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EVALUATING AND INTERPRETING APPLICATION UNIFORMITY OF CENTER PIVOT IRRIGATION SYSTEMS With rising fuel prices it is increasingly important that irrigation systems apply water uniformly in order to achieve maximum benefit from the water applied. When irrigation systems are used to apply fertilizers and pesticides application uniformity becomes even more critical. Consequently, it is important for center pivot owners and operators to periodically check the uniformity of their systems. The uniformity of water application under a center pivot is determined by setting out cans or rain gauges along the length of the pivot and letting the system pass over them. The distance for the center of the pivot and the amount of water collected is recorded for each can or gauge. From this information, a coefficient of uniformity can be calculated. There are a number of methods for determining application uniformity, but the easiest and most widely used is the Christiansen Uniformity coefficient. The mathematical equation for the Christiansen Uniformity coefficient is: U c = 100 (1- A/B) where U c is the Christiansen Coefficient, A is the sum of the absolute value of the deviation of the average value from each individual data point, and B is the sum of the observations. A second method is the Heerman and Hein Uniformity coefficient but space will not allow discussion of this calculation. You may decide that the calculations are too cumbersome. If this is the case, a plot of the data will reveal the places along the pivot where high or low spots occur. Actual plots of the data are shown in Figures 1 and 2 to illustrate the point. In some cases it is obvious that only certain sprinklers need attention (Figure 1) while in other cases (Figure 2) it may be necessary to renozzel the pivot to correct the uniformity problem. Rain gauges, oil cans, or some other containers of uniform size may be used to perform a uniformity test. If rain gauges are used, the amount collected may be recorded directly in inches. If other containers are used the volume collected can be determined by pouring the contents into a small graduated cylinder calibrated in milliliters. This volume can be converted into inches of depth by measuring the inside diameter (in inches) of the top of the container and calculating a conversion factor as follows: C.F. = 12.87 x D x D By dividing the amount of water collected in milliliters by this conversion factor, the depth in inches can be determined. Performing the test
Set out cans or rain gauges in a straight line from the pivot point. Place the first gauge about 150 to 200 feet from the pivot. It is not necessary to place gauges any closer to the pivot than this because the uniformity under the first tower is usually not good, and the area represented is small (less than three acres). Place gauges no more than 50 feet apart. A 10 to 30 foot spacing is preferred. Placing the gauges closer together provides a more accurate representation of the application uniformity. If the end gun will be operating, placing of cans should be extended about 150 feet past the end gun. This will ensure that the gauges are located under the full throw of the end gun. Once the gauges are in place, the pivot should be brought up to normal operating pressure and passed over the gauges completely. The speed of the pivot is not important. However, operation at normal percentage timer setting will tell the operator whether he is applying the amount of water he thinks he is. Remember that on a hot, dry day evaporation losses may be as high as 25%. If time is critical, the uniformity test may be preformed at a percentage timer setting of 100%. This will allow the pivot to pass over the gauges in a shorter period of time. After the pivot has passed over the gauges the enclosed work sheet may be used to record the distance of each gauge from the pivot and the depth or volume of water collected in each gauge. When calculating the uniformity coefficient disregard all gauges at the end of the system in which the volume of water collected begins to drop drastically. Page Break % Timer Setting: Distance to Last Tower: Travel Speed: ft. min/sec System Brand Name: Sprinkler Type: (circle one) Impact Spray Nozzle UNIFORMITY DATA Test Name: Test I.D.: Gauge Diameter: Gauge Spacing: Distance to First Gauge: Gauge No. Volume Gauge No. Volume Gauge No. Volume 1 31 61 2 32 62 3 33 63 4 34 64 5 35 65 6 36 66 7 37 67 8 38 68 9 39 69
10 40 70 11 41 71 12 42 72 13 43 73 14 44 74 15 45 75 16 46 76 17 47 77 18 48 78 19 49 79 20 50 80 21 51 81 22 52 82 23 53 83 24 54 84 25 55 85 26 56 86 27 57 87 28 58 88 29 59 89 30 60 90 % Timer Setting: Distance to Last Tower: Travel Speed: ft. min/sec System Brand Name: Sprinkler Type: (circle one) Impact Spray Nozzle UNIFORMITY DATA Test Name: Test ID: Gauge Diameter: Gauge Spacing: Distance to First Gauge: Gauge No. Volume Gauge No. Volume Gauge No. Volume 1 31 61 2 32 62 3 33 63 4 34 64 5 35 65 6 36 66 7 37 67 8 38 68 9 39 69
10 40 70 11 41 71 12 42 72 13 43 73 14 44 74 15 45 75 16 46 76 17 47 77 18 48 78 19 49 79 20 50 80 21 51 81 22 52 82 23 53 83 24 54 84 25 55 85 26 56 86 27 57 87 28 58 88 29 59 89 30 60 90
Page Break INTERPRETATION OF UNIFORMITY COEFFICIENTS 90 100 Excellent: No changes required. 85 90 Good: No changes required unless a problem is obvious. 80 85 Fair: No improvements needed but system should be monitored closely. Below 80 Poor: Improvements needed, particularly if chemicals are to be injected. POSSIBLE CAUSES OF POOR UNIFORMITY Common Problems Clogged Nozzles Sprinklers Not Turning Inadequate System Pressure Elevation Differences Sprinkler Installed in Wrong Order End Gun Not Adjusted Properly Wrong End Gun Nozzle Correction Measures Remove and clean nozzles. Repair sprinklers. Could also be caused by inadequate pressure. Increase pressure if possible. Pressure regulators may be required. Obtain computer printout from manufacturer and install sprinklers correctly. Adjust part circle stops on end gun. Replace with correct nozzle.
Worn Nozzle Excessive Wind* Replace sprinkler nozzles. Check uniformity while wind velocity is low. *It is generally not recommended to inject chemicals into a center pivot system when wind velocities exceed 10 mph.