Em4885 irrigation management practices to protect ground water

4 CHAPTER
Infiltration rates and application rates are described in terms of inches per hour. That is, application
and infiltration rates refer to a depth of water that is either applied or infiltrated per unit of time.
It is important to realize two important facts:
1. At any point in a sprinkle system the application rate usually will remain constant.
2. The infiltration rate of soil will be constantly changing no matter how little the change.
Thus, although the infiltration rate of a soil may be described as .5 inch/hour, that does not mean
that the infiltration rate will be equal to .5 inch/hour for one hour. It may infiltrate water at the rate
of 1 inch/hour for 10 minutes, at the rate of .75 inch/hour for another 10 minutes, etc. The average
infiltration rate for the whole may be .5 inch/hour. But again, for any one time frame, it may be
more or less.
Thus, even though the application rate of a center pivot system may be 1 inch/hour at the outer
reaches of the span, and the soil’s infiltration rate may be described as .5 inch/hour, there may not be
excessive runoff because the center pivot passes over any one part of the soil in 20 minutes. And in
that 20 minutes the soil’s instantaneous infiltration rate may be at or near 1 inch/hour. Thus, there
will not be excessive runoff.
Disregarding the special case of center pivot machines for a moment, there are three aspects of
overall distribution uniformity with sprinkle systems: 1) pressure uniformity, 2) device uniformity,
and 3) wind effects.
Pressure uniformity
The flow of water through an individual sprinkler or sprayer depends on the pressure at
the device and the size and type of opening. Pressure uniformity refers to how uniform
the pressure is throughout a sprinkle system. It is usually practically and economically
unfeasible to design piping systems for 100% pressure uniformity. There will be some
pressure variance even if pressure regulators are used (commonly in situations involving
large changes in elevation). Competent and experienced irrigation engineers/specialists
should be retained to design all sprinkle irrigation systems.
Device uniformity
If different devices are used at different locations, the flow of water at those locations
will be different, even if the pressure was 100% uniform throughout a piping system.
Commonly, device uniformity refers to having the same sprinkler or spray device with
the same nozzle size at all positions in the irrigation system. Note also that it is important
that the nozzles are not worn.
Device uniformity is not a concern in some center pivot designs as different size devices
will be used at different positions on the span. However, the devices that are used should
be maintained in good operating condition.
50