Temporal Subsurface Flow Patterns from Fifteen Years in North-Central Iowa

Subsurface drainage in the Upper Midwest is of importance to agricultural production. However, proper management of these systems through in-field management, drainage management, or edge of field practices is needed to limit negative environmental impacts particularly from nitrate-nitrogen leaching losses. One management practice being considered is drainage management where the outflow of subsurface drainage is managed to conserve water and decrease the overall outflow of subsurface drainage. To understand how and when drainage management may be utilized in the upper Midwest it is important to review long-term drainage data to understand the timing, duration, and volumes of subsurface drainage in these climates. An on-going drainage study from north-central Iowa allows for reviewing fifteen years of subsurface drainage which encompasses a range of climatic conditions. This information has been reviewed with the objective of understanding the timing, duration, and drainage volumes considering temporal drainage flow patterns. In particular, the monthly and seasonal flow patterns have been investigated using this long-term drainage record. On this site with a relatively narrow drain spacing of 7.6 m, drainage volume was approximately 40% of the precipitation. The time period from April through June had approximately 50% of the average annual precipitation and approximately 70% of the average annual drainage. In addition, the percent of annual drainage occurring after August 1 was only approximately 7%. The timing of subsurface flow in these areas specifically during the spring coincides with time of planting, crop germination, and early crop development has implications when considering drainage management practices and the effectiveness of these practices to limit flow and therefore nitrate-nitrogen leaching losses. To minimize outflow of drainage water, these drainage management systems would need to allow for adequate flexibility to ensure crop production while effectively managing subsurface drainage flow to potentially minimize the outflow of water