SKIP irrigation pattern can provide more uniformity
By Jeremiah Vardiman
Did you know the movement pattern of set-move irrigation systems, such as wheel-line and hand-line, affects the uniformity of the irrigation and thus the uniformity of a crop?
The SKIP irrigation pattern, which is described below, provides higher yields and better crop quality. Knowing some basic information, such as movement patterns, system offsets, soil type and basics in soil water, can greatly improve the management of a field and provide maximum production.
Striving for uniformity in irrigation water is a huge piece of the puzzle to creating uniform crops, which ensures maximum production from all areas of the field. However, the misunderstanding about uniform irrigation with wheel-line and hand-line systems is sprinklers do not apply water in a perfectly uniform manner. This can be managed by the movement pattern of the system and utilizing offsets.
TAXI, WIPE and SKIP are typical movement patterns used with wheel-lines and hand-lines. The TAXI pattern irrigates every riser down the field – risers one through 12, sequentially – then the system is walked empty back to the beginning of the field before the cycle is started over, as shown in figure one. This is a viable pattern, however, it is unfavorable because of the time needed to walk the system back across the field to start over, especially a hand-line that is typically loaded and unloaded on a trailer.
The WIPE pattern irrigates every riser down the field – risers one through 12, sequentially – then after a 12- to 24-hour delay, the irrigation begins the opposite direction in risers 12 to one, sequentially. This system addresses the labor issue of the TAXI pattern, but results in too much water at the end of the field when the system is turned around and water stressed plants on the opposite end because of the long interval time between irrigations. This option is not recommended because of over watering and yield loss due to water stress.
Unlike the other patterns, the SKIP pattern balances labor needs with uniform irrigation application. The SKIP pattern irrigates every other riser on the way down the field, typically odd risers, then irrigates the missed, or the even risers on the way back. This pattern results in more frequent irrigations with smaller amounts of water in the overlap areas and ends, resulting in uniform coverage in a timely manner and less likelihood of over watering an area or water stressing the crop. The SKIP pattern provides higher yields and better crop quality when compared to TAXI and WIPE. Numbering or color coding the risers can make this pattern easier to implement in a field.
The movement pattern evenly distributes the water across an entire field, but it does not entirely account for the imperfect application of water from the sprinklers. To improve application uniformity, the irrigation can be offset, which is when the wheel-line or hand-line is offset from the riser. The offset is typically 20 to 30 feet to the right or left of the riser. This offset is maintained for the duration of the irrigation cycle, which is one entire irrigation pass on the field. The next irrigation cycle should be set at the riser or offset to the opposite side of the riser.
An offset’s shifted position averages application depths to an entire field, increasing application uniformity. This management technique is particularly important for systems running on the lower side of operation pressure, typically 40 pounds per square inch (psi) or constant day/night diurnal wind patterns. Extreme diurnal wind patterns can also be compensated for by changing the start time by 12 hours. For example, instead of starting riser number one at 6 a.m., delay start time until 6 p.m.
The next factor is determining irrigation set times, which is the duration of the irrigation per location. Typically, 12- or 24-hour set times are desired to minimize the labor to move sets. An eight-hour set time is another option, but is not as common because of the additional set in a day. Set times are often determined on an irrigator’s time availability or personal choice.
Determining irrigation sets
Matching the irrigation set to the type of soil and its water-holding capacity or available water is the ideal way to select an irrigation set. Different soil types have different water-holding capacities, as shown in table one. A basic summary is that sand does not hold as much water as clay or silt.
Water-holding capacity is the difference between field capacity and permanent wilting point, shown in figure three. One must know the basics of soil water to understand this range. Water, either from precipitation or irrigation, is held within the pore spaces between soil particles. These pore spaces fill with water until they reach saturation, and the excess water will drain out over time.
After the excess water is drained, the soil holds a certain amount of water against gravity’s downward pull. This amount is called field capacity. The opposite end of field capacity is permanent wilting point – when the soil has dried to the point the suction of the soil on the water is greater than a plant’s ability to absorb it – which is also the point where plants will not recover no matter the amount of water applied.
The available water or water-holding capacity needs to be maintained at a certain percentage to prevent significant yield loss. For example, pasture grass needs 50 percent or more of the available water to prevent yield loss.
The rooting depth of the crop is another factor to available water. Deeper root crops have access to more water than shallow-rooted crops.
Understanding plants require the same amount of water and nutrients no matter the soil type. The same water is required on lighter soils as with heavier soils. The difference is lighter soils need irrigated more often and in smaller amounts.
To estimate water-holding capacity, identify the soil type in table one, then multiply by the rooting depth of the crop. For instance, pasture grass with a rooting depth of three feet, grown on silty loam, would be 1.8 inches per foot by three feet, or 5.4 inches of available water. The crop needs 50 percent or more of this water maintained. Thus, 2.7 inches of water can be depleted before irrigation is needed.
The last step is measuring how much water is applied during a set. Take a straight sidewall can or bucket, such as a coffee can, and place under the sprinkler path for the duration of the set. Calculate the amount captured in inches and compare this to the depletion number. If a 12-hour set applies 1.5 inches, this irrigation will not completely fill the soil capacity. In this example, a 24-hour set may be better.
Managing the move pattern, irrigation offsets, and selecting the correct irrigation set for your soil will maximize production of a field, in an irrigation perspective. There is no time better than now to adjust a wheel-line or hand-line system.
Jeremiah Vardiman is a University of Wyoming Agriculture and Horticulture Extension Educator. He can be reached at firstname.lastname@example.org.