By Taylor Herbert
More than 25% of groundwater in the state of Minnesota is pumped for irrigating agricultural crops. This makes irrigation the second-largest user of groundwater in the state (Freshwater Society, 2013). Currently, Minnesota has approximately 600,000 acres of irrigated agricultural cropland, a number that increased by 4% from 2007 to 2012 (USDA NASS 2012). Many of these irrigated acres are in Minnesota’s Central Sands region. The coarse-textured or sandy nature of the region’s soils means that they don’t hold large quantities of water and have a rapid drainage rate to groundwater compared to high clay soils. At the same time, many communities in this region depend on groundwater as their drinking water source. Balancing agriculture’s economic needs while protecting rural drinking water supplies leads to two critical challenges in agricultural watershed management. First, is the maintenance of groundwater quality. Water percolates through the soil profile quickly in coarse-textured soils, taking agricultural chemicals (fertilizers) with it. In particular, leached nitrogen poses environmental, human health, and economic risks to communities that use groundwater for drinking. Fertilizer loss also represents a financial loss to the farmer as valuable nutrients are leached beyond the root zone. Second, is the overall availability of groundwater. High groundwater withdrawals during the crop growing season can temporarily reduce the discharge of groundwater into nearby streams and lakes, impacting aquatic and cause interference with nearby private and municipal wells.
A meaningful way to address these issues is by implementing proven advanced irrigation management techniques and technologies such as variable rate irrigation (VRI). With funding from the AGRI Sustainable Agriculture Demonstration Grant from MDA, we implemented a project focused on evaluating the ability of precision irrigation technology to address both groundwater quality and water quantity issues. The project is focused on evaluating the impact of VRI technology on water and savings, corn yield, and nitrogen (N) leaching in comparison to uniform water management. VRI technology addresses the reality that soil physical properties can vary significantly within a single field–from rapidly draining sands to poorly drained clays. Uniform rate irrigation (URI) does not account for this variability, leading to potential over- or under-application of irrigation water and subsequent negative impacts on crop yields. By addressing in-field variability with VRI, we can optimize irrigation, maximize crop growth, and minimize negative environmental consequences. For this project, management zones for VRI management were created based on soil electrical conductivity (EC), soil type, elevation, and previous yield. An irrigation rate of 100% was applied across the entire URI plots regardless of the soil variability.
After a single year of the project, we found some interesting results. Overall, because of very dry conditions in the growing season, greater irrigation in URI plots did not cause any significant grain yield loss as the water use by the crop was high. However, if precipitation had been in the normal range, we would have expected higher yields in VRI plots and lower yield in URI. On average VRI treatment used 43% less water as compared to URI while producing almost similar yield. The URI treatment produced an average of 258 bu/ac and VRI produced 242 bu/ac while using 11.6 inches and 6.6 inches water respectively. These results indicate that VRI could be beneficial in terms of saving water and reducing irrigation induced environmental pollution.
Analysis of soil water samples did not show a statistically significant difference between the nitrate-N concentrations in the VRI and URI zones. Averaged across the entire season, soil water in the VRI treatment contained 59 ppm while that in the URI treatment contained 54 ppm nitrate-N. Although the nitrate concentrations in the VRI zones were slightly higher, this could be explained by the fact that the VRI zones received less irrigation than the URI zones. Since the nitrate levels are a concentration measurement, less water entering a system with the same amount of nitrogen in the soil would be expected to have higher concentrations of nitrate in the water leaving that system. The volume of water percolating the soil in the URI system is much higher and might have a reduced concentration of nitrate, but the net nitrate loss would be higher under URI.
We look forward to continuing this work in 2022. Special thanks to the AGRI Sustainable Agriculture Demonstration Grant and MDA for funding this research and to our farmer cooperator for collaborating on this project.