Measuring Tile Drainage in Iowa

Although recent rains have brought Iowa out of drought, they also led to flooding and significant increases in nitrate loads to surface waters – the same nitrate leading to a dead zone in the Gulf of Mexico the size of New Jersey. Tile drainage, the underground piping in cropland, has contributed to the flow of nitrate from fertilizer and manure. 

USDA’s five-year national census of agriculture, released earlier this year, shows that Iowa still has the largest number of acres drained by tile lines. It also showed a surprising 7.7% reduction in tile-drained acres in Iowa and several other Midwestern states. This runs against anecdotal evidence of increased tile installation across the state, especially in eastern Iowa (I’ve seen plenty of the large black coils in fields over the last few years). There is no evidence (reported, anecdotal, or otherwise) of old tile drainage being removed without being replaced. Even with the reported reduction, Iowa has by far the most tile-drained acres in the country and almost half of Iowa cropland is tile-drained. 

There are a few potential explanations for the decrease. First, the census showed a 3.3% reduction in cropland in Iowa compared to five years earlier; it makes sense for fewer acres to have less drainage. Second, the census doesn’t get responses from everyone, creating some variability in the survey results – and the response rate dropped by 10% compared to the previous census. And at the county scale, there is even more questionable variability: did Buena Vista County really reduce tile drainage by 72,000 acres? Probably not, and the same is true in several other counties. Cedar County, on the other hand, might have had the 8th highest growth in newly tiled acres nationwide.  

Iowa still has outdated drainage laws that have converted nearly all wetlands across the state into cropland. Since IEC raised that issue four years ago, the state has not moved to adopt requirements that even the Dakotas have in place. The few remaining farmland wetlands in Iowa are now at risk due to a lawsuit challenging the federal protections in place. Meanwhile, the drainage systems we have in place can lead to sinkholes and fish kills, as reported last month, and state officials are clamoring to install more taxpayer-funded water quality wetlands at a cost of millions. 

We need a better system to manage water in Iowa, including natural infrastructure solutions like wetlands. Taking lessons from other Midwest states would be a good first step. And when paired with a broader shift to more diverse cropping systems, we can build resilient communities and achieve both economic and environmental benefits.