Preseason Sprinkler Management

Making the best use of a limited water supply

Published in the July 2013 Issue Published online: Jul 07, 2013 Dr. Howard Neibling
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Nelson Irrigation

Regular and preseason maintenance to repair leaks, replace worn nozzles or replace older center pivot application packages can help you make the best use of a limited water supply, minimize pumping costs, reduce disease potential and reduce nitrate leaching.

Based on a joint University of Idaho/Idaho Power study last summer, leaks and worn nozzles on wheel lines can increase water application per line by up to 48 percent, with an average of about 25 percent. Of the center pivots tested, 60 percent needed water application package repair or replacement to increase water application uniformity to an acceptable level. Irrigation system maintenance is particularly important this year because of short water supplies.

Idaho Power and other electric power providers in southern Idaho currently have cost-share programs to help pay for a number of qualifying improvements.

Leaks and worn nozzles on set systems like hand or wheel lines require excess water to be pumped per acre to meet crop needs. In addition to the extra water, power costs are increased and system water application uniformity is reduced because of lower system pressure, resulting from a higher pumping rate.

Poor water application uniformity on both set systems and pivots results in significant areas of the field receiving at least 25 percent more or less water than intended. The season-long, cumulative effect of this under- or over-irrigation can reduce potato yields on the affected areas. Chronically wet areas caused by leaks or excess water application enhance deep leaching of nitrogen, which can reduce early- and mid-season growth.

A study funded by Idaho Power was conducted during the summer of 2012 to determine water and energy savings that can be achieved by replacing or repairing pressurized irrigation equipment. Systems evaluated were located in the King Hill to Burley area of southern Idaho. Field measurements of water losses related to leaks in various system components were performed. Energy savings due to each item were calculated based on the head and flow rate of each system tested. Types of systems evaluated were hand line, wheel line and center pivot.

Leaks and Worn Nozzles

Excess water and power usage due to leaks varied considerably based on individual grower attention to maintenance, with average excess applied water of 16 percent on standard wheel line, 12 percent on Thunderbird wheel line and 36 percent on hand lines.

This translated to an additional 203, 144 and 381 kWh/ac energy usage for standard wheel line, Thunderbird wheel lines and hand lines, respectively, when individual system characteristics were considered. This assumed a 2,000-hour irrigation season, lift from a surface-water-supply holding pond and pressurization to 60 psi at the pump.

Based on study results, set system nozzles must be worn to an average excess flow level of at least 10-15 percent before growers see the need for system repair. Because some systems tested were nearly new, the actual threshold for needed repair may be even higher. Incentive programs to encourage replacement or repair of worn or damaged parts are currently funded and can encourage repair and therefore save water and energy.

Measured levels of excess flow from worn nozzles were 14 percent, 11 percent and 16 percent, with excess energy use required of 148, 156 and 148 kWh/ac for standard wheel lines, Thunderbird wheel lines and hand lines, respectively. This is a lower rate than observed in a 1978 Idaho study, where average excess flow from 9 lines tested was 119 percent of design.

Refurbished set-move systems had minimal leaks and excellent water application uniformity. In this study and in a 2007 Utah State study system, performance was related more to maintenance than system age.

Center Pivot Uniformity

Catch funnels were set at 10-foot intervals along the entire length of the lateral. Cans were set in a line sufficiently ahead of the lateral to allow for the complete set of funnels to be placed before the lateral reached the line. After the lateral passed over the funnels, water collected in each was read and recorded. Catch-can data were used to calculate a uniformity index (area-weighted Coefficient of Uniformity or CU). The higher the index, the more uniform the water distribution.

Average area-weighted CU was 83 percent for both high- and low-pressure pivots. CU ranged from 78 to 93 on the high-pressure machines, and from 69 to 94 on low-pressure machines. To put these numbers in perspective, a study on irrigation uniformity by Dr. Brad King, USDA-ARS Kimberly, Idaho, stated that "new pressure-regulated, low-pressure center pivots and linears were capable of achieving CU's of 90-95 percent," and that "a CU value of 85 percent is generally considered to be the minimum value below which a system needs updating or maintenance."

Using this threshold, 75 percent of the high-pressure machines and 60 percent of the low-pressure machines tested needed updating or maintenance.

Catch-can data are shown for several center pivots in Figures 1-3. In Figure 1, one pivot has considerable variability along the entire lateral, giving an unacceptable CU of 74, while the other pivot has much less variability along the majority of the pivot but several high deviations near the outer end. Because the CU calculation considers catch data more heavily as distance from the pivot increases, these few high points bring the CU down to 84 (just below the cutoff for repair or replacement). Except for these few high points, this pivot uniformity would be acceptable.

In Figure 2, one pivot has good uniformity along the entire lateral (CU=92) while the other pivot has visually just as good a uniformity along the majority of the lateral but has significant deviation from the target application on the outer span. This pattern indicates incorrect nozzle size on the outer span. The areas of excess application near the outer end of the lateral in Figure 2 and under the first span of the lateral in Figure 3 represent locations where disease is most likely to begin and where nitrate leaching will probably occur.

A comparison of measured nozzle discharge along a line with no maintenance for 15 years and another lateral of about the same age with two-month old R2000 nozzles is shown in Figure 4.

Several very-high nozzle discharges in the old line were due to incorrect nozzle size (e.g. 5/32 inch, not 9/64). The relatively consistent increase in nozzle discharge for most nozzles shows the long-term effect of nozzle wear. The combination of worn nozzles and wrong-sized nozzles produced an excess flow of 47 percent.

One of the additional energy and excess water costs of low system pressure resulting from leaks, worn nozzles or other system problems is that most growers irrigate to give good crop appearance on the majority of each field. If uniformity is poor, most growers tend to over-irrigate some areas to assure adequate water on the majority of the field. This is true in both center pivot and set systems. The result is more water use and energy consumption.

In this sense, improving application uniformity generally results in a significant reduction in energy consumption. Because improving system uniformity reduces the degree of over- and under-watering, crop yield will be higher on a more uniform system because the degree of under-watering is less.

Figure 1 Figure 2 Figure 3 Figure 4a Figure 4b

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