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Fluid Journal : Early Spring 2013
12 The Fluid Journal Early Spring 2013 Table 1. Water and chemical travel times through pipelines and drip lateral lines for selected vineyard and orchard field sites. Site Mainline and submain pipeline Lateral line Total Length Travel time Length Travel time Travel Time (ft) (min) (ft) (min) (min) 1 1,000 22 175 10 32 2 1,500 30 340 10 40 3 5,000 65 340 10 75 4 1,400 15 630 23 38 5 700 8 625 23 31 6 820 17 600 28 45 lateral would be 60 gph or 1 gpm. For typical drip tubing with a nominal inside diameter of 5/8-inch, the resulting flow velocity would be 60 feet per minute (fpm) or 1 foot per second (fps). The flow velocity in the drip line depends on the flow rate and the tubing size. For the same size drip tubing, the higher the flow rate, the higher the flow velocity. Using the above drip lateral as an example, downstream of the first emitter, the flow rate in the drip tube would be 59 gph; downstream of the second drip emitter, the flow rate would be 58 gph; and so on. Since the flow rate decreases along the drip lateral, so does the flow velocity. The slowest-moving water is between the next-to-last and the last emitter. In our example, the flow rate in this section is only 1 gph. For the same 5/8-inch drip tubing, the flow velocity would be only about 1 fpm in this last drip line section. The total travel time of water along a drip lateral line therefore depends on four factors: • The length of the drip lateral • The number of emitters installed in the lateral line • The discharge rate of the emitters • The inside diameter of the drip tubing. Knowing these factors, the drip lateral line travel times can be calculated. But the easiest way to deter¬mine the travel time is to measure it in the field. Field measurement. Microirrigation water travel times can be measured by "tracing" the movement of injected chlorine through the system. Injecting chlorine into the irrigation system is a recommended microirrigation system maintenance procedure. The presence of chlorine in the discharge from emitters can be easily monitored using a pool or spa chlorine test kit. The chlorine's passage through the microirrigation system can be readily traced using this technique, and the water travel time easily determined. The recommended procedure is as follows: Step 1: Start up the microirrigation system and allow it to come to full pressure. If the microirrigation system has not been flushed recently (pipelines and lateral lines), this should be done now. Allow the microirrigation system to return to full pressure after flushing. Step 2: Begin injecting chlorine so that the chlorine concentration in the irrigation water is approximately 10 to 20 parts per million (ppm). Note the time when chlorine injection begins. Step 3: Go to the emitter at the head of the lateral farthest (hydraulically) from the injection point. Using the chlorine test kit, monitor the discharge from that emitter and note the time when the chlorine registers on the test kit. The time from the start of the injection to when the chlorine registers on the test kit is the travel time of water through the mainline-submain system. Step 4: Go to the last emitter at the tail end of the lateral you just monitored (the lateral farthest hydraulically from the injection point). Monitor discharge from this last emitter until chlorine registers on the test kit and note the time this occurs. The time from the start of the injection to when the chlorine registers on the test kit is the travel time of water through the entire microirrigation system. As part of a field study of microirrigation systems, travel time and chemigation uniformity information on a number of drip systems was collected. Table 1 shows the travel times for these evaluations. This data show that there is no standard water travel time to the far point in a drip irrigation system. Travel time should be measured for each individual microirrigation system, but it only needs to be measured once. Post-injection. To ensure chemigation uniformity it is important that irrigation continue following an injection. This accomplishes two things. First, it allows the injected material to be cleared from the microirrigation system. Second, it maximizes the chemigation uniformity, since all emitters will have discharged nearly the same amount of injected material by the time the irrigation stops. Clearing the microirrigation system of the injected material is often important to minimize emitter clogging. For example, leaving fertilizer in the system may encourage biological growth (e.g., biological slimes), which can lead to emitter clogging. Leaving materials containing calcium (e.g., gypsum or calcium nitrate) in the system may lead to chemical precipitation of calcium carbonate (lime), which may also cause emitter clogging. Time and temperature enhance chemical precipitation. The exception to this recommendation may be the injection of system maintenance products such as chlorine or acid. It may be desirable to leave these products in the system at shutdown to maximize their effects and minimize clogging problems. Just as it takes time for the injected material to travel through the microirrigation system once injection starts, it takes an equal or greater amount of time for the injected material to clear out of the system. The injected material first clears from the head of the system, and the last point to clear is the emitter hydraulically farthest from the injection point. This is just the opposite of what occurs when injections began, and it balances the amount of injected materials discharged from emitters throughout the microirrigation system. This gives a uniform chemigation application. Field evaluations have been done on a single drip lateral line to evaluate the impact on chemical application uniformity of varying the injection and post-injection irrigation times. The
Late Spring 2013