Delaying drainage from prairie potholes protects water quality
By Ann Perry USDA-ARS
By Ann Perry USDA-ARS
Nov. 6, 2013 – Field drainage installations called blind inlets have been adapted by U.S. Department of Agriculture (USDA) scientists for managing agricultural pollutants in the Lake Erie watershed.
These systems, which were developed by Agricultural Research Service (ARS) scientist Doug Smith and others, are so effective that farmers who install them are eligible for financial assistance through USDA’s Environmental Quality Incentives Program. ARS is USDA’s chief intramural scientific research agency. Prairie potholes in crop fields have no natural outlets and are prone to ponding, so farmers often install tile risers to drain away excess water. These risers are perforated vertical pipes that extend a foot or more above the soil and connect directly to subsurface tile drainage networks.
Water drained from potholes by the risers is discharged via the subsurface drains into the nearest field ditch without any filtration or processing. Often agricultural pollutants drained from the potholes in northeast Indiana’s St. Joseph River watershed eventually reach Lake Erie.
Smith, who works at the ARS National Soil Erosion Research Laboratory in West Lafayette, Ind., studied how water quality differed between pothole drainage that passed through tile risers and drainage that passed through blind inlets. A blind inlet is similar to a French drain that channels excess water away from a building foundation. Smith constructed blind inlets by digging a square pit three feet deep at the lowest point of each pothole and then strategically arranging layers of gravel, tile lines, and related drainage materials in each pit.
Water samples collected from drainage channeled through risers consistently showed the highest levels of total phosphorus lost from the fields—as high as 1.73 ounces per acre. In addition, almost all these samples had higher levels of sediment, soluble phosphorus, total phosphorus, and nitrogen than water that drained through blind inlets. Phosphorus loads in samples from the blind inlets were 78 percent lower on average than phosphorus loads in samples from the tile risers, and average sediment loads were 79 percent lower.
Results from this study have been accepted for publication in Soil Use and Management.
Read more about Smith’s studies in the October 2013 issue of Agricultural Research magazine