Distribution Uniformity (DU) is a measure of how evenly
water is applied across a field during irrigation. For
example, if one inch of water is applied in one part of
the field and only half an inch is applied in another part
of the field, that is poor DU. DU is expressed as a percentage
between 0 and 100%, although it is virtually impossible
to attain 100% in practice. DUs of less than 70% are considered
poor, DUs of 70 - 90% are good, and DUs greater than 90%
are excellent. In short, bad DU means that either too much
water is applied, costing unnecessary expense, or too little
water is applied, causing stress to crops.
Irrigation Efficiency is different than DU. Irrigation
Efficiency refers to how well the irrigator matches water
applications to crop water need, and generally answers
the question of how much to apply, and how often. This
is generally referred to as irrigation scheduling. For
instance if a crop needs 24” per year and the irrigator
applies close to that amount (less any distribution inefficiencies)
then the irrigation efficiency would be high. In contrast,
if the irrigator applied 48” per year with a high
DU system, and the crop really only needed 24”, the
irrigation efficiency would be very low, nearly 50%.
Obviously, there must be good DU before there can be good
irrigation efficiency, if the crop is to be sufficiently
watered. So although good DU alone is no guarantee of good
irrigation efficiency, it is a good place to start. Following
are profile views of two adjacent sprinklers in a field
and the root zone underneath them. The spray patterns from
the adjacent sprinklers must overlap to result in the same
amount of water falling in all parts of the field.
In the figures below, the horizontal, dashed line depicts
the depth of the actual soil water deficit at irrigation.
This
is the amount of water that the grower applies to soak
into the soil to satisfy crop water use requirements. The
dark brown area depicts the actual depth of water infiltrated
during the irrigation. Over irrigation is indicated whenever
the actual depth of irrigation is below the soil water
deficit line (the horizontal, dashed line). Conversely,
under-irrigation is indicated whenever the actual depth
of irrigation line is above the soil water deficit.
In Figure 1, the farmer has irrigated to sufficiently
water the entire field. The poor DU, indicated by the uneven
infiltrated water, has resulted in excessive watering in
some areas of the field. That is, more water was applied
than necessary to satisfy the crop.
As a simple example, let’s say the field in Figure
1 requires 400 acre-feet of water and that the cost of
water is $10/acre-foot. Currently, the farmer is applying
500 acre-feet of water to ensure that the center of the
field is adequately watered. If the farmer’s system
had a better DU, he could reduce his water output by 100
acre-feet. His water expense would decrease from $5000
to $4000 giving him a savings of $1000.
It is also likely that a more even application of water
would result in a healthier, more consistent crop with
higher yield and quality. In Figure 2, the farmer has acted
to prevent excessive watering by shortening run times,
without any other changes. But as a result part of the
field now remains under-irrigated. Although under-irrigation
prevents waste, it is undesirable because the crop will
likely suffer from lack of water. In addition, salts may
not be properly leached from the root zone leading to further
crop decline.
Using the same example as above, the farmer has cut back
his run time and is now applying 400 acre-feet to his field,
but because his system lacks a high DU, the center of the
field is now being under watered. Before he cutback his
run-time, the farmer’s field generated $50,000 of
revenue. Now that crop stress exists, the field is only
generating $40,000 worth of revenue. So although the farmer
is saving $1,000 in water expenses, he is losing $10,000
of revenue for a total loss of $9,000.
Figures 3 and 4 show that good DU allows for more even
applications of water, but in the case of Figure 3, irrigation
efficiency is poor because more water was applied than
was needed.
Again, using our example field, the farmer is now using
a well-designed and maintained sprinkler system that provides
good DU and the potential for high irrigation efficiency.
He should be able to apply 400 acre-feet of water evenly
throughout his field, allowing for healthy crops and even
better yields. However, because the farmer is running the
system twice as long as necessary, the potential is not
realized. He is now applying 800 acrefeet of water, doubling
his water expense to $8,000 and putting his crops at risk.
Figure 4 depicts good irrigation. Both the DU is high
(even application) and the depth is right (irrigation efficiency).
The field has been slightly over irrigated but this is
probably necessary to leach salts from the soil profile.
In this case, the farmer has applied the right amount of
water and has not been forced to waste money to over irrigate,
nor has he been forced to sacrifice revenue by under irrigating
and producing a substandard crop.
DU may be determined theoretically by using sprinkler
uniformity software, or it can be determined by taking
actual irrigation application measurements in the field.
In either case, The Center for Irrigation Technology (CIT)
has software and guidelines to help farmers and designers
achieve the best DU possible.
More information can be found on CIT’s website at
http://cati.csufresno.edu/cit .
Remember: better DU means more profits through cost
savings, healthier crops and better yields. Measure your
DU now and
see which category you fall into:
below 70%: bad
70-90%:
good
above 90%: excellent
Much of this information was provided by The Center for
Irrigation Technology and supplemented by Rain Bird Corporation.
Sprinkler
Irrigation Uniformity Tech Tip