A new study is one of the first to use segmented regression to study changes in water table elevation trends. Researchers set out to evaluate the effects of droughts, crop prices and local groundwater management on tipping points in groundwater decline, particularly across the High Plains Aquifer. Previous studies assigned a linear or logistic regression to the decline, suggesting that depletion is inevitable and extending the life of the aquifer would be difficult or impossible. However, by finding breakpoints in the data, the new study suggests there are opportunities to enhance the lifespan of the resource through improved management.
Researchers from the Nebraska Water Center, Michigan State University and the University of Florida found that when conditions were excessively dry, wells declined faster. When corn prices were extremely low, there was recovery or slowing of decline. “This is likely because farmers determined that the cost of irrigation was too high when compared to the crop value,” said Erin Haacker, lead author and research associate at the Nebraska Water Center. “As one would expect,” Haacker said, “much wetter conditions were also associated with recovery.”
According to the report, when water management districts were first established, the area saw water tables decline rapidly or recover much less quickly. One reason for this may be because pumping restrictions are often based on historical rates of pumping in the area. Five years or more after the district was established, the water tables saw recovery or declined less rapidly. Based on these findings, researchers estimate that both establishment of management districts and faster declines in water tables are associated with drought.
The study explored thresholds for changes in water table elevation trends to inform predictions of the pattern of decline and recovery. Despite the critical role groundwater plays in providing water for irrigation, groundwater management is not as established as surface water management in the U.S. and elsewhere. However, these management districts sometimes impose pumping restrictions or other regulations that could affect water table elevations.
Researchers studied an 82-year record of water table data from the High Plains Aquifer and 41 groundwater management areas within it. The High Plains Aquifer is the source for nearly a third of all groundwater used for irrigation in the U.S., and 98% of water extracted from it is used for irrigation. Researchers note that the institution of pumping restrictions by management areas is relatively recent and highly variable across the aquifer. Haacker said the research may be useful in future studies that would evaluate the effectiveness of programs in which irrigators voluntarily imposed pumping limits.
This work was supported by the US Department of Agriculture [USDA-NIFA 2016-68007-25066; USDA-NIFA 2015-68007-23133] and the National Science Foundation [NSF 1039180]. Any opinions, findings, and conclusions or recommendations expressed here are those of the authors, and do not necessarily reflect the views of the USDA or NSF.
Full article: https://doi.org/10.1016/j.agwat.2019.04.002