Irrigation Water Management

Irrigation Efficiency & Distribution Uniformity

2-Water Destination Diagrams





Index

DU - Distribution Uniformity
Pre-Infiltration Losses
Water Destination Diagram








The Distribution Uniformity (DU)
describes how evenly water is made available to plants throughout
a field. Distribution Uniformity is defined as:




  

    

        

          
            
 
        

      

  





Simplified, DU is a ratio of the
"minimum" to the "average" amounts of water
received by plants in a field or system. A DU of 1.0 would mean
that the "minimum" received equals the "average"
received and infers that plants throughout the entire field receive
the same amount of water. In fact, no system is capable of applying
water so that every plant in a field receives the same amount
of water. The absolute minimum recieved is not used to compute
the Distribution Uniformity. In this case "minimum"
is defined as the average of the lowest quarter (or smallest 25%)
amount recieved by the plants in the system being evauated. There
are many factors that contribute to the non-uniform application
of irrigation water. The major components of system or global
Distribution Uniformity are listed below.



Components
of Irrigation System DU for Sprinklers:




  

    
Undertree
    Sprinklers
  

  
  
Hand
  Move Sprinklers

  
  
Components
  of System DU for Drip Irrigation:

  
  
Components
  of System DU for Surface Irrigation

  
  


  The Irrigation Training & Research Center (ITRC) and the
  National Resources Conservation Service (NRCS) have developed
  methods for systems analysis. The ITRC at Cal Poly offers a 2
  1/2 day course emphasizing on definitions and techniques of evaluation.
  This course ranges from how to take a pressure measurement to
  what specific measurements are needed for evaluation of six distinct
  irrigation methods (furrow, border strip, hand move/side roll
  sprinkler, linear move sprinkler, undertree sprinkler, and drip/micro).
  For more information on this course you can contact the ITRC
  at (805) 756-2434 or sgartner@calpoly.edu



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Water Destination Diagram



What does it mean when a system
is described as having a DU of 0.5? A water destination graph
can be used to illustrate graphically the impact of having DU
of 0.5.



Using the following information
we will construct a "Water Destination Diagram."



DU = 0.5 
Applied water = 9 in., 
Pre-infiltration losses =12%, 
Runoff =15% of the applied

Solution:
Pre-Infiltration losses = 12/100 X 9in = 1.08 in
Runoff losses = 15/100 X 9in = 1.35 in
Average Water Infiltrated (Dave)= Total Applied - PI Losses - Runoff = 9in - 1.08 in - 1.35 in = 6.5in
Dmin= Dave X DU = 6.5in X 0.5 = 3.3 in




  

    

        

          
            
 
        

      

  





"25% of the area receives at
least 8.73 inches of water."



"50% of the area receives at
least 6.5 inches of water."



"100% of the area receives
at least 2.25 inches of water."

 



It is a reasonable, and very helpful,
simplification to assume that the diagonal line from the maximum
to the minimum reading is a straight line:




  

    

        

          
            
 
        

      

  





The places where the straight diagonal
line crosses the 50% and 87.5% of area positions (across the horizontal
axis) are called the maximum, average and minimum depths infiltrated,
or Dave and Dmin in symbols. In the above example,

Dave
= 8



Dmin = 4



The average depth infiltrated is
defined at the 50% mark. Whereas the minimum depth infiltrated
is defined at the 87.5% mark. Any readings taken at the 100% mark
are defined as absolute minimum depth infiltrated and the absolute
maximum depth infiltrated is defined at the 0% mark.



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DU - Distribution Uniformity



DU, Distribution Uniformity, is
defined as:




  

    

        

          
            
 
        

      

  





For perfect uniformity, that is,
where all positions receive the same amount of water, Dmin and
Dave are equal. The destination diagram is a flat, horizontal
line crossing the graph at the depth all placed received.




  Perfect Uniformity, DU = 1.00

Dave = Dmin
   
  

    

        

          
            
 
        

      

  





A more realistic example is shown
below:




  

    

        

          
            
 
        

      

  






  

    
      

        

          
            
 
        

      

  





An example of better uniformity
is this:




  

    

        

          
            
 
        

      

  






  

    
      

        

          
            
 
        

      

  





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Pre-Infiltration Losses



The water destination diagrams shown
above display the fate of the water that has infiltrated into
the ground. But we know that not all the water pumped may make
it into the ground - there is runoff, evaporation and spray loss
too.



It is customary to show the amount
of these pre-infiltration losses above the Depth = 0 line, as
shown here:




  

    

        

          
            
 
        

      

  





The Pre-Infiltration Loss (PI Loss)
is usually indicated as a per cent of the gross amount applied,
or in inches if known directly. It is important to note that the
PI Loss is NOT a % of the NET amount (the beneficial amount).
It is always calculated as a percentage of the gross amount pumped
(the total applied). Runoff is included in pre-infiltration losses
but is handled separately because it may be recovered for re-use.



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