Em4885 irrigation management practices to protect ground water

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CHAPTER 4
Wind effects
Wind can have the largest effect on distribution uniformity with sprinkle irrigation
systems. Water is sprayed through the air with these systems and excessive wind will
move the water droplets while they are in the air. Thus, the water droplets will not fall in
patterns as planned. Spray patterns from adjacent sprinklers are intended to overlap to
some degree. The measure of how well the patterns overlap as designed is referred to as
“overlap uniformity” or “catch-can uniformity” (since the uniformity is measured with a
grid of cans to see how much water falls in different locations between sprinklers).
Center pivots as exceptions
Center pivot machines may be exceptions to both device and pressure uniformity as
described above. This is due to the decrease in opportunity time as distance from the center
pivot increases. As the opportunity time decreases, the application rate must increase
so as to maintain the same total application at all points in the field. For example, at a
point 100 feet away from the pivot, the opportunity time may be 1 hour. That is, water
from sprinklers strikes the soil at this point for one hour for every revolution. On the other
hand, the soil at a point 800 feet away will only receive water for about 7.5 minutes.
Thus, if the total application was to be .25 inch, the application rate at the near point
would have to average .25 inch/hour and the application rate at the far point would have
to average 2 inches/hour.
The difference in application rates may be achieved by:
1. Varying spacing of sprinklers while the nozzle size is held constant.
2. Holding sprinkler spacing constant while the sprinkler head and nozzle size are varied.
3. Some combination of the above, along with pressure regulation.
Thus, device pressure uniformity may or may not be a factor depending on the design.
However, all devices should be in good operating condition and the whole system
maintained so as to deliver design performance.
IP 2.03.01 - Have an Irrigation Engineer/Specialist Check Hand-Line and
Side-Roll Sprinkle Field Layouts to Ensure Correct Combinations of Spacing,
Operating Pressure, Sprinkler Head, and Nozzle Size/Type
Objective
Ensure proper overlap of adjacent sprinkler patterns by utilizing the correct combination
of spacing, sprinkler head, nozzle size, and operating pressure.