WHO WANTS TO BE AN IN-CANOPY IRRIGATOR?

Freddie R. Lamm, Research Irrigation Engineer¹
flamm@oznet.ksu.edu
Northwest Research Extension Center, Colby, Kansas
K-State Research and Extension
Phone 785-462-6281 Fax 785-462-2315

ANSWER SHEET FOR
WHO WANTS TO BE AN IN-CANOPY IRRIGATOR?

QUESTIONS FOR
WHO WANTS TO BE AN IN-CANOPY IRRIGATOR?

CORRECT ANSWERS AND RATIONALE FOR
WHO WANTS TO BE AN IN-CANOPY IRRIGATOR?

The correct answer is C. Apply water at a rate less than the soil intake rate. Traditionally, sprinkler irrigation systems have been designed to uniformly apply water to the soil at a rate less than the soil intake rate to prevent runoff from occurring (Heermann and Kohl, 1983). These design guidelines need to be either followed or intentionally circumvented with appropriate design criteria when designing and managing a center pivot irrigation system using LEPA and other in-canopy sprinklers. Answer A (Always place sprinkler nozzles at a height below the corn ear height) is incorrect because it ignores the fact that impact sprinklers might be the most appropriate selection in many cases. Answer B (When irrigating use a timer setting not more than 7%) is incorrect because timer settings are changed as needed to adjust irrigation amount. Answer D (Use lower capacity sprinklers on sandier soils) is incorrect because lower soil water holding capacities for sands generally increase the need for higher capacity sprinklers that can more closely match peak water needs.

The correct answer is A. Higher uniformity due to application closer to soil surface is not an advantage of in-canopy sprinkler application. Distortion of the spray pattern by the crop canopy will lower the uniformity of broadcast applications (Lamm, 1997). In the case of LEPA applications in the bubble mode to alternate furrows, uniformity would still be lower than broadcast applications over a short distance, but are not considered to be a detriment. Answers B, C and D (Lower wind evaporation losses, less canopy interception losses and Insecticides may be applied to crop canopy) are all considered to be advantages. For further discussion of this topic, check out references Lamm (1997) or Yonts et al. (1997) or point your Internet browser to http://www.ianr.unl.edu/pubs/irrigation/g1337.htm

The best answer is D. Raise the outside span of nozzles above the canopy because a larger wetted radius occurs when sprinklers are raised above the corn canopy. A larger wetted radius reduces peak application rates that may be exceeding the soil infiltration rate. Both A and B (Speed the system up by reducing cycle time or slow the system down by increasing cycle time) are sometimes attempted by irrigators but are incorrect. Increasing or decreasing the center pivot sprinkler speed DOES NOT affect the peak application rate (Figure 1). It only changes the duration of the application period. Increasing the system speed may reduce the total amount of runoff, but will not eliminate runoff. Increasing the system speed will increase the ratio of evaporative losses to applied irrigation, thus lowering application efficiency. Long slow center pivot sprinkler cycles have been promoted by some consultants as a way to promote soil cracking which could be a positive influence on infiltration. However, this increases the duration and resulting total amount of runoff (Figure 1) Answer C (Accept some runoff while reducing evaporation losses) is accepting defeat. Runoff losses can easily be larger than the evaporation savings the irrigator is trying to gain. For further discussion of this topic, check out the reference Kranz et al. (1991) or point your Internet browser to http://www.ianr.unl.edu/pubs/irrigation/g1043.htm

The most effective scheme is B (Plant corn in circular rows and space nozzles at twice the row spacing) because it allows water to be applied relatively uniformly to the cropped area and reduces the potential for runoff. Straight planted corn rows will distort the pattern severely when corn rows are parallel to center pivot sprinkler travel. Nearly all the center pivot sprinkler capacity will be applied to a few irrigation furrows when the corn rows are perpendicular to sprinkler travel direction. (Lamm, 1998) Nozzles at twice the row spacing still allow plants equal opportunity for the irrigation water. Answer A (Space nozzles 10 ft apart at a height of 4 ft.) is incorrect because K-State research (Lamm, 1997) has shown 4 ft. to be the worst height because of the large ear and leaf mass at this location. Answer C (Replace nozzles annually) is generally not required every year. Nozzle wear is most often associated with heavy pumping of sand. Answer D (Use short duration cycles later in the crop season) is incorrect because earlier in the season is generally the time period shorter duration cycles are utilized to firm wheel tracks.

Answer A (Result in 50% water savings when compared to above-canopy irrigation) is incorrect. Savings in the 5-15% are more realistically possible. Answer B (Help improve soil structure due to less crusting) is incorrect because the typically higher peak application rates associated with in-canopy irrigation would tend to degrade the soil surface, leading to more soil crusting. Answer C (Help reduce nitrogen costs) is incorrect. Nitrogen fertigation has been successfully used with LEPA fertigation in some K-State research, but there isn't evidence at the present time indicating it would be more efficient than traditional sprinkler irrigation methods.

The correct answer is A. Pressure at the end tower must be less than 20 psi. It is not a guiding principle, because the pressure should be less than 10 psi. Answers B, C, and D (Be capable of conveying and discharging water into a single crop furrow, each plant has equal opportunity for irrigation water, result in zero runoff from irrigation water application point) are all guiding principles (Lyle, 1992).

The correct answer is D. All of the above which included crop row orientation with respect to center pivot sprinkler travel, time of the irrigation season and height of the nozzles. Circular crop rows which result in sprinkler nozzles traveling parallel to corn rows will result in less distortion of the spray pattern. Spray patterns for widely spaced spray nozzles at a two ft. height are distorted for approximately 60 days, while spray nozzles just below the truss rod are distorted for only about 30 days. This difference in time periods for the different heights can ultimately reduce corn yields for rows furthest from the spray nozzle (Lamm, 1998). This is also noted in Figure 3 where the yield reduction effect for distant rows was greater for the lower heights.

The correct answer is D. Both B and C: Is the predominate flow path for a fully developed corn canopy and Increases with decreased plant spacing. Although most people would think that water falling through the plant leaves would be the primary way water reaches the soil, the fully developed corn plant is actually a very good funnel and channels the water along the stem. Higher plant population results in more "funnels." At a typical irrigated corn plant spacing of 7.9 inches, stemflow accounts for 53% of the water reaching the soil after tasseling when being irrigated near the top or above the corn canopy (Lamm, 1990). This flow mechanism conceivably could affect application of crop amendments when using the center pivot sprinkler.

The correct answer is C. When properly designed and managed. Answer A (For all corn production systems due to high corn water use) is incorrect because sometimes runoff on tight soils and/or sloping ground can easily exceed any possible evaporation savings. Answer B (On all sandy soils but less frequently on silt loam soils) is incorrect because system flowrates, which are higher on sandy soils to meet peak water needs, may exacerbate runoff problems.