Water table, drainage, and yield response to drainage water management in southeast Iowa

Subsurface drainage is an important practice for optimizing crop production, but it accelerates nitrate-nitrogen (NO3-N) loss to downstream water bodies. As a result, there is a need for practices that can maintain crop production while decreasing subsurface drainage volume and NO3-N export. The objectives of this work were to evaluate the impact of drainage water management through controlled drainage, shallow drainage, conventional drainage, and no drainage on subsurface drainage volumes, water table depths, crop yields, and NO3-N export. This research was conducted at the Iowa State University Southeast Research Farm and consisted of four management schemes with two replicates for a total of eight plots. Plots consisted of a corn (Zea mays L.)–soybean (Glycine max L. Merr.) rotation with half of the plot planted in corn and half planted in soybeans each year. Findings from four years show that undrained plots had a high occurrence of elevated water tables. Controlled and shallow plots had elevated water tables in the early spring and early fall in accordance with the rainfall and management protocols for controlled drainage. Water table response was quick, with drawdown to tile depth within one to two days after significant rain events. During the period of the study, drainage water management through controlled drainage or shallow drainage reduced overall drainage volume by 37% and 46%, respectively. Average annual NO3-N loss for the study period was reduced by 36% and 29% for controlled drainage and shallow drainage, respectively. Over the four-year period, corn yields in the controlled plots were significantly lower than conventional drainage; however, yields were not statistically different from shallow drained plots. There was no statistically significant difference between drained plots in terms of soybean yield for the study period. Undrained plots, however, had significantly lower yields for corn when compared with shallow and conventional treatments and for soybeans when compared to all treatments. This study highlighted the potential for use of drainage water management practices in reducing subsurface drainage volume and downstream NO3-N loss. In addition, the study highlighted the overall importance of drainage on maintaining crop yields.