Loam at wilting point
• Sand holds less total water than loam soils

56.
1”water
Assuming a safe
depth of water to apply,
avoid limits of desiccation
tolerance
Can assume 2” water to
apply for loam soils,
most woody plants
Assume 1” for
sandy soils and
drought sensitive
plants
2”water

57.
Depth of water to apply for different
plant types
Plant Traits Depth of Water to Apply, cm
(inches) (Large X = recommended depth)
Plant Type General rooting
depth
1.3
(0.5 )
2.5
(1.0)
5.0
(2.0)
7.5
(3)
Annuals 15-30 cm
(6-12 in)
X x -- --
Turfgrass 15-60 cm
(6-12 in)
x X -- --
Herb. Perennials,
sandy soil woodies
30-60 cm
(6-12 in)
x X -- --
Woody Plants 60-120 cm
(24-48 in)
-- x X --
Desert Plants 30-300 cm
(12-144 in)
X (cacti) -- -- X
Soil Traits
Depth of applied (or
rain) water pene-
tration into the soil
Silt Loam 14
(6)
28
(11)
55
(22)
83
(33)
Loamy Sand 32
(12)
62
(25)
125
(50)
187
(75)

58.
Tree crown size x depth of
water in root zone = volume
of water to apply
 Rooting depth is genetic;
turfgrass shallow, woody
plants deeper, drought
adapted plants deepest
 Rooting depth x available
soil water = water available to plant
1”water
2”water
 Root zone water x tree size (crown cross section
area) = volume (gallons) to apply
 Water in root zone also determines irrigation
schedule (

59.
Incomplete plant cover; estimate water demand
of individual plants
• Estimate water use of
individual plants;
projected crown area x
depth of water
• Assume 2 inches to apply;
volume needed depends
on crown size
– Radius2 x 3.14 x 2 x 0.623 =
gallons to apply 2 inches of
water
– Simplified: diameter2
=gallons needed to apply 2
inches
White fir: 20 ‘
diameter=400 gal
Bigtooth maple: 10 ‘
diameter=100 gal
Mountain lover: 3 ‘
diameter=9 gal

60.
Estimating water demand of single,
isolated tree
GALLONS OF WATER BY CROWN DIAMETER AND DEPTH OF WATER
Crown diam.,
feet
0.05 0.1 0.15 0.2 0.3 0.5 1 2
depth of water to apply, inches
1 0.0 0.0 0.1 0.1 0.1 0.2 0.5 1.0
2 0.1 0.2 0.3 0.4 0.6 1.0 2.0 3.9
4 0.4 0.8 1.2 1.6 2.3 3.9 7.8 15.6
8 2 3 5 6 9 16 31 63
12 4 7 11 14 21 35 70 141
20 10 20 29 39 59 98 196 391
Range, daily turf water demand Extended Extended trees
Range, daily tree water
demand turf
Sandy
soil Loam soil
Irrigating isolated tree; water application does not need
to be perfectly uniform; just need to close

61.
At end of 19 days, estimated
water to refill root zone
depleted
-Apply 9 gallons to shrub
-Apply 50 gallons to tree
NOTE: these are best guess
estimates; need to be tempered
by good judgment
2”water
Day1:fullrootzone,0.20”ET,
demand=0.1”=1.9”remaining
Midsummer,conventionalspecies,PF=0.5%ofETo
1.9”water
Day2:drierrootzone,0.3”ET,
demand=0.15”=1.75”remaining
0.03”water
Day2:fullrootzone,0.28”ET,
0.03”remaining
After 17 days,
average ET 0.2”day,
1.7” of water used…

62.
Water
use
Water
use
Water
use
Irrigate
to refill
root
zone
3. Root depth: irrigate before plant
performance is affected; desiccation
tolerance; 2” enough for most trees
2” water

63.
Reno Irrigation
Schedule
• For any tree with 2”
water in root zone,
when to irrigate
Average +sd -sd
1-Apr 1-Apr 1-Apr
28-Apr 22-Apr 5-May
20-May 11-May 31-May
8-Jun 27-May 21-Jun
24-Jun 11-Jun 9-Jul
9-Jul 25-Jun 25-Jul
24-Jul 8-Jul 11-Aug
8-Aug 21-Jul 31-Aug
25-Aug 4-Aug 27-Sep
14-Sep 19-Aug
12-Oct 5-Sep
25-Sep
23-Oct
1-Nov 1-Nov 1-Nov

64.
How to apply water
based on tree size
• You have estimated
volume water needed
based on tree size
• Run irrigation system
long enough to apply that
amount
– Learn how much water
your water delivery system
(drip, sprinkler, hose)
applies per 10 minutes
• Be sure to apply across
entire root zone

65.
65
Rooting depth
• Tree root zones
adaptable; can take up
water from part of root
zone
• Doesn’t have to be
perfect, but cover root
area under tree crown as
possible

66.
Keeping your tree alive
1) 1 measurement: area of cross section tree
crown (dripline area/projected crown area);
estimate gallons needed from table or web site
2) 1 choice: depth of root zone water, 1” or 2”;
follow the schedule for dates when to irrigate to
replace 1” or 2”
3) Challenge: how to apply that water (#1) to tree
root zone
4) Observe plants to ensure performance

67.
Conclusion
• SLIDE Rules make landscape irrigation water use
estimation more accessible
– #1. Reference ET basis for estimating landscape
water use; useful for turf, less so for non turf
– #2. Plant Factors estimate water use as fraction of
ETo for turf, non turf, and desert plants
– #3. Hydrozone goal; use plants of same PF in zone
– #4. Density; > 80% ‘big leaf’ water use; <80%
single plant water use
• Ultimately, satellites will measure actual water use of
large, oasis turf; eliminate need for weather station
ETo