Groundwater issues

Two critical challenges in agricultural watershed management in Minnesota are:

1. Groundwater quality: Water percolates quickly through the soil profile in coarse-textured soils, moving some agricultural nutrients from applied fertilizers below the root zone. Leached nitrogen poses environmental, human health, and economic risks to communities that use groundwater for drinking. The nutrient loss also represents a financial loss to the farmer as some of the nutrients they paid for are lost.
2. Water availability: High groundwater withdrawals during the crop growing season can temporarily reduce groundwater discharge 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. In the dry years that Minnesota has experienced in the last three years, ground and surface water availability is a huge concern in some irrigated parts of the state.

On-farm variable rate irrigation study

A meaningful way to address groundwater 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 conducted an on-farm research project in Stearns County, Minnesota during the 2021 and 2022 growing seasons to evaluate the ability of precision irrigation technology to address both groundwater quality and water quantity issues.

Figure 2. Soil moisture sensor installed in the field.

The project was focused on evaluating the impact of VRI technology on water savings and corn yield 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), where water is applied uniformly without considering the spatial variability of the field, can lead to potential over- or under-application of irrigation water and subsequent negative impacts on crop yields. Over-irrigation can cause issues like waterlogged soils and leaching of essential nutrients while under-irrigation causes symptoms like reduced crop growth and pollination.

By addressing in-field variability using VRI technology, we can optimize irrigation, maximize crop growth, and minimize negative environmental consequences. For this project, irrigation management zones for VRI management were created based on soil electrical conductivity (EC), soil type, and elevation. Many studies have shown that EC is related to soil properties and crop yield potential and could be used for site-specific management in precision agriculture. An irrigation rate of 100% was applied across the entire URI plots regardless of the soil variability (see figure 1 below). Irrigation events were scheduled based on the soil moisture sensors (Figure 2) installed in each treatment. When sensor-measured water content in the lowest EC (zone 1) locations (sandy soils and low water holding capacity) dropped close to 50% of available water holding capacity, irrigation was triggered. In each irrigation event, the irrigation amount was based on the water holding capacity as well as irrigation system capacity.

In 2021, the yield varied from 250 bu/ac to 259 bu/ac in management zone 1, 243 bu/ac to 259 bu/ac in zone 2, and 234 bu/ac to 255 bu/ac in zone 3. Comparing the three zones, the lowest yield was obtained in zone 3, which was a high EC zone with loamy soil and very low elevation (depressional). Because of lower elevation in zone 3, the soil was mostly above the field capacity both in VRI and URI plots, resulting in lower yield in these areas. The highest yield was obtained in management zone 1 in both VRI and URI. Our results indicate that the yield distribution within each irrigation treatment followed a similar pattern in that the management zones with lower EC had higher yield. Overall, because of very dry conditions in the growing season, higher irrigation in URI plots did not cause any significant grain yield loss as the water use (crop evapotranspiration) by the crop was high. However, if the weather conditions would have been normal or precipitation would have been in the normal range, we would have expected higher yields in VRI plots and lower yield in URI.

In terms of irrigation water application (Figure 3 and Table 1), the VRI treatment used an average of 43% less water compared to URI while producing similar yield. On average, the URI treatment produced 258 bu/ac and VRI produced 242 bu/ac while using 11.6 inches and 7 inches water, respectfully. Partial economic analysis with a corn price of $5/bu and irrigation price of $16/ac-in shows that both VRI and URI had almost the same net income (Table 1). These results indicate that VRI could be beneficial in terms of saving water and reducing irrigation-induced environmental pollution. Irrigation water productivity (IWP) in the VRI and URI treatments was also calculated (Figure 3b). The IWP is equal to the amount of grain produced (bu/ac) by the irrigation water divided by the amount of irrigation water applied (inches). In 2021, the IWP in the VRI treatment was 65% higher than the IWP in the URI treatment. On average, the IWP was 35 bu/ac-in in the VRI treatment and 22 bu/ac-in in URI.

Similar results were obtained in 2022, which was also dry compared to normal growing seasons. Figure 3c and Table 2 show corn yield with various irrigation treatments and management zones. The yield was slightly lower in 2022 compared to 2021, possibly due to reduced irrigation in 2022 than in 2021 (approximately 5 inches lower irrigation in URI) and slightly lower temperatures in 2022. The yield varied from 226.5 to 228 bu/ac in zone 1, 224 to 229 bu/ac in zone 2 and 225 to 222 bu/ac in zone 3 in 2022. On average in 2022, the lowest yield was obtained under zone 3 in URI and zone 2 in the VRI treatment. Statistically, the yields were not significantly different from each other in 2022. There was also no significant yield difference between zones within a treatment. On average, the IWP for URI was 35 bu/ac-in, which was 30% lower than the VRI IWP of 47.3 bu/ac-in. In terms of irrigation amounts, on average, VRI reduced the irrigation amount by 27% compared to URI while only reducing corn yield 1 bu/ac.

Over both years, the yield between URI and VRI management was not significantly different, whereas the IWP was significantly higher in the VRI treatments compared to URI. This suggests that VRI is superior in fields similar to the study field.

Table 1.  Seasonal irrigation amounts (inches) applied, yields (bu/acre), Irrigation water productivity (IWP), gross income ($) and net income ($) in each irrigation treatment and zone in 2021