Considerations For Center Pivot Irrigation System Designs By Ben Palen

The focus of the article will be on the considerations for designing a LEPA (Low Elevation Precision Application), center pivot irrigation system.   

 

There is no doubt that LEPA systems are a vast improvement in the evolution of center pivot irrigation. They are in wide use in the US, and around the world, and in many cases water application efficiencies have improved to the mid to high 80^th percentiles as compared to 60-65% with prior designs.  In a nutshell, LEPA at its optimum is designed to discharge water about 18 inches from the ground, under low pressures, and with drop hose spacing of about 30 inches in most cases. 

Before a farmer can make a complete evaluation of the viability of a LEPA system on his/her farm, some questions need to be asked:

• ·         What is the condition of the soil?  What is the organic matter level?  These have to do with the ability of the soil to absorb water with little or no runoff.   As an example, a current project includes LEPA evaluation on a large Saudi Arabian farm, and one of the issues is the low organic matter content of the soil, and its hydrophobic nature, which means that water infiltration can be limited.  Further, the ability of the water applied to wick, e.g., move laterally and vertically, in the soil, is a critical component of the LEPA analysis, as noted in more detail below. 
•  ·         What crops will be grown, and in what configuration?   LEPA tends to work best with crops planted in rows with similar spacing to the drop hoses, or with a crop such as wheat that is relatively easy to germinate.  It has proven to be problematic for small seeded crops such as lettuce, onions, and carrots, and with crops such as potatoes that need good soil cover during the growing period.
•  ·         The reason for these issues is that LEPA tends to concentrate a lot of water (relatively speaking) in a small area, and fairly rapidly.  If the soil is not in good condition to absorb the water, then not enough will reach the plant roots.  Further, uneven germination of small seeded crops will likely result. For potatoes, the stream of water from a LEPA system can lead to soil washing off of the potatoes, and leaching of nutrients below the root zone of the potatoes.

With these factors in mind, one must ask the questions of whether the benefits of LEPA can be attained with a slightly modified version of the technique. The answer to that is yes, and our example below is from the Saudi farm.

That farm has a variety of crops, including some with small seed stock, and many areas where the soils are slow to allow water to infiltrate. In addition, the slope of portions of some of the fields has led to issues with run off of water.

I sought to design a system with many of the benefits of LEPA, but without creating some of the issues noted above.  Using an existing center pivot with eight foot spacing between drop hoses, a discharge height of about six feet, and 35 psi water pressure at the pivot, we undertook the following approach:

-          Lowered hoses to about 24 inches from the ground

-          Kept the drop hose spacing, but installed new nozzle packages from Nelson, along with 15 psi pressure regulators

-          Reduced pivot point water psi to about 20 pounds

The Nelson nozzles were the Rotators or Accelerators.  I have found both of these products to provide a very good, efficient water application pattern. 

We used this approach because it was the best all purpose solution. The farm needed flexibility to grow a variety of crops under the pivots, and we used this design to ensure uniformity of water application, a gentle water droplet size, and reduced evaporative loss.   Whereas the system was operating at 60% efficiency before the upgrades, we raised that percentage to about 85, all with some simple modifications that cost less than $30 per drop hose.  All of this still leaves open the option of converting to a true LEPA system with closer drop hose spacing at some point down the road.   This is but one part of a farming program that is designed to improve soil characteristics over the next few years, with better water efficiency going hand in hand with that.