August 19, 2019
A 65-year comparative analysis between U.S. yields of irrigated and rainfed crops has sounded a message to farmers, land managers and policymakers: Mind the gap.
The University of Nebraska–Lincoln’s Suat Irmak and Meetpal Kukal analyzed the annual yields of nine crops — corn, soybean, spring wheat, winter wheat, sorghum, cotton, barley, oats and alfalfa — on a county-by-county basis from 1950 to 2015.
Irmak and Kukal found that the yield gaps — differences in food produced with irrigation versus rainfall alone — generally widened over that span, a trend they suspect stems partly from climate change and technological advances in irrigation management.
But the rates at which those gaps widened, and how consistently they did so over time, differed substantially among the crops and the regions that grew them.
“You get more yield from irrigated than rainfed (agriculture), but the magnitude of yield increase is a function of several variables,” said Irmak, Eberhard Distinguished Professor of Biological Systems Engineering. “It’s not surprising that as precipitation increases, the yield gap decreases. But that also has spatial and temporal attributes, so it’s not really constant in all regions or for all crops.”
Irrigation most benefited corn yields, according to the study, boosting them 270% nationally over the 65-year span.
Adapted from figure in Environmental Research Communications
Maps illustrating the average difference in irrigated vs. rain-fed yields based on data collected from 1950 to 2015. A county colored yellow, for instance, indicates that a crop yielded between 210% and 250% more food when irrigated than when relying only on rainfall. Data were standardized to better compare irrigation effects across crops and regions.
Growing season
The unique growing season of winter wheat meant that its yields rose only 25% with irrigation, the smallest gain among the nine crops. Yet even crop-specific yield gaps varied noticeably by location. Two corn-growing areas separated by about 700 miles, for instance, saw a seven-fold difference in irrigation-related yield gains.
Having mapped such differences across roughly 80% of the United States’ cultivated land, the researchers said they hope the findings can help guide future crop production while calibrating water management and irrigation use nationwide.
Regions or states with historically wider gaps, especially those growing the most irrigation-thirsty crops, might consider irrigating more or investing in soil management practices that help conserve moisture, Irmak said. Those that have historically seen little difference — or, in a few areas, seen the gaps between irrigated and rainfed yields narrow — might decide to allocate their resources in other ways.
The county-level precision of the study’s data might also help individual farmers better estimate the potential return on investing in irrigation.
“We have those answers for different crops in each county, so they can go back and do some analysis,” Irmak said. “If they want to buy a center-pivot for approximately $150,000 and convert their land to irrigation, they can quantify the (return on investment). Of course, these are ranges (of values), but they can say, for instance, ‘I can increase my yield by about four to six tonnes per hectare. Depending on grain prices, over the course of five to eight years, I can pay for my pivot.’”
Irmak and Kukal likewise quantified the number of years that given counties failed to generate any meaningful yields of a crop when relying only on rainfall, which some farmers might factor into long-term risk assessments, the researchers said.
“There’s a bigger risk with rainfed (agriculture),” Irmak said. “There’s an overall, long-term average value for rainfed yields, but that doesn’t mean you are going to get that yield every year. That’s why we consider irrigation an insurance policy that normalizes things and provides stable productivity under varying climatic conditions.”
Adapted from figure in Environmental Research Communications
A bar chart illustrating the average difference in irrigated vs. rain-fed yields based on data collected from 1950 to 2015. A value of 0.7, for instance, indicates that a crop yielded 70% more food when irrigated than when relying only on rainfall. Data were standardized to better compare irrigation effects across crops and regions.
Climate trends
The researchers, who in 2018 published studies that quantified the lengthening of growing seasons and the agricultural effects of climate trends across the United States, said they expect farmers and other ag professionals to welcome the depth and complexity of the new analysis.