Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: June 11, 1999
Publication Date: N/A
Nitrogen losses from agricultural lands have been considered to be a major source of nutrients into the rivers, streams, and lakes of the Midwest. One of the problems has been the lack of a data base relative to different farming systems that would help to establish information that producers could use to assess the impact of current farming practices on nitrate-nitrogen losses. In 1990 we began a study across an intensively cropped watershed in central Iowa to evaluate the variation in nitrate-nitrogen loads from a subsurface drained watershed. This study was conducted in the Walnut Creek watershed near Ames. Stream discharge within Walnut Creek is driven by the discharge from the subsurface drainage systems. Water use by corn and soybean crops has been measured since 1991 and revealed that precipitation was almost equally divided between crop water use and evaporation during the off-season and subsurface drainage. These amounts vary depending upon the total precipitation for the year; however, less than 10 percent moves below the subsurface drains into ground water. Crop water use patterns vary throughout the growing season in response to tillage and crop residue management practices. Nitrate-nitrogen discharge is closely coupled with the precipitation patterns. Observations across the watershed revealed that precipitation and subsurface drainage were closely related at all spatial scales. These relationships showed that total annual discharge was linear to total precipitation for a given land area. Nitrate-nitrogen concentrations varied throughout the year with the highest concentrations in the early spring and the lowest during the late summer.