Extensive subsurface "tile" drainage systems on Corn Belt farmlands have important implications for nutrient pollution in surface water, notably the hypoxic "dead zone" in the Gulf of Mexico. Unfortunately, existing drainage data are outdated and inconsistent. Can a map-based GIS analysis lead to a better assessment of tile drainage?

Key Findings

  • Map-based GIS analysis using soil and land cover data can provide a good representation of land that would benefit from drainage, and in densely tile-drained regions may be an improvement over previous estimates. Refinements could be made through further exploration of smaller geographic areas using more detailed data and maps.
  • Improved drainage data will contribute to a better understanding of the large-scale environmental impacts of tile drainage-related nutrient pollution, and the cost-effectiveness of nutrient abatement strategies. To that end, we offer a range of drainage estimates and a revised national county-level database of agricultural tile drainage for collaborative validation and review.
  • Ultimately, without actual measurements, ad hoc efforts to estimate tile drainage extent will only be stop-gap measures in solving the drainage data problem. Pressing water quality concerns such as Gulf hypoxia highlight the need for another large-scale drainage survey, which could be included in the USDA’s next agricultural census.

Executive Summary

Extensive agricultural subsurface "tile" drainage in the Midwestern U.S. has important implications for nutrient pollution in surface water, notably the seasonal hypoxic "dead zone" in the Gulf of Mexico.

However, drainage data limitations have constrained efforts to effectively factor tile drainage into regional economic and environmental impact studies.

Better drainage data would be a valuable addition to future modeling applications.

In light of this need, a methodology incorporating a geographic information system (GIS) analysis based on soil and land cover maps was used in addition to existing data to create a set of county-level tile drainage extent estimates.

These estimates are downloadable for review in order to evaluate existing data and the results of this GIS analysis, with the goal of arriving at an improved picture of tile drainage in the U.S.