Irrigation Water Management

Irrigation Scheduling

Let's take a moment to review the soil, plant water factors we have considered up till now and that have brought us to this point:

• Soil Considerations
  • Available Water Holding Capacity (AWHC) (in/ft) or (%) = Field Capacity (FC) - Permanent Wilting Point (PWP)
    
• Crop Rooting Depth (RZ)
  • AWHC (in/ft) x RZ (ft) = AWHC (in)
    
• Crop, System and Management Considerations
  • Choose MAD (%)
  • AWHC (in) x MAD (%) = max allowed SMD
    AWHC (in) x MAD (%) = SMD just prior to irrigation
    
• To achieve perfect timing (also known as "good management")
  • Dmin = SMD ( + LR if there is a salinity problem)
    
• Distribution Uniformity (DU)
  • Dave = Dmin ÷ (DU/100)
  • Dave = infiltrated amount
    
• The gross amount the pump must deliver is Dave, the average amount infiltrated plus any pre-infiltration losses
  • gross applied amount = Dave ÷ (1 - PI Loss/100)

(1) How much water to apply?       and

(2) When to irrigate?

[there are several versions of this question, such as . . .]

How many days between irrigations?

How many days until the next irrigation will be needed?

Note:
"How much water to apply?" is answered by the foregoing review items, ending with the expression for the gross pumped amount.

"When to irrigate?" is a TIME question. Our basic expression for TIME questions is this:

Time = Amount/Rate          so, for irrigation time,

Irrigation Frequency = [max allowed SMD (in)]/[ET crop (in/day)]

So the only piece missing is ET crop. How is this determined?