Efficiency of Overhead Irrigation Systems
This article is a summary of part of the content in a publication from the Proc. of the 30th Annual Central Plains Irrigation Conference held Feb. 20-21, 2018. The authors are Steve Melvin and Derrel Martin of the Univ. of Nebraska. The publication is titled “In-Canopy vs. Above-Canopy Sprinklers: Which is Better Suited to Your Field?”. As you may know, Nebraska is the leading state in the US for irrigated corn and soybean acreage, with 57% (5.278 million acres) of its corn acres and 49% (2.787 million acres) of its soybean acres irrigated in 2017. Thus, I consider irrigation information from Univ. of Nebraska personnel to be highly authoritative.
The following points are highlights contained in the above article, and are related to the efficiency of overhead irrigation systems. The specific data and findings reported in this article are from research conducted in the Great Plains region of the US, so they may not directly transfer to overhead irrigation conditions in the sub-humid Midsouth. However, they do highlight items that should be considered for overhead irrigation applications in any environment.
In the above article, the authors highlight consequences of sprinkler design choices and how those decisions affect uniformity of water application, application efficiency, and the quantity of irrigation water lost from overhead system applications. Selecting the proper sprinkler package for an overhead irrigation system is essential to ensure the most efficient operation of these systems. An important component of this process is the placement of sprinklers in or above the canopy of the crop being irrigated; i.e., the mounting height of the sprinklers in relation to the top of the crop being irrigated.
• Water losses from pivots or low application efficiency result from several factors that include: 1) droplet evaporation while the water is traveling through the air; 2) drift of water droplets from the site of application; 3) evaporation of applied water from the wetted crop leaves; 4) evaporation of applied water directly from exposed soil; and 5) non-uniform application which results in runoff from the field or into low field areas.
• Improving application efficiency of irrigation water is the goal of lowering the sprinkler devices into the crop canopy.
The authors cite results from research conducted at Bushland, Texas. The details of that research follow.
• Experiments were conducted during the middle of the day with temperatures of 88°F, wind speeds ≥ 15 mph, and ~36% relative humidity. Water supply rate was about 6 gal./min./acre with one inch of water applied after a corn canopy had reached its full height.
• Sprinkler package efficiencies were: 1) about 88% from impact sprinklers on top of the pivot pipeline; 2) about 92% from sprinklers at truss-rod height; and 3) about 98% from LEPA systems that were operated perfectly.
• The 2% loss from the LEPA system was from soil evaporation during the day of application; the 8% loss from the sprinklers at truss-rod height was 1% from droplet evaporation, 3% from canopy evaporation during water application, and 4% from canopy drying after application; the 15% loss from the sprinklers on top of the system was attributed to 3% from droplet evaporation and drift, 8% from canopy evaporation during application, and 4% from canopy drying after application.
• The field of application was essentially flat; thus, runoff was minimal, and the losses during the experiment were attributed to evaporation effects.
The authors provided additional points of interest to consider since essentially all losses during the experiment were attributed to evaporation.
• Evaporative losses are the same regardless of the water application depth assuming the canopy is thoroughly wetted with each depth of water applied. Thus, when only 0.5 inch of water is applied, a 4% loss would be the same amount of water lost as when 1.0 or 2.0 inches are applied. However, the percentage of loss from a 0.5 inch application would be twice as great as that from a 1.0 inch application and four times as great as that from a 2.0 inch application.
• Evaporation of water from drops that travel through the air before reaching the crop canopy is strongly affected by the droplet size. In essence, larger drops are less affected by wind and evaporate at a much lower rate than smaller droplets.
The authors concluded that the water loss from above-canopy sprinklers to the soil is small and a small amount of runoff that causes water to move within a field (such as could occur if the standing crop interferes with the stability of sprinklers located within the canopy) will more than offset gains from locating sprinklers in the canopy. Thus, the primary objective for an overhead irrigation system sprinkler package should be to have the water applied uniformly across the field in order to achieve uniform infiltration into the soil; i.e., soil in all portions of an irrigated field should be uniformly wetted.
I encourage you to access the above linked article for additional details about efficiency of overhead irrigation systems and how a sprinkler package should be carefully selected to match the conditions during application of water with these systems.
Composed by Larry G. Heatherly, Apr. 2018, larryheatherly@bellsouth.net