Going by soil moisture data alone? Soil moisture data are useful, but they can’t tell you everything. Other strategies for growers and researchers, like plant and weather monitoring, can inform water management decisions. In this webinar, world-renowned soil physicist, Dr. Gaylon Campbell shares his newest insights and explores options for water management beyond soil moisture. Learn the why and how of scheduling irrigation using plant or atmospheric measurements. Understand canopy temperature and its role in detecting water stress in crops. Plus, discover when plant water information is necessary and which measurement(s) to use. Find out: - Why the Penman-Monteith equation, with the FAO 56 procedures, gives a solid, physics-based method for determining potential evapotranspiration of a crop - How the ATMOS 41 microenvironment monitor combined with the ZL6 logger and ZENTRA Cloud give easy access to crop ET data - How v can be controlled by manipulating plant water potential using appropriate irrigation strategies - Why combining monitoring soil water potential with deficit irrigation based on ET estimates provide an efficient and precise method for controlled water stress management - And more…

2. HOW TO USE PLANT-WATER
RELATIONS AND
ATMOSPHERIC DEMAND
FOR SIMPLIFIED WATER MANAGEMENT
Gaylon S. Campbell, PhD
METER Group, Inc. USA

3. SOIL MATRIC POTENTIAL
UNDER POTATOES
TEROS 21
Matric Potential Sensor

4. ENVIRONMENTAL DATA
DIRECTLY TO THE CLOUD
4
Solar Radiation
Wind
Temperature
Humidity
Rain
ZL6 Data Logger
- solar charging
- cell enabled
- data directly to the cloud
Soil Moisture
ATMOS 41 Microenvironment Monitor

5. ESTIMATING
EVAPOTRANSPIRATION (ET)
SOME TERMS
5
• Evaporation – water evaporated from soil
• Transpiration – water evaporated from plants
• Reference ET – ET from a reference crop (usually grass) never short of water
• Crop ET – ET of crop under standard conditions (no disease, full production)

6. FAO 56 APPROACH
TO COMPUTING CROP ET
FAO 56: Crop evapotranspiration 6

7. PREDICTING ETO
AN ENERGY BALANCE PROBLEM
7
• 2.44 MJ of energy are required to evaporate 1 mm of water from 1 m2.
• Daily input of solar radiation is > 20 MJ/m2 in summer, < 3 in winter
• Some of the available energy heats the air, but, for well-watered
vegetation, most goes to evaporating water

8. THE PENMAN-MONTEITH
EQUATION
FOR REFERENCE ET
8
etemperaturandhumidityondependsD
heightcropandcondstomatalwindondependsg
etemperaturandradiationsolarondependGR
windandetemperaturondepend
ss
s
p
D
g
s
GR
s
s
E
v
n
a
v
n
o
.,
,
*
*
,
*
*
*
*
g
g
g
g
g
lg
++
+
+
-
+
=
depend on temperature and wind
depend on solar radiation and temperature
depends on wind, stomatal conditions, and crop height
depends on humidity and temperature

9. P-M MODEL PARAMETERS
FROM AVAILABLE DATA
Need for P-M
• Rn – net radiation
• D – vapor deficit
• gv – vapor conductance
• g* – apparent phychrometer constant
• G – soil heat flux
Data available
• Solar radiation, Ta, RH
• Ta, RH
• Wind speed, canopy height
• Wind speed, canopy height
• Fraction of Rn
9

10. FAO 56: Crop evapotranspiration
TYPICAL CROP COEFFICIENT
FUNCTION

12. Matric Potential

13. WATER BUDGETS
FOR FIVE FIELDS
13
Field ET Irrigation Drainage Leaching
Fraction
1 216 249 52 0.21
2 232 271 39 0.15
3 219 218 13 0.06
4 234 329 89 0.27
5 237 310 73 0.24

14. WATER FLOW
IN THE SOIL-PLANT-ATMOSPHERE
CONTINUUM (SPAC)
Low water potential
High water potential
Soil-root resistance
Stem resistance
Leaf resistance

15. WATER POTENTIAL
15
• Water potential gradients provide the driving force for water flow in the
SPAC
• Water flows from high to low potentials
• Water potential describes the availability of water for biological processes

