“This won’t hurt” – Yes it will

The University of Nebraska research station at North Platte reports that corn needs about 33.2 inches of water to produce the maximum yield.  Soybeans require 30.1 inches, and wheat requires 27.1 inches.  Less water means less yield.  Less yield means less efficiency and productivity.  Farms that do not generate the maximum yield are inefficient and are usually taken over by more efficient, usually larger farms.  

Dr. Derrel Martin of the University of Nebraska has begun a series of educational classes aimed at showing the farmers how they can reduce their irrigation needs to as little as 6 inches with little affect on the yield. 

Ray Supalla, also of UNL, has based his economic studies of the region on the data being reported by Dr. Martin. 

Dr. Martin bases his assertions on data generated by a study done by Gary Hergert, Norman Klocke, James Petersen, Paul Nordquist, Richard Clark, and Gail Wicks, entitled “Cropping Systems for Stretching Limited Irrigation Supplies” and published in the Journal of Production Agriculture in 1993.

Based on this study, Dr. Martin generates graph 1. It is used by Dr. Supalla in his economic study.  This graph is used to illustrate that a reduction in water allocation from the top end will have minimal impact on the yield.  For example, it contends that going from 22 inches to 18 inches will have almost no affect on yield. 

The basis for this graph comes from the Hergert study.  This graph is based on three data points: 0, 6, and x inches.  “x” varies by year and is as little as 4 and as high as 13 inches.  “x” is adjusted to assure there is enough water in the soil so the plant will not be stressed.

Top line = Pivot Irrigated, bottom line gravity.                   Graph 1 Apologies for the poor quality of the graph.  It is a screen shot from one of Dr. Supalla's presentations.

Graph 2 is taken from one of Dr. Martin’s presentations and implies that the first six inches of irrigated water causes a 10.9 or 12.3 bushel per inch yield response; the difference is caused by the type of crop rotation – corn on corn or a wheat, corn, soybean rotation.   

Graph 2

There is a major problem with this graph’s assertion.  It assumes precipitation will always be average.  Crops do not care if the water comes from the sky, the stream or the aquifer.  To ignore between 60% and 75% of the water used by a crop and focus only on the water applied by irrigation is not a fair or accurate way to draw a conclusion. 

We have created graph 3.  It uses the same data, but includes precipitation.  We believe it is a more accurate reflection of the Hergert report.  

Our Table 1 below summarizes the data from the Hergert study for corn.  There are three irrigation regimens considered: no irrigation (just the yield from normal precipitation), precipitation plus 6 inches applied, and precipitation plus as much as the corn needs.  Corn need is measured by using a moisture probe.  Any time the soil wetness drops below 50%, water is applied.  In this table, we show the variable amount as “x” or “no stress.”   Each inch of water, even the last inches, still contributes to the yield in a significant way.  So far, no study has controlled for precipitation, and it is not possible to say how much yield the last inch of water contributes as compared to the first.  This study only shows yield results on three types of irrigation regimens.  

As can be seen from Table 1, when precipitation is taken into consideration, the results give a different impression from what Dr. Martin suggests.

The idea that one can reduce the amount of water needed by the crop and still get almost the same yield is not supported by the data.  It is not supported even by the study used by Dr. Martin to make his point.  This would cause the Supalla study to seriously underestimate the economic impact. 

Studies are needed that will look at the interaction between irrigation and precipitation, when the water is delivered to the crop, and what each of the last few inches of water contributes to the yield.  These studies have not been done, yet policies are being based on non-existent data.

Corn needs 33 inches, soybeans need 30 inches, and wheat needs 27.  Irrigation requirements are dependent on precipitation.  If the precipitation is not there, then irrigation is required to make up the difference.  Otherwise, the yields and the farm will suffer.  It is important that the policy makers have accurate information as they make their decisions.  Otherwise, they will end up with unintended results.  When Dr. Martin and “educators” arrive to tell you how you can work with less water, make sure you understand the data they are basing their reports on and point out how dependent the allocation is upon precipitation.