Em4885 irrigation management practices to protect ground water

4 CHAPTER
Note that the opportunity times at the top of a furrow and the bottom of a furrow will
never be the same since it takes time for the water to travel down the furrow. However,
infiltration rates decrease with time during an irrigation. Thus, if the opportunity times are
equalized to some degree, the difference in total infiltration between the top and the
bottom of a furrow will be acceptably small. And, as the recommended advance ratios
above indicate, the difference in opportunity times from top to bottom of a furrow can be
larger with a heavier soil than they are in a coarse soil for acceptable uniformity.
Common practices to improve down-row uniformity include increasing furrow flow rates,
cutting the length of furrows, and using surge-flow techniques. All of these are aimed at
reducing the difference in opportunity times between the top of the furrow and the bottom.
Achieving good down-row uniformity involves the potential for significant amounts of
surface runoff. Assuming that the grower does not want to lose this runoff, it can be
controlled by using cutback flows (IP 2.02.10), a runoff reuse system (2.02.11), surge-flow
techniques (IP 2.02.03), or some combination.
Cross-row uniformity
Cross-row uniformity refers to the difference in infiltration from furrow to furrow.
Differences in overall infiltration can occur due to differences in infiltration rates or
differences in opportunity time.
Usually, tractor tires do not run in every row of a furrow irrigated field. Thus, some
furrows are compacted by the tractor tires and some are not and there will be different
infiltration rates in those furrows. Additionally, as a tractor makes a turn in a field, a
“guess row” is created that may have a different infiltration rate from either the compacted
or the uncompacted rows. A schematic diagram showing the effects of poor cross-row
uniformity due to compaction from tire tractors is seen in Figure 4-11.
Head of Field
Bottom of Field
FIGURE 4-11. Schematic diagram showing effects of poor cross-row uniformity due
to compaction from tractor tires
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Area of generally lower
water infiltration
direction of
water flow