ENH142 Annual Maintenance and Evaluation of Overhead Irrigation Systems 1 Gary W. Knox2 1. This document is ENH 142, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date revised: March 2000. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. Gary W. Knox, Associate Professor, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean. Basic maintenance practices far irrigation systems include flushing water lines and checking valves and sprinklers. Examine valves to make sure they work properly, and lubricate them only if necessary. All irrigation lines should be flushed to remove any sediment which may have built up and which could clog sprinklers. Flush irrigation lines systematically, starting with the main lines, then moving to sub-mains and manifolds, and finally flushing the laterals. Check sprinklers for damaged oscillating arms or worn nozzles. A damaged oscillating arm could alter the angle at which the jet of water hits the arm, resulting in different turning characteristics of the sprinkler. Worn nozzles reduce sprinkler efficiency and increase sprinkler discharge. Nozzles can be checked for wear by using a drill bit the same size as the nozzle. The drill bit is inserted into the nozzle, and if the bit fits snugly, there is no wear; a loose-fitting bit indicates nozzle wear, and the nozzle should be replaced. After the basic components of your irrigation system have been checked, you should evaluate your system for proper water pressure, application rate, and application depth. The required equipment includes pressure gauge, soil auger or shovel, 12 containers of the same size (jars, for example), ruler, 5-gallon bucket, piece of hose, watch that displays seconds, and paper and pencil to record data. Water pressure should be measured in main lines, laterals, and at the sprinklers. A pressure gauge with a pitot tube attachment will allow you to measure water pressure at the sprinkler nozzle. Otherwise, connect the pressure gauge before the water is on. With the irrigation system in operation, measure water pressure in main lines at the pump, at the high point of the line, and at the point farthest from the pump. Measure water pressure in the laterals at the high point of the line, the end sprinkler, and the sprinkler closest to the main line. Pressure readings should be close to the sprinkler manufacturer's recommended pressure for proper sprinkler performance. if water pressure varies more than 10% from the average pressure, poor water distribution could result, and you should examine your irrigation system closely to detect and correct the problem. There are several possible causes for differences in water pressure. Some of which are poor irrigation system design, improper operating pressure, inadequate-sized pump, or excessive sprinkler discharge due to oversized or worn nozzles. Correcting a pressure problem could involve changing lateral length, sprinkler spacing, number of sprinklers per lateral, nozzle size, or lateral pipeline size.
Annual Maintenance and Evaluation of Overhead Irrigation Systems 2 To determine the application rate of your irrigation system, evenly distribute the same-sized containers in the square or rectangular shaped area formed by four sprinklers. Operate the system for a measured amount of time and then measure the amount of water in each container with a ruler (the more exact the better). From this information, you can determine your application rate in inches per hour. To determine your application rate in gallons per minute per sprinkler, place a piece of hose over a sprinkler nozzle. Direct the stream of water from the hose into the 5-gallon bucket and measure the time required to fill the bucket. To determine the discharge per sprinkler, use the formula shown in Equation 1. Equation 1 You should also evaluate the application rate of your irrigation system as compared to the infiltration rate of your soil. "Infiltration rate" refers to the rate at which a soil can absorb water. Turn the irrigation system on and, while it is running, observe the rate at which the water enters the soil. Does water form puddles or run off in small streams? If so, it indicates that your system applies water faster than soil absorption, To avoid irrigation runoff, you should irrigate for shorterperiods more frequently, reduce the nozzle size or operating pressure to reduce sprinkler output, or increase sprinkler spacing. To determine the depth of watering, run the irrigation system as for a "typical" irrigation. After the irrigation cycle is complete, use the soil auger or shovel to determine the depth of water penetration. Ideally, you should irrigate so that water penetrates slightly deeper than the depth of the root system of your crop. This assures adequate watering of your crop with leaching of excess salts. If water penetrates too deeply, shorten the operating time of your system or reduce the nozzle size to decrease water output. For deeper watering, irrigate for longer periods of time or increase nozzle size to increase water output (providing you can maintain pressure in the laterals).