Impacts of agricultural drainage on groundwater recharge

Science Center Objects

Artificial subsurface drainage is being increasingly utilized on agricultural land in southeast Minnesota. This region is underlain by thinner glacial deposits than are found in the historically drained areas of the State. Due to these thinner deposits, drainage in this area may have a greater impact on recharge to the underlying bedrock aquifers, a critical resource to the region.

The overall goal of the project is to characterize and measure the water budgets for these three agricultural fields. Quantification of the effects of subsurface drainage on the flowpaths of water at the field scale will lend insight to the potential effects that subsurface drainage may have on groundwater recharge around similar sites in the surrounding area. 

This project established two separate field-scale monitoring sites in agricultural fields with subsurface drains in southeast Minnesota. A third field-scale monitoring site was established in a non-drained agricultural field as a control. Study sites were selected in areas with a shallow depth to bedrock and low surface slope to limit surface runoff. Other considerations included fields with similar agricultural practices (for example, tillage practices, nutrient management, and crop rotation) and landscape characteristics. The overall goal for each of the three study sites is to quantify the field-scale water budget. A comprehensive water budget requires a full quantification of the water balance in the soil, where any change in the water storage in the soil can be quantified by the following:

ΔS = (P + I + U) - (ET + PR + D + Ro)

where: 

  • P represents the total precipitation across the monitoring site
  • I is irrigation
  • U is the upflux or capillary rise of water from shallow groundwater
  • ET is evapotranspiration
  • PR is deep infiltration or potential groundwater recharge
  • D is subsurface drainage
  • Ro is surface runoff

By choosing a site with low surface slope, Ro was limited. For inputs other than U, P will be measured at each site and I will be zero. ET will be quantified for each study area by a Penman-Monteith or similar calculation. D was measured by monitoring drainage outflow for one or more tiles at each field site, and using DRAINMOD or a similar modelling program to estimate subsurface drainage for the entire field site. All aspects of the controlled study area will be quantified and compared against the water budgets of the other sites.

Illustration of subsurface-drained agricultural field

A hypothetical configuration of one of the two drained field sites, including a weather station for measuring climate data, a piezometer network for continuous water level measurements, soil moisture probes, and subsurface drainage flow. The third undrained field site will have a similar configuration, with the absence of subsurface drainage flow.

Read more in the Minnesota Ground Water Association's white paper: Drain Tiles and Groundwater Resources: Understanding the Relations (2018)

 

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