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Blaine HansonDepartment of Land, Air and Water Resources
University of California, Davis
Irrigation Water Management -
Science, Art, or Guess?
When should irrigations occur?How much water should be applied?How long should the irrigation last?
Irrigation water management: questions to answer
Avoid Deficit Irrigation
Avoid Overirrigation
Methods
ET based (sometimes called the water balance approach)Plant basedSoil moisture measurementsExperienceGuess
ET based method
Calculate the crop evapotranspiration (ET) between irrigations and apply that amount plus any needed for irrigation efficiencyAssumes that the ET between irrigations equals the soil moisture depletion (does not apply under shallow ground water conditionsET = Kc x ETo x IN
ET = crop evapotranspirationKc = crop coefficientETo = reference crop ET (California Irrigation Management Information System)IN = days between irrigations
Plant‐based
Use leaf‐water potential measurements to determine when to irrigation
Pressure chamberTime consuming
Use ET = Kc x ETo to determine how much water to apply
Soil moisture measurements
Approach 1 ‐measure changes in soil moisture content between irrigations –assumes that the ET between irrigations equals the soil moisture change between irrigations
When to irrigateHow much to apply
Approach 2 ‐measure soil moisture tension to determine when to irrigate
Soil moisture tension ‐ tenacity with which water is retained in soil (the drier the soil, the higher the soil moisture tensionUse reference crop ET and crop coefficients to estimate the amount of water to apply
Not recommended for drip and microsprinkler irrigation
Recommended method for drip and microsprinkler irrigation
Select an appropriate irrigation frequency
Use the ET‐based approach to determine how much water to apply
Do not use soil moisture measurements to determine how much water to apply
Monitor soil moisture to ensure adequate irrigation over time
Periodic measurements Limited dataMay be difficult to identify trends in soil moisture
Continuous measurement – trends are readily evident in data
Methods for monitoring soil moisture
Soil probe
Tensiometers – soil moisture tension
Electrical resistance blocks – soil moisture tension• Watermark sensor• Gypsum block
Neutron moisture meter – soil moisture• Radioactive source – licensing, storage• Not appropriate for grower use
Dielectric moisture sensors – soil moisture
Soil probe
0
10050
Vacuum Gauge
Porous Ceramic Cup
Reservoir
Cap
Water
Tensiometer
Electrical Resistance Blocks
Watermark Block
Gypsum Block
Neutron moisture meter
Dielectric Soil Moisture Sensors
Sensors measure an electrical property of soil called the dielectric constant ‐ varies with soil moisture content
Calibration equation relates dielectric constant to soil moisture content
GroPoint Dielectric Soil Moisture Sensor
Echo Dielectric Soil Moisture Sensor
Enviroscan Dielectric Sensor
Conclusions of USDA‐ARS Research on dielectric soil moisture sensors
All dielectric sensors studied required site specific calibrationNone of the sensors performed within the accuracy specifications published by the manufacturersSensors were sensitive to soil bulk electrical conductivityCalibrations of sensors were more difficult to establish than for the neutron probeSensor readings were sensitive to small scale variations of soil properties due to their very small zone of influence. These small scale variations were smaller than the scale at which plants respond to soil moisture
Drip/microsprinkler irrigation: where should the sensors be installed?
Soil moisture under drip/microsprinkler irrigation varies with distance and depth from the emitter
Root density also can vary with distance and depth from the emitters
Installation too close – very wet soil
Installation too far – little or no response to irrigation
0 2 4 6Distance (feet)
0
2
4
6
8
10
12
14
Dis
tanc
e (fe
et)
-100102030405060708090100110
Site 2 (Young Navel)
Microsprinkler
High
Low
0 2 4 6 8Distance (feet)
0
2
4
6
8
10
12
14
16D
ista
nce
(feet
)
-100102030405060708090100110
Site 1 (Valencia)
Microsprinkler
High
Low
FanjetSpinner
Microsprinkler water application patterns
2 3 4 5 6 7 8 9Distance from micorsprinker (feet)
-3
-2
-1
Dep
th (f
eet)
2 3 4 5 6 7 8 9Distance from microsprinkler (feet)
-3
-2
-1
Dep
th (f
eet)
00.10.20.30.40.50.60.70.80.911.11.21.31.41.51.61.7
July 24 (just after irrigation)
July 29 (just before next irrigation)
High
Low
Soil water content
Soil moisture – microsprinkler (citrus)
20 30 40 50 60 70 80 90 100Distance From Tree (inches)
-70
-60
-50
-40
-30
-20
-10D
epth
(inc
hes)
Soil moisture – drip irrigation
Tree Trunk
Tree Skirt
Wetted Area
High Root Density
Low Root Density
Root pattern – flood irrigation of trees
Tree Trunk
Tree Skirt
Wetted Area
High Root Density
Low Root Density
Root pattern – microsprinkler‐irrigated tree
Tree Trunk
Tree Skirt
Wetted Area
High Root Density
Low Root Density
Root pattern – drip‐irrigated tree
01-Mar-03 01-May-03 01-Jul-03 01-Sep-03 01-Nov-03 01-Jan-04
Soil
moi
stur
e te
nsio
n (c
entib
ars)
0
20
40
60
80
100
246
Distance From Drip Line (feet)
Grapes
Soil moisture tension – drip‐irrigated grapes
Courtesy – Blake Sanden,UCCE, Kern County
How frequently should soil moisture be measured ‐ periodic vs. continuous measurements
Periodic • Measurement intervals – once or twice per week• Appropriate for sprinkle, furrow, and flood irrigation• Not recommended for drip or microsprinkler irrigationContinuous • Measurement interval – few minutes to once per day• Recommended for microirrigation
What Do The Readings Mean?
