Journal of the NACAA
ISSN 2158-9429
Volume 3, Issue 1 - July, 2010


Center Pivot Irrigation and Fertilizer Use Evaluation

Nelson, R.M., Agriculture/Youth Agent, Utah State University Extension, Beaver County
Hill, R.W., Professor, Biological and Irrigation Engineering Department, Utah State University,


An education program was conducted in Beaver County, Utah to help farmers evaluate their center pivot irrigation systems. Twenty-two center pivots were evaluated in 2002; twelve of the original 22 pivots were re-evaluated in 2008 for comparison. During the evaluations, catch cans were set out at a 20 foot spacing in a line from the center of the pivot to the outer edge of the irrigated portion of each field. Each pivot was allowed to pass completely over the line of cans. Distribution uniformity (DU) and coefficient uniformity (CU) were calculated for each pivot. The average DU in 2002 was 61 % whereas 70 % was the average in 2008. Nine of the twelve pivots which were evaluated both years showed an increase in DU. Six of which showed increases over 17%. Soil samples were collected under all of the pivots tested to determine if farmers were applying the correct amount of fertilizer to meet the crop's needs. The soil tests from 2008 indicated that 72 % of the farmers were applying less fertilizer and manure than they were in 2002. One reason for the decrease in phosphorus levels may be because the price of fertilizer was very high in 2007 and 2008.


In semi-arid environments such as Utah, water is one of the most limiting factors to crop production and the single most important factor influencing plant growth and crop yields. Agricultural related irrigation accounts for 80% of Utah’s presently developed water supply. Improving irrigation efficiency may provide two major benefits to agriculture: (1) increased agricultural productivity and profitability and (2) improved water quality. A sound, thorough, understanding of how water influences crop growth is essential for good water management.

Water is a very important resource in Beaver County, Utah. Just as in most western states, there is not enough to meet demands. In 2007 there were 36,590 acres irrigated in the county. Of these, 15,900 were irrigated by center pivots. Traditionally, flood irrigation has been the most prevalent irrigation system in the county. In the last 30 years 85 % of the irrigation systems have been converted to sprinkler. Wheel lines have been used heavily throughout the county but now many of them have been converted to center pivots. We have found that many farmers lack understanding of irrigation techniques with these systems which only reach peak efficiency when managed properly.

Irrigation audits were conducted on 22 center pivot sprinkler systems throughout Beaver County, in 2002. Irrigation uniformity of the pivots varied from a low of 53% Coefficient of Uniformity (CU) to a high of 91 % (CU). We met with each producer and gave them suggestions on how to make their system more efficient. We found that some of the farmers were running their pivots too fast thus not applying enough water in each pass over the field. We also conducted soil tests in these same fields and found that 56 % of the farmers were not applying enough phosphorus for the crop. 8% of them were applying too much manure or fertilizer. In the
six years since this evaluation several things have changed in Beaver County. The number of pivots in the county has increased 30%. The price of fuel has more than doubled for planting and harvesting crops. Electrical costs for pumping irrigation water have also increased. The cost of fertilizer has also doubled and it has become more difficult and more expensive to hire farm workers.


The pivot evaluations were conducted by students from Utah State University. Catch cups were placed at 20 foot intervals on a radial line from the center to beyond the outer wetted edge. The distribution uniformity (DU) and coefficient uniformity (CU) were calculated for each pivot. A value of 100% (for either DU or CU) would represent a pivot that was applying exactly the same amount along its entire length. After the pivots were evaluated each producer was give a printout that listed the pivot pressure, flow rate, average catch, DU and CU.

Figure 1. A representation of one producer’s printout of the distribution uniformity from 2002 to 2008.


Nine of the twelve pivots which were evaluated both years show an increase in distribution uniformities, six of which showed increases over 17%. Of the three pivots which showed a decrease, two decreased by 2% or less. A DU value of 80% or greater is preferred. The DU values have increased from 61% on average in 2002 to 70% in 2008. This increase, on average may save $1745 per pivot per season.

Figure 2. Summary results comparing 2002 evaluations to the evaluations completed in 2008.


75% of the pivots which were evaluated both years showed an increase in distribution uniformity. Farmers can save up to $1745 per pivot per year by running them more efficiently. Farmers can improve the uniformity efficiency by changing the sprinkler heads regularly. This follow-up irrigation study shows that 3/4 of the farmers improved the way they were operating their center pivot as a direct result of our evaluation program. One of the reasons this study was usefull for local farmers is because if you can't measure it you can not improve it. Most of the farmers thought their pivots were very effecient and did not worry about them until out study showed they had a room for a lot of improvement.

Fertilizer Use

Crops require proper nutrition for good production. Commercial fertilizer and manure can be applied to meet the needs of the crop. The key to good nutrient management is soil testing. Over applying fertilizer and manure not only cost farmers extra money, but may contaminate ground water and cause a buildup of nutrients in the soil. As a part of the irrigation grant we conducted soil tests under the pivots that were evaluated.


Soil samples were taken for each crop grown under each pivot. Using a soil auger a number of samples were taken randomly from each field, from the surface down to 12 inches. All of these samples were then combined and one pint was sent to the Utah State University Analytical lab. Each sample was tested for pH, salinity, texture, phosphorus and potassium. The majority of the crops grown under the pivots were alfalfa but there were some small grains, corn and pasture grasses.


As shown in Figure 3, phosphorus levels were higher in 2002 than in 2008. The soil tests from 2008 indicated that 72% of the farmers were applying less fertilizer and manure than they were in 2002. In 2008, 78% of the fields tested did not have enough phosphorus to meet the needs of the crop which could limit production. In 2002, 56% of the fields tested did not have enough phosphorus. One reason for the decrease in phosphorus levels may be because the price of fertilizer was very high in 2007 and 2008.

Figure 3. Soil phosphorus levels in 2002 compared to levels in 2008.


A number of farmers could increase crop yield by applying more phosphorus. Research shows that soil testing will pay for itself many times over. Farmers will not be paying for fertilizer they do not need and they will be applying enough fertilizer to provide sufficient nutrients to maximize crop production. This fertilization study showed that the majority of the farmers are applying insufficient fertilizer or manure for the crop.


This work was funded by a research grant from USU Extension. We would like to thank the USU Irrigation Department and the graduate students for all their work on the project.


  1. Hill, R. W., and R. M. Nelson, 2001. Sprinklers, crop water use, and irrigation time, Beaver County. Online., ENGR/BIEWM/24.
  2. 2007 Utah Agricultural Statistics and Utah Department of Agriculture and Food annual report. United States Department of Agriculture, Utah Agricultural Statistics Service, P.O. Box 25007, Salt Lake City, Utah 84125-0007.
  3. Susong, D. D., 1995. Water budget and simulation of one-dimensional unsaturated flow for a flood and a sprinkler-irrigated field near Milford, Utah. Technical Publication No. 109, State of Utah, Department of Natural Resources, 1995.