1. Irrigation Scheduling Overview and Tools
William Northcott
Department of Biosystems and Agricultural Engineering
Michigan State University
June 26th, 2009

2. Irrigation Scheduling
Process of maintaining an optimum water balance in the soil profile for crop growth and production
Irrigation decisions are based on an accounting method on the water content in the soil

3. Irrigation Scheduling
Components
Plant Growth and Water Use
Soil Water Holding Capacity
Rainfall / Irrigation
RECORDKEEPING

4. Plant Growth and Water Use
Fundamentally crops use water to facilitate cell growth, maintain turgor pressure, and for cooling.
Crop water use is driven by the evaporative demand of the atmosphere.
Function of temperature, solar radiation, wind, relative humidity.
Example, a fully developed corn crop in Michigan can use as high as 0.35 inches per day. (~9,500 gallons / acre)
Optimum crop growth and health occurs when the soil moisture content is held between 50 – 80% of the “plant available water”

5. Estimating Plant Water Use
Crop water use = Evapotranspiration (ET).
A “potential reference ET (PET)” can be calculated based on weather conditions.
The standard method – Penman – Monteith.
Based on temperature, solar, humidity, wind, rainfall
“Well watered grass”
Michigan Agricultural Weather Network (MAWN) calculates hourly PET at each station.

6. Estimating ET for Different Crops
Combining a “Crop Coefficient Curve” with the reference ET.
Crop Curve is a relationship between the specific plants’ growth characteristics and its water use relationship to the reference crop.

7. Crop Curve

10. Soil Water Holding Capacity
Soil act as a reservoir to hold water for plant use.
The capacity for a soil to hold water is primarily based on the soils texture but can be modified by attributes such as soil organic matter.

11. Available Water
Soil Texture
Range
Average
in./in.
Very coarse-textured sands and
fine sands
0.06
Coarse-textured loamy sands
and loamy fine sands
0.08
Moderately coarse-textured
sandy loams and fine sandy loams
0.13
Medium textured very fine sandy loams,
loam and silt loams
0.16
Moderately fine-textured sandy clay loams,
clay loams, and silty clay loams
0.18
Fine-textured sandy clays, silty clays,
and clay
0.17
Reference: USDA, NRCS, Engineering Field Manual
Available water for each soil group by soil horizon from NRCS Soil Surveys

12. Optimum Moisture Range
for Crop Growth

13. 80%
50%

14. Example
A loamy fine sand with a water holding capacity of 0.08 in/in on a corn crop with a rooting depth of 36” has a storage capacity of 0.08 in/in * 36 in. = 2.88 inches of storage capacity in the profile.

15. Soil Water Balance
Scheduling is very much a “checkbook” type method for accounting on a daily basis the following components
Rainfall / Irrigation in (Rain gage / Pumping Records)
Evapotranspiration out (Ref Et * Crop Coefficient)
Storage in the soil reservoir (H2O capacity * Root Depth)
Water holding capacity
Increasing soil storage capacity with crop rooting depth

16. Rainfall measurement Measure in each field
Should be read each day that a rain event occurs
Record time reading is taken – should be consistent
Keep Clean
Install away from obstructions
Basic gauges must not be allowed to freeze

17. Rain Gauges Basic unit – 2 inch opening Cost less than $10.00

19. Tools Available Scheduling Tools Daily ET estimates
Checkbook registers
Scheduling software
Daily ET estimates
Simple Estimates
Complex Estimates

20. Checkbook Register

21. Minnesota Data Michigan Data Weeks from Emergence 1 2 3 4 5 6 7 8 9 10
Weeks from Emergence 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Max Temp
Eto
0.25
0.38
0.57
0.75
0.94
1.12
1.2
1.1
0.86
0.62
0.41
0.63
0.07
0.02
0.03
0.04
0.05
0.08
0.06
0.12
0.09
0.11
0.13
0.14
0.10
0.15
0.17
0.18
0.16
0.19
0.20
90 +
0.21
0.24
0.23

26. Questions

27. Summary of Websites
Irrigation Scheduling Checkbook Method – University of Minnesota is an Extension
Michiana Irrigation Scheduler
MSU Excel Version of Scheduler
Irrigation Scheduler V 4.0