18-Ju
n
25-Ju
n
28-Ju
n2-J
ul
10-Ju
l
12-Ju
l
16-Ju
l
19-Ju
l
23-Ju
l
30-Ju
l
6-Aug9-A
ug
13-A
ug
16-A
ug
20-A
ug
23-A
ug
27-A
ug1-S
ep3-S
ep6-S
ep
10-S
ep
13-S
ep
17-S
ep
20-S
ep2-O
ct0
50
100
150
200W
ater
mar
k B
loc k
Re a
din g
(ce n
tibar
s)
Depth (feet)6 12 18 24
Walnut (flood irrigated)
Soil moisture tension – flood irrigated walnut
Plot 4 - 6"
0
20
40
60
80
100
12"
Tens
iom
eter
Rea
ding
s (c
entib
ars)
0
20
40
60
80
100
24"
1-Au
g2-
Aug
5-Au
g6-
Aug
8-Au
g11
-Aug
12-A
ug13
-Aug
15-A
ug16
-Aug
19-A
ug20
-Aug
22-A
ug23
-Aug
26-A
ug27
-Aug
29-A
ug30
-Aug
2-Se
p3-
Sep
5-Se
p6-
Sep
9-Se
p10
-Sep
12-S
ep13
-Sep
16-S
ep17
-Sep
19-S
ep20
-Sep
23-S
ep24
-Sep
26-S
ep27
-Sep
30-S
ep1-
Oct
3-O
ct4-
Oct
6-O
ct10
-Oct
11-O
ct13
-Oct
14-O
ct15
-Oct
17-O
ct18
-Oct
21-O
ct22
-Oct
24-O
ct25
-Oct
28-O
ct29
-Oct
31-O
ct1-
Nov
4-N
ov5-
Nov
7-N
ov8-
Nov
4-D
ec13
-Dec
0
20
40
60
80
100
Soil moisture tension – microsprinkler‐irrigated citrus
Date
Soil moisture tens
ion (cen
tiba
rs)
Continuous monitoring of soil moisture
Sensor typeCompatible for continuous measurement – multiple measurements per day to once per dayPlacement of sensors (distance, depth) relative to drip emitters/microsprinklers
Data loggerComputer/software for evaluating dataWireless system or computer downloadConsultant or grower
Sensors for continuous monitoring of soil moisture
Watermark electrical resistance blocks – soil moisture tension
Dielectric moisture sensors – soil moisture
AMD 400 Data Logger
Irrometer Monitor Data Logger
A - Almond - 47.9 inches
01-Jan-04 01-Mar-04 01-May-04 01-Jul-04 01-Sep-04 01-Nov-04So
il M
oist
ure
Tens
ion
(cen
tibar
s)0
20
40
60
80
100
120
140
160
183660
B - Almonds - 59.7 inches
01-Jan-04 01-Mar-04 01-May-04 01-Jul-04 01-Sep-04 01-Nov-040
20
40
60
80
100
120
140
160
183660
Depth (inches)
Depth (inches)
Courtesy – Blake Sanden,UCCE, Kern County
Soil moisture tension –drip‐irrigated almond
Peach Orchard - Flood Irrigation
Day of Year
100 120 140 160 180 200 220 240 260
Soil
Moi
stur
e Te
nsio
n (c
entib
ars)
0
20
40
60
80
100
120
140
160
180
200
123
Depth (feet)
1 May 1 Jun 1 Jul 1 Aug 1 Sep
Soil moisture tension – flood irrigated peach
Wireless Systems
Advantage – frequent access to data from office
Sensors
Data logger
Transmitter device at logger – radio, cell phone
Receiver device – office, network
Computer
Automating the microirrigation system
Automation – controller turns the irrigation system on and off
Devices: soil moisture sensor, data logger, wireless system, controller
What triggers the irrigation?• Soil moisture level• Predetermined amount of
evapotranspiration• Days between irrigation
anrcatalog.ucdavis.edu