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Irrigation
Scheduling
Phil Tacker
Extension Engineer
University of Arkansas
Irrigation Scheduling
Determining when to irrigate
and sometimes how much
irrigation water to apply
We irrigate most of our crops
in the Arkansas Delta
• Rice: 1.5 million acres - all irrigated
• Corn: 350,000 acres – 80% irrigated
• Cotton: 1 million acres - 75% irrigated
• Soybeans: 3 million acres - 62% irrigated
• Milo: 250,000 acres – 50% irrigated
4.5 million irrigated acres – 74% of total acres
Going by the calendar, crop
appearance, soil cracks etc.
usually not good enough.
Output from Watermark Sensors - Cotton
3.5
Final Irrigation
Tension (bars)
3.0
2.5
No Irr.
2.0
1.5
60% ET
1.0
0.5
100% ET
0.0
7/22 7/30 8/7 8/15 8/23 8/31 9/8 9/16 9/24
Crop/Canopy Temperature Monitoring
n:
e
t
red
Indicator Arm
Indicator
arm
Irrigate
Irrigate
Soil Water
Soil
water
Is Full
is full
Computerized Irrigation Scheduler
Distributed through internet:
(www.aragriculture.org/computer/schedule)
and is available on CD
Keeps track of the Soil Water (Moisture) Deficit
Room for more water.
Soil holds all the water it can.
Change in Water Use With Crop Age
Typical* Water Use (in.)
0.4
Last Ef f ect ive Flower
Cotton
0.3
(80 DAP - mid-August )
Plant ing
First Square
(32-38 DAP)
First Flower
(57-63 DAP)
0.2
First Open Boll
Emergence
(>100 DAP)
(4-10 DAP)
0.1
0
27-Apr
* based on Keiser
30-yr mean Tmax
25-May
22-Jun
20-Jul
Date (for May 1 planting)
17-Aug
Arkansas Irrigation Scheduler
Designed for minimal input data,
don’t need an expensive automatic weather station.
Measure Rain in Field
Scheduler Program
Scheduler Program
Field Verification of Scheduler
Scheduler Used in Crop Verification Programs
Arkansas County PA 2003
Planting Date: 6/8
Harvest Date: 10/14
Yield: 58.0 bu/ac
Asgrow 5501 RR
Double-Cropped
4 Furrow Irrigations
Total Rainfall: 10.39”
Irr. Terminated: 9/17
2.50
2.25
2.00
R1
R6
Target Deficit = 1.75”
1.75
1.25
Rainfall
1.00
Deficit
Irrigation
0.75
0.50
0.25
Date
18-Sep
11-Sep
04-Sep
28-Aug
21-Aug
14-Aug
07-Aug
31-Jul
24-Jul
17-Jul
10-Jul
03-Jul
26-Jun
19-Jun
0.00
12-Jun
Inches
1.50
Irrigation Methods
• Flood
• Furrow
• Pivot
• Border
Scheduler Program
Irrigation Scheduler Assumptions
• Good surface drainage – minimum standing
water 24 hrs after rain and/or irrigation
• No surface runoff until deficit is depleted
– crusting
– application rate
– effective rainfall
• Crop uses water at rate of non-stressed plant
• The soil moisture deficit returns to zero when
a field is surface irrigated
Scheduler updated periodically
• Investigating water use functions
– to see if they need updating for southern conditions
– to see if corn/cotton need to consider relative
maturity of cultivar (like soybeans)
• Changes currently being considered
– adjustments for early/late cotton
– direct input of reference (potential) ET for users
with access to better weather data
– drip irrigation scheduler
Program Use
• Primarily in Arkansas
• Some program users in:
MS, TN, LA, MO, KY
• Adaptable to other areas
Sometimes not able to keep
on schedule
• Insufficient water
• Insufficient time/labor
• Insufficient commitment
Summary
In order to manage irrigation so it gives
the best results, it is important to use an
irrigation scheduling method that you feel
is practical and that helps you irrigate
timely enough to satisfy the crop’s water
needs during the season.
Acknowledgments
Arkansas State Cotton
Support Committee
Arkansas Soil & Water Conservation Commission