Can variable rate irrigation combat groundwater issues in Minnesota?

By: Taylor Becker, Extension educator & Vasu Sharma, Extension irrigation specialist 

Did you know that more than 25% of groundwater in Minnesota is pumped for agricultural crops? This makes irrigation the second-largest user of groundwater in the state. Can variable rate irrigation (VRI) technology help Minnesota farmers save water without sacrificing yield?

Minnesota’s Central Sands region

Minnesota has over 600,000 acres of irrigated cropland, and many of these irrigated acres are in the state’s Central Sands region. The coarse-textured or sandy nature of the region’s soils means that they do not hold large quantities of water and have a rapid drainage rate to groundwater compared to high clay soils.

These soils present challenges in agricultural irrigation and nutrient management. It takes less precipitation to saturate these soils and they hold less plant-available water than loams. Low organic matter levels also mean that less nitrogen is provided by these soils and higher rates of nitrogen fertilizer are required. Attaining economically viable yields requires careful irrigation and well-timed fertilizer applications.

Groundwater issues

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:

1. Maintenance of groundwater quality: Water percolates through the soil profile quickly in coarse-textured soils, taking some agricultural nutrients from applied fertilizers with it. In particular, leached nitrogen poses environmental, human health, and economic risks to communities that use groundwater for drinking. Nutrient loss also represents a financial loss to the farmer as some of the nutrients they paid for are lost beyond the root zone.
   
   
2. 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 life and recreational activities on waterways, and leading to water supply shortages for nearby private and municipal wells.

Field map representing different irrigation zones.

Different colors indicate different electrical conductivity range.

Variable rate irrigation on-farm study

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 the Minnesota Department of Agriculture (MDA), we are currently evaluating the ability of precision irrigation technology to address both groundwater quality and water quantity issues. The study is being conducted on a grower’s field in Stearns County.

The project is focused on evaluating the impact of VRI technology on water 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 sandy soils 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.

2021 study results

After a single year of the project, we found some interesting results. Overall, because of very dry conditions during the 2021 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, the VRI treatment used 43% less water compared to URI while still producing very high corn yields. The water savings achieved with VRI amounted to 16 million gallons of water (5 inches of water per acre on a 120 acre irrigated field), a substantial environmental impact. 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 nutrient loss.

Table 1. 2021 on-farm study results comparing URI and VRI (Stearns County, Minnesota)

	Uniform rate irrigation (avg)	Variable rate irrigation (avg)
Irrigation (inches)	11.6	6.6
Yield (bu/ac)	257.7	242.4
Irrigation water productivity (bu/ac-in)	22.2	36.7

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 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 through 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.

If you’re interested in learning more about irrigation best management practices in Minnesota, there are limited spots remaining for the March 23 and March 30 (two-day program) Minnesota Irrigator Program in St. Cloud.  

You can also learn more about this study and other U of M irrigation and nitrogen management research in this February 2022 presentation from Extension irrigation specialist Vasu Sharma.

A version of this article was originally published by the University of Minnesota’s Water Resources Center.

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