Irrigation Water Management

Irrigation Efficiency & Distribution Uniformity

5.5" of water is pumped through an irrigation system for application. Pre-Infiltration Losses are 0.5", Runoff is 0.0". The system DU is measured to be 0.70. Draw the water destination diagram for this irrigation.

Solution:

Average Water infiltrated = Total Applied - PI Losses = 5.5" - 0.5" = 5".
Dave = 5".
From the formula for DU, DU = (Dmin/Dave), so
Dmin = Dave x DU = 5" x .70 = 3.5"

So we can draw the water destination diagram:

To determine AE, we need to compare this irrigation to its target. Suppose that on the day of irrigation, SMD = 3" and LR = 0.5". Thus the target = SMD+LR = 3" + 0.5" = 3.5". Let's draw these figures on the diagram:

What can we say about this irrigation, relative to its target?

Timing?
Since target = SMD+LR = 3.5" = Dmin, we have perfect timing.

Beneficial Use?
Total Beneficial Use = SMD (stored for crop ET crop) + LR (for salt control)
Total Beneficial Use = 3" + 0.5" = Less Than 3.5".
NOTE: There is a slight under irrigation at the absolute driest location in the system.
it amount to about 0.03" as an average over the entire field.

Total Applied?
Total water applied = pumped = 5.5". Also,
Total water applied = PI Loss + total infiltrated = 0.5" + 5" = 5.5".

Application Efficiency?
AE = 100 x (Ben IW/Total IW)
AE = 100 x (3.5"/5.5")
AE = 100 x 0.64 = 64%

Total Deep Percolation?
     = Beneficial Deep Percolation + Non-Beneficial Deep Percolation
     = 0.5" + 1.5 " = 2"

The irrigation system applies 5" total to a dry soil for which SMD = 7. Salinity is not a problem here (winter rains are sufficient for leaching) so LR = 0". Pre-Infiltration Losses are 10%. The system DU is measured to be 0.75. Draw the water destination diagram and evaluate this irrigation.

Solution:

PI Losses = 10% x Total (Gross) application = 10% x 5" = 0.5"

Note:
When Pre-infiltrationI Losses are expressed as % they are always referring to a % of the Total (Gross) Application.

Water infiltrated = Total Pumped - PI Losses = 5" - 0.5" = 4.5".
Since water infiltrated always = Dave, Dave = 4.5".
From the formula for DU, DU = (Dmin/Dave), so
Dmin = Dave x DU = 4.5" x .75 = 3.38"

So we can draw the water destination diagram:

To determine AE, we need to compare this irrigation to its target. Since on the day of irrigation, SMD = 7" and LR = 0",
target = SMD+LR = 7" + 0" = 7".

What can we say about this irrigation, relative to its target?

Timing?
Since target = SMD+LR = 7", but Dmin = 3.38", we have under-irrigation.

Beneficial Use?
All the water that infiltrates is stored for crop ET crop.
Total Beneficial Use = 3" + 0.5" = 4.5".

Total Applied?
Total water applied = pumped = 5". Also,
Total water applied = PI Loss + total infiltrated = 0.5" + 4.5" = 5".

Application Efficiency?
AE = 100 x (Ben IW/Total IW)
AE = 100 x (4.5"/5")
AE = 100 x 0.90 = 90%

Note:
The efficiency is greater than the DU (AE > DU) because the field is under-irrigated. High efficiencies do not necessarily mean high yields. Under-irrigation can result in both high efficiencies and low yields.

Total Deep Percolation?
Total Deep Percolation = 0"

The irrigation system applies 4.5" total to a soil for which SMD = 2". Salinity is achieved by leaching, and for this irrigation, LR = 0.5". Pre-Infiltration Losses are 0.5". The system DU is measured to be 0.75. Draw the water destination diagram and evaluate this irrigation.

Solution:

Water infiltrated = Total Applied - PI Losses = 4.5" - 0.5" = 4".
Dave = 4".
From the formula for DU, DU = (Dmin/Dave), so
Dmin = Dave x DU 1/100 = 4" x = 3."

So we can draw the water destination diagram:

To determine AE, we need to compare this irrigation to its target. Since on the day of irrigation, SMD = 2" and LR = 0.5",
target = SMD+LR = 2" + 0.5" = 2.5".

What can we say about this irrigation, relative to its target?

Timing?
Since target = SMD+LR = 2.5", but Dmin = 3", we have over-irrigation.

Beneficial Use?
Total Beneficial Use = SMD (stored for crop ET crop) + LR (for salt control)
Total Beneficial Use = 2" + 0.5" = 2.5".

Total Applied?
Total water applied = pumped = 4.5". Also,
Total water applied = PI Loss + total infiltrated = 0.5" + 4" = 4.5".

Application Efficiency?
AE = 100 x (Ben IW/Total IW)
AE = 100 x (2.5"/4.5")
AE = 100 x 0.56 = 56%

Note:
Even with good DU (DU = 0.75) AE can be low if you over-irrigate.

Total Deep Percolation?
Total Deep Percolation =
     = Beneficial Deep Percolation + Non-Beneficial Deep Percolation
     = LR + Deep Perc due to Poor Timing + Deep Perc due to DU
     = .5" + .5" + 1.0"