16. MID-DAY
WATER POTENTIALS IN THE SPAC
16
–100,000
Soil
Root
Xylem
Leaf
Field Capacity
(kPa)
Permanent wilt
(kPa)
Atmosphere
–1,000
–700
–30
–30
–3,000
–2,500
–1,700
–1,500

17. –10,000
Soil
Root
Xylem
Leaf
Field Capacity
(kPa)
Permanent wilt
(kPa)
Atmosphere
–200
–150
–100
–30
–1,800
–1,700
–1,600
–1,500
PRE-DAWN
WATER POTENTIALS IN THE SPAC

18. HSIAO
PLANT RESPONSE TO WATER STRESS
Ann. Rev. Pl. Phys. 24:519 (1973) 18

19. PLANT WATER RELATIONS
SOME OBSERVATIONS
• Leaf water potential varies widely from night to day, every day—
low leaf water potential doesn’t necessarily mean a plant is stressed
• Highest leaf water potential depends on SWP;
lowest depends mainly on ETo until stomates close
• Stomatal conductance decreases when demand exceeds supply
(high ET, dry soil)
• Growth is fastest when soil is wet, demand is low
• Cell expansion is mainly at night with plentiful soil moisture
• Photosynthesis is not affected until the soil dries significantly
19

20. TWO IRRIGATION SCENARIOS
Irrigate for maximum vegetative
growth and biomass production
• Monitor soil moisture and keep it
above -100 kPa
• Monitor ET and keep it near potential
• Monitor irrigation and keep it close to ET
• Plant monitoring is not useful since no
stress should develop
Irrigate to control vegetative
growth; focus on assimilate
partitioning to fruit
• Monitor reference ET and irrigate at
some fraction of ET
• Monitor irrigation to be sure the right
amount is being applied
• Monitor soil moisture to assure that
stress levels are not exceeded
• Monitor stress levels in plant
20

21. MONITORING THE PLANT
21
• Leaf or stem water potential
• Stomatal conductance
• Canopy temperature

22. CONDUCTANCE VS. LEAF
WATER POTENTIAL
MERLOT
22
0
0.2
0.4
0.6
0.8
1
1.2
1.4
-2000 -1800 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0
RelativeConductance
Leaf Water Potential (kPa)

23. Tc – Ta
WATER RELATIONS INFORMATION
23
Tc - Ta depends on stomatal conductance (gv ), but many other things, too
Works best when vapor deficit is large and radiation and wind fairly constant
vH
aHp
n
ac
gg
p
D
gc
GR
s
TT
/00066.0*
**
*
=
÷
÷
ø
ö
ç
ç
è
æ
-
-
+
=-
g
gg
g

24. Data from Mark Blonquist, Apogee Inst.
25
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 5 6 7 8
WaterStatusIndex
Day
Removal of soil water
Precipitation
-2
-1
0
1
2
3
4
5
6
7Tc-Ta[C]
Corn Measured Tc - Ta; water
status index is relative
distance between
theoretical bounds.
Theoretical Tc - Ta for non-
transpiring crop (calculated
from surface energy
balance).
Theoretical Tc - Ta for fully
watered crop (calculated
from surface energy
balance).

25. • Controlling irrigation based on leaf water potential, stomatal
conductance, or canopy temperature is hard
• The outcome of any of those methods is that the crop uses less water
• Why not just manage the irrigation to supply less water than the plant
“wants,” and let the plant figure out the right potential and conductance?
DEFICIT IRRIGATION
FOR MANAGING PLANT WATER POTENTIAL

26. DEFICIT IRRIGATION
COMBINING SOIL MOISTURE AND ET
27
• Goal: Irrigate at 100% of ET
until completion of bloom,
then at 70%
• Use moisture measurements
to monitor soil water status
and make adjustments

27. DEFICIT IRRIGATION
MATRIC POTENTIAL
9/30/20 28
TEROS 21
Matric Potential Sensor

28. CONCLUSIONS
9/30/20 29
• The Penman-Monteith equation, with the FAO 56 procedures gives
a solid, physics-based method for determining potential ET of a crop
• The ATMOS 41 microenvironment monitor with ZL6 logger and
ZENTRA Cloud gives easy access to crop ET estimates
• Combining ET, irrigation, and matric potential provides a powerful tool for
managing irrigation for maximum production with minimum water waste
• Combining soil moisture monitoring with deficit irrigation, based on ET
estimates, provides efficient and precise control of plant
water stress

29. QUESTIONS?