This document summarizes a workshop on irrigation best practices presented by Caleb Carter. It discusses determining soil water capacity and managing irrigation to stay within plant available water levels. Key points covered include measuring soil properties, monitoring irrigation systems, managing uniformity and runoff risk, and scheduling irrigations based on soil type and crop water needs. The goal is to apply only as much water as crops can use through proper system setup, monitoring, and scheduling.

1. Caleb M. Carter
UW Extension Educator
Master Hay Grower Workshop
Riverton, WY
March 25, 2015
Irrigation: making the
most of your water

2. Available soil moisture
• Field Capacity
• 50% Available
Water
• Permanent
Wilting Point
• MAD

3. The sweet spot
Saturation Field
Capacity
Permanent
Wilting Point
• Texture
• Rocks
• Organic matter
• Bulk density
• Structure
• Rooting depth

4. Effects of irrigation on
infiltration
Soil Texture
Basic
Infiltration
Rate (in/hr)
Fine sand 0.50 – 0.75
Sandy loam 0.35 – 0.5
Silt loam 0.25 – 0.40
Clay 0.10 – 0.20

5. Web soil survey
Obtain info on:
• Soil type
• H2O capacity
• infiltration
http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

6. Active root zone

7. IRRIGATION SYSTEM
CHECKUP

8. Potential problems in the field
• Inappropriate monitoring of systems to ensure
proper operation
• Sprinkler installation or maintenance problems
• Sprinkler placement that decreases uniformity and
does not provide evaporation savings that many
expect
• Runoff due to inappropriate sprinkler selection or
system operation
• Inadequate pressure at pivot point to provide
desired flow rate and uniformity
• System capacity is not appropriate

9. Pivot characteristics
Span
Span
end (ft)
Area within the
span (acres)
Discharge from
span (gpm)
1 180 2 14
2 360 7 42
3 540 12 71
4 720 16 99
5 900 21 127
6 1080 26 156
7 1260 30 184
OH 1310 9 56
Total 124 750
7 x 180 ft. spans
Span
1 2 3 4 5 6 7

10. Pivot efficiency
Good uniformity

11. Pivot efficiency
Poor uniformity

12. Pivot efficiency
Different soils?

13. Need to know
• Depth per circle
• Uniformity of application
• Pressure/flow monitoring
• Pump/pivot interaction
• Managing limited capacity
• Control runoff
• Select appropriate sprinkler packages

14. How much water are you
applying?
System
capacity
(gpm/ac)
System flow rate for land
acres of:
Depth
applied
(in/day)
Depth
applied
(in/wk)
Time to
apply one
inch (days)120 130 160 240
3.0 360 390 480 720 0.16 1.1 6.3
3.5 420 455 560 840 0.19 1.3 5.4
4.0 480 520 640 960 0.21 1.5 4.7
4.5 540 585 720 1080 0.24 1.7 4.2
5.0 600 650 800 1200 0.27 1.9 3.8
5.5 660 715 880 1320 0.29 2.0 3.4
6.0 720 780 960 1440 0.32 2.2 3.1
6.5 780 845 1040 1560 0.34 2.4 2.9
7.0 840 910 1120 1680 0.37 2.6 2.7
7.5 900 975 1200 1800 0.40 2.8 2.5
8.0 960 1040 1280 1920 0.42 3.0 2.4
8.5 1020 1105 1360 2040 0.45 3.2 2.2

15. Sprinkles
• Follow manufacturer
recommendations
• Narrow spacing for
expensive sprinklers
may not be advisable
• Get in field to see how
good the coverage is
• Check runoff when
lateral aligns with row
direction

16. Monitor pressure/flow rate

17. Monitoring pressure
• Buy a pressure gauge and make sure your
system is operated at the designed pressure.
• Pressure regulators are important to low-
pressure sprinkler packages and will not
function properly unless the delivery line
pressure is at least 5 psi greater than the
pressure rating of the regulator.
• A non-functioning regulator means non-
uniform water application.

