Hossein Sadeghi 2017-02-22 23:38:08
Center pivots currently irrigate more than 10 million hectares (25 million acres) in the U.S. and are steadily replacing traditional flood irrigation and other types of pressurized irrigation systems. Despite their great advantages, the water application uniformity under these machines is not consistent and varies across time and space. A typical center pivot with mid-elevation spray drops will achieve a sprinkler discharge efficiency (SDE) of 80% to 90%. This means that 10% to 20% of the applied water is lost before it can be caught and stored by the soil. The most important factors affecting the SDE are wind drift and evaporation losses, so the SDE varies greatly with environmental conditions, including day/night differences. In addition to the water lost, this variation of the SDE causes poor application uniformity under center pivots, as well as inaccurate estimates of actual water application depths. To mitigate these effects, growers often start their center pivots at inconvenient times so that the system does not irrigate the same area of the field at the same time of day. They also deliberately over-irrigate some areas of the field to ensure that the entire field is adequately irrigated. On a large scale, this can result in several disadvantages, including an overall reduction in yield, loss of crop quality, greater energy consumption, and increased risks of runoff, nutrient leaching, and soil loss. Finding practical solutions to reduce the temporal and spatial variation of center pivots is therefore essential and requires knowledge about the continuous variation of the SDE in different weather conditions. Unfortunately, due to its limitations in setup and read times, the traditional “catch can” method cannot provide this information. The strip method As a current associate in research at Washington State University and former doctoral student under the supervision of ASABE member Troy Peters, I have been testing a new technique—called the “strip method”—to continuously measure the dynamic variations of the SDE over long sampling periods and short timing intervals. Our research has shown that the strip method works well even in very hot or windy weather conditions. In the strip method, the applied water is captured by long collection strips that are perpendicular to the sprinkler mainline and oriented toward outlets where the outflow is continuously measured by tipping-bucket flow gauges. The strips are made from boards that define the edges of the measurement area, and an impervious tarp is placed over these boards and on the surrounding soil. The strips have a triangular cross-section at their downstream end to direct runoff toward the outflow measurement point. At the outlet of each strip, a pit is dug to install a tipping-bucket flow gauge below grade. The tipping buckets are covered with triangular wooden shields to prevent biases due to wind, dust, evaporative loss, and falling soil. A trench is dug to drain off the wastewater and surface runoff from the bare soil. The SDE is calculated by dividing the average flow rate from all three strips by the average sprinkler nozzle flow rate and correcting by the ratio of strip width to sprinkler head spacing. Experiments were carried out over the 2013-2014 growing season in Prosser, Wash., for periods lasting between five hours and six consecutive days. The system collected more than 12,000 values of five-minute average SDE data for a total duration of roughly 1,040 hours and applied more than 16,250 m3 of water during this time. Depending on the weather conditions, the five-minute average SDE (SDE5min) could vary between 74% and 98%. The experiments also showed that the minimum SDE5min always occurred during the day, while the maximum SDE5min occurred either at night or before 7:00 a.m. Wind speed, temperature, and relative humidity were the best explanatory variables for predicting the SDE. Overall, the average daily SDE varied between 82% and 92%, indicating that about 8% to 18% of the daily applied water was lost or drifted during a 24-hour cycle. As part of our research, a dynamic model was developed to help predict the SDE as a function of weather parameters. The model predicted the SDE with an absolute error of just over 4%. This model can be used by the pivot controller, which would also record the measured weather parameters to automatically adjust the center pivot travel speed to correct for day/night differences and changing weather conditions. Many advantages These results can open up a new window for improving the large-scale application uniformity of center pivots. This can have numerous advantages: • Growers may realize higher yields, since large areas of the field are no longer over- or under-irrigated due to poor uniformity because of the changing SDE. • When all plants are irrigated uniformly, there will be improved crop quality across the field, resulting in the grower’s ability to maximize returns, especially for vegetables like potatoes, carrots, and onions. • Water management can become simpler, since growers won’t have to start their pivots at odd times of day or night to prevent the pivot from consistently being in the same area of the field at a particular time, which leads to over- or under-irrigation. • There will be a decrease in water and nutrient losses due to leaching in areas that were unnecessarily over-irrigated in order to adequately irrigate all areas of the field. • There will be energy savings due to the decreased need to run pumps and rotate the pivot for additional passes. These energy savings will be particularly noticeable in high-lift operations. ASABE member Hossein Sadeghi, Postdoctoral Research Associate, Department of Biological Systems Engineering, Washington State University, Pullman, USA, email@example.com. For further information, contact ASABE member Troy Peters, Associate Professor, Department of Biological Systems Engineering, Washington State University, Pullman, USA, firstname.lastname@example.org.
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