Irrigation Water Management

Irrigation Scheduling

Suppose we need to schedule a hand move sprinkler irrigation system for the following situation:

• field size = 80 acres
  • Field is 1320 ft x 2640 ft
• Crop = alfalfa
  • RZ = 5 ft
  • peak crop ET crop = 0.26 in/day
• Loamy soil
  • AWHC = 1.7 in/ft
• MAD = 55%
• No salt problem
  • LR = 0
• For hand move sprinklers, we will assume
  • PI Loss = 5%
  • DU = 75%
  • Sprinkler Spacing 30'X40'
  • Sprinkler Flow Rate = 3.5 gal/min 65 psi
• Hours of Operation
  • 11 or 23 hours per day
    This allows one hour per set to move the pipe

How may days before the next irrigation

There are two major concerns:

• How much water can be replaced at the direst location (Dmin) in an 11 or 23 hour set?
• How much water can be stored in root zone each irrigation?

Root zone soil moisture depletion

Root zone Available Water Holding Capacity

Root zone Available Water = Root zone Depth (ft) X Available Water Holding Capacity(in/ft)
Root zone Available Water = 5 (ft) X 1.7 (in/ft) = 8.5 in.

Allowable Root Zone Depletion

Allowable Root Zone Depletion =Rootzone Available Water x Management Allowable Depletion (MAD(
Allowable Root Zone Depletion = 8.5 in. X 55%/100 = 4.68 in.

Water replaced

11 hour set

Gross Water Applied = (96.3 X 11 hrs X 4.5 gal/min)/(30 ft X 40 ft) = 3.97 in.

Net Water Applied= Average infiltrated (Dave)= 3.97 X (1-5%/100) = 3.77 in.

Min Infiltrated = Net Water Applied X DU
Min Infiltrated = 3.77 X .75 = 2.82 in
SMD That can be replaced = Min. Infiltrated = 2.83 in.

# Days = SMD / ET Crop
# Days = 2.83 in. / .28 in/day = 10.1 use 10 days

Note :

• The Min. Infiltrated or the SMD that can be replaced at the driest location is 2.83 in. is smaller than the the Allowable Root zone Depletion of 4.68 in. This means that an 11 hour irrigation with a 10 day interval is acceptable. This would tend to create less crop stress than was anticipated with the selected MAD of 55%.
• Irrigating more frequently (less that a 10 day interval) would over irrigate and cause deep percolation.
• Irrigating less frequently (more than a 10 day interval) would create a larger SMD, which would not be replaced with an 11 hour irrigation.
  

23 hour set

Gross Water Applied = (96.3 X Hours of Operation X Sprinkler Flow Rate )/(Sprinkler Spacing X Lateral Spacing)
Gross Water Applied = (96.3 X 23 hrs X 4.5 gal/min)/(30 ft X 40 ft) = 8.31 in.

Net Water Applied= Gross (1-losses)
Net Water Applied= 8.31 X (1-5%/100) = 7.89 in.

Min Infiltrated = Net Water Applied X DU
Min Infiltrated = 7.89 X .75 = 5.92
SMD That can be replaced = Min. Infiltrated = 5.92 in.

# Days = SMD / ET Crop
# Days = 5.92 in. / .28 in/day = 21.1 use 21 days

Note :

• The Min. Infiltrated or the SMD that can be replaced at the driest location is 5.92 in. is larger than the the Allowable Root zone Depletion of 4.68 in. This means that an 23 hour irrigation on a 21 day interval is not acceptable. This would tend to create more crop stress than was anticipated with the selected MAD of 55%.
• Irrigating more frequently (less that a 21 day interval) would over irrigate and cause deep percolation.
• Irrigating less frequently (more than a 21 day interval) would create an even larger SMD, which would not be replaced with an 23 hour irrigation and create even larger SMD and soil moisture stress.