18. Volume
• Acre-inch: The volume of water required to
cover 1 acre 1-inch deep.
• Acre-foot: The volume of water required to
cover 1 acre 1-foot deep.
• 1 cubic foot = 7.48 gallons = 62.4 pounds
• 1 acre-inch = 3,630 cubic feet = 27,154
gallons
• 1 acre-foot = 12 acre inches = 43,560 cubic
feet = 325,851 gallons

19. Flow
• 1 acre-inch/hour = approximately 450 gallons
per minute = 1 cubic foot per second

20. Runoff potential
0.0
0.5
1.0
1.5
2.0
2.5
Rate(inches/hour)
Time (hours)
Infiltration rate
-How fast soil takes in water
Application rate
-How fast water is applied

21. Runoff potential
0.0
0.5
1.0
1.5
2.0
2.5
Rate(inches/hour)
Time (hours)
Infiltration rate
-How fast soil takes in water
Application rate
-How fast water is applied
Surface
storage
amount
ponded
Surface
saturates

22. Runoff potential
0.0
0.5
1.0
1.5
2.0
2.5
Rate(inches/hour)
Time (hours)
Infiltration rate
-How fast soil takes in water
Application rate
-How fast water is applied
Surface
storage
amount
ponded
Surface
saturates
Runoff Potential
Water that can move in
or leave the field
Runoff begins

23. Runoff potential

24. Runoff potential

25. Reducing runoff
• Short term
– Speed up the pivot
• Long term
– Increase wetted diameter
– Reduce gpm
– Increase surface storage
– Increase infiltration rate

26. Time of wetting
Initiation time
Rain gage

27. Time of wetting
Initiation time
Rain gage
End time
Time of wetting = end time – initiation time
Sprinkler height?

28. Measuring uniformity

29. Hard to manage what you
don’t measure
• Schedule irrigations
• Measure soil water
• Monitor pivot
• Control

30. WHEEL AND HAND LINES

32. Taxi system/Wipe

33. Skip

34. The offset

35. Sprinkler type

36. Pressure

37. HOW MUCH TO WATER?

39. Feel method
• If it makes a ball
and falls apart
when you bounce it
in your hand,
needs irrigation

40. Irrigation scheduling example
• Alfalfa – 5 ft. rooting
depth
• Soil – loamy sand

41. Irrigation scheduling example
• 5 x 0.5 = 2.5 in AWC
• MAD = 50%
• 2.5 x 0.50 = 1.75 in.
PAW
• Assume 3 in. applied
in 24 hrs.
• Will need to go to 12
hr. sets

42. Available water capacity (AWC)
Texture Soil Textural Class Estimated Average Plant AWC (in/ft2)
Sandy Soils Coarse Sands 0.5
Loamy sands 1.0
Loamy fine sands 1.25
Loamy very fine sands 1.25
Fine sands 1.25
Very fine sands 1.25
Loamy Soils Moderately coarse Sandy loam 1.5
Fine sandy loam 1.5
Medium Very fine sandy loam 2.0
Loam 2.0
Silt loam 2.0
Silt 2.0
Moderately fine Clay loam 2.2
Sandy clay loam 2.2
Silty clay loam 2.2
Clayey Soils Fine Sandy clay 2.0
Silty clay 2.0
Clay 2.0
http://www.nrcs.usda.gov/wps/portal/nrcs/detail/mt/soils/?cid=nrcs144p2_057354

43. Irrigation scheduling example
Soil Texture Available Water (AW) in/ft
Coarse Sand 0.2 ‐ 0.8
Fine Sand 0.7 ‐ 1.0
Loamy Sand 0.8 ‐ 1.3
Sandy Loam 1.1 ‐ 1.6
Fine Sandy Loam 1.2 ‐ 2.0
Silt Loam 1.8 ‐ 2.5
Silty Clay Loam 1.6 ‐ 1.9
Silty Clay 1.5 ‐ 2.0
Clay 1.3 ‐ 1.8
Peat Mucks 1.9 ‐ 2.9

44. Pivot example
• Pivot output in acre-
in?
• Soil?
• Catch: has to wait 24
hrs. between circles
• What is peak ET in
summer?
• 0.8 acre-in/24 hrs.
• Sandy loam:1.5 AWC
• MAD:1.5 x 0.5 = 0.75
PAW

46. Fall irrigation
Why?
• Increase fall weed
flush
• Can facilitate fall
tillage
• Fall plant growth
– Tiller production
• Leaching of salts
• Storing soil moisture

47. What it is all about…

48. Summary
• Get sprinkler chart & ensure package is properly installed
• Determine if system capacity is adequate for your location
• Ensure that pump and pivot are properly matched
• Buy good pressure gauge and operate system at design
pressure
• Operate system when crops are small and look for broken,
plugged sprinklers or pressure regulators, other leaks
• Observe water application in the outermost span on the
steepest portion of the field to see if you have runoff, if
problems exist:
– Reduce the application depth
– Use reduced tillage to enhance surface storage and infiltration
– Eventually evaluate if different sprinkler package would help
– Select devices with large droplet size when renozzling
• Routinely maintain mechanical/electrical components

49. Questions?
Caleb Carter
UW Extension Educator
Goshen County
(307) 532-2436
ccarte13@uwyo.edu
http://www.uwyoextension.org/highplainscropsite/