Winter 2021 - 42

IRRIGATION
In addition to pistachio, distribution uniformity is
important across all crops at all ages, including
young almond, seen here. Photo: Crystal Nay
Distribution uniformity
A uniform distribution allows the grower more control over
the field and the inputs that are applied. Irrigation technology
continues to provide promising offerings both in data collection
and control, but underlying it all is the simple, yet not
necessarily easy to achieve, concept of distribution uniformity.
" There's no point in more technologically advanced
precision irrigation management if growers are not evaluating
and maintaining distribution uniformity, " said Mallika
Nocco, University of California (UC) Cooperative Extension
specialist and director of the
Conservation Irrigation Lab
located at UC Davis.
Calculating distribution
uniformity involves some
in-field work, and growers
can do it themselves or
contract it out. The flow of
the emitters is measured,
and distribution uniformity
is calculated by the average flow of the lowest 25% emitters
divided by the average flow of all the emitters.
For example, a grower has 16 emitters, and the lowest
four of them have an average flow of 0.87 gallons per
hour (gph). The average flow of all the emitters combined
measures 0.97gph. By using the DU equation, this grower
has a 90% distribution uniformity. If the average flow of
42 WINTER 2021
Distribution uniformity (DU)
=
average flow of lowest 25% emitters
average flow of all emitters
those same bottom 25% dropped to 0.67gph, with the same
0.97gph field average, this would result in closer to a 70%
distribution uniformity - cause for concern since there
would be trees in the field being vastly underwatered,
others probably overwatered, and could be the result of
unequal drainage, spacing, pressure or clogs and breakage.
" Similarly, fertigation protocols would be compromised
with some trees receiving too much nitrogen and others not
enough - poor distribution uniformity means that both the
water and everything in it is not being applied in the way you
intend it to be, " said Nocco.
This concept helps to
ensure growers have an even
application season across
the orchard. And while
maintaining distribution
uniformity is a necessary step,
it does not guarantee high
irrigation efficiency.
" High irrigation efficiency
comes from a combination of high distribution uniformity
and consistently meeting crop water demand in the root
zone, " said Nocco.
Where there is poor distribution uniformity and poor
irrigation efficiency, a lot of water is being applied, but not
much of it is being used properly by the trees. There may be a
blockage or breakage somewhere in the system, but in any case,
Winter 2021

Table of Contents for the Digital Edition of Winter 2021
