1.
WHY WATER CONTENT CAN’T
TELLYOU EVERYTHING YOU
NEED TO KNOW
Colin Campbell, Ph.D.
METER Group, Inc. USA

2.
Without more information,
we cannot tell how many logs will keep
us adequately in our comfort range.

3.
?

4.
DO YOU NEED TO UNDERSTAND
TEMPERATURE TO USE IT?
Typically we only connect these
values with comfort
Easy to remember ranges if you
are familiar with the units
˚F
˚C
K

5.
= water potential

6.
WATER POTENTIAL IS THE
SAME WAY
-30 to -50 kPa
Sterling Taylor, 1972,Physical Edaphology:
The physics of irrigated and non-irrigated
soils

7.
WHY ISN’T WATER POTENTIAL
MORE COMMONLY USED?
1. Water potential is difficult to understand
2. Water potential is hard to measure
3. Even using water potential, we still don’t know HOW MUCH water to add
We’ll now focus on each of these challenges

8.
WATER POTENTIAL
UNIT OVERLOAD
kPa cbars
Bars
mm water
atm
MPa
What do they all mean?!!
J/kg
pF

9.
“WATER POTENTIAL IS HARD TO
UNDERSTAND”
Temperature
Defined by the third law of thermodynamics
with respect to molecular movement at
absolute zero…
Simply learn relevant temperature unit and
scale and accept values for optimal ranges
Water potential
Energy required, per quantity of water, to
transport, an infinitesimal quantity of water
from the sample to a reference pool of pure,
free water
Simply learn relevant water potential scale
and units and accept literature values for
optimal ranges

10.
“WATER POTENTIAL IS HARD TO
MEASURE”
True –
• Many field sensors are inaccurate
• Some are expensive
• Inherent sensor-to-sensor variability
• The most accurate sensors have limited range (tensiometers)
A poorly calibrated water potential sensor is just a water content sensor
• It will give a relative response but will not tell you if it’s in the right range

11.
“WATER POTENTIAL IS HARD TO
MEASURE”
METER Group’s TEROS 21
• Individually calibrated
• Cover broad water potential
range
• Low sensor-to-sensor variability
• Sensor consistency evident in field
data

12.
“WATER POTENTIAL LACKS
CONNECTION TO HOW MUCH
WATER TO APPLY”
Let’s consider some scenarios
Good:simply use water potential to know when plants need water
Great:Use water potential and water content together to get complete soil
moisture picture

13.
STRESS AND YIELD
POTATOES
Site Days in stress Yield (Mg/ha)
9 42 31.47
12 53 32.77
10 44 37.44
11 0 39.67
6 0 40.08
7 16 40.33

14.
FIELD MOISTURE STATUS

15.
IRRIGATION STEERING WITH
WATER POTENTIAL
Vine kill
Southern Idaho, USA - Irrigated seed potatoes
Data courtesy of BKR Farms from ZENTRA Cloud

16.
RESULTS FROM 2020
Potatoes
• Improved yields
• Higher quality
• Reduced variability
• Lower disease
Inputs
• Reduced water and fertilizer
applications
• Lower pumping costs
• Less frequent field walks
Challenges
• Requires knowledge of water
requirements

17.
GREAT:UNITE WATER CONTENT
AND WATER POTENTIAL
Water potential
Water content
15 cm
6 cm
30 cm
15 cm
6 cm
Soil Type Crop Type
Loamy sand Turf grass

18.
OVER-IRRIGATION
15 cm
15 cm
6cm
6cm
25cm

19.
OPTIMAL IRRIGATION
15 cm
15 cm
6cm
6cm
25cm

20.
UNDER IRRIGATION
-500 kPa
15 cm
15 cm
6cm
6cm
25cm

21.
OPTIMAL MOISTURE RANGE
15 cm deep layer: Maximum of 12 mm of irrigation to fill up profile
- - - - -

22.
WATER AVAILABILITY IN POTATOES
Rough guess: 4% VWC range w/ 50 cm root depth  20 mm water refill

23.
We don’t know if we’ll be
comfortable just by knowing how
many logs we’ve added to the fire.
We won’t know if the soil is optimal
for plant growth just by knowing
water content.

24.
QUIZ
SHOULD I WATER?
Data courtesy of BKR Farms from ZENTRA Cloud

25.
QUIZ
SHOULD I WATER?
Data courtesy of BKR Farms from ZENTRA Cloud

26.
QUIZ
SHOULD I WATER?
Data courtesy of BKR Farms from ZENTRA Cloud

27.
SUMMARY
Water potential should be understood like temperature
• Rarely consider its complex definition
• Memorize critical range for comfort and apply strategies to keep within boundaries
Water potential is difficult to measure accurately, but not impossible
• Some sensors have large variability
• Individual calibration of TEROS 21 sensors shows excellent consistency
Combining water potential and water content is powerful
• Straightforward to irrigate with only water potential, but miss how much water to add
• Together, they give a complete moisture picture
• Comfort range generates the metaphorical logs to add

28.
QUESTIONS