SUBSURFACE DRAINAGE IN IOWA AND THE WATER QUALITY BENEFITS AND PROBLEM

Abstract

It is estimated that there are approximately 3.6 million ha of land with artificial subsurfacedrainage in Iowa, with 2.4 million ha of that within the 3000 organized drainage districts (totalland area of the state is 14.6 million ha). This drainage has made otherwise wet soils veryproductive. Much of this drainage was installed early last century and is reaching the end of itsservice life. One challenge will be the repair/replacement of these drainage systems. Becausesubsurface drainage short circuits some infiltrating water back to surface water resources, thereis also a water quality challenge. Research has shown that during rainfall-runoff events, thepresence of artificial subsurface drainage generally delays and reduces the volume of surfacerunoff. Therefore, total losses of sediment, phosphorus, ammonium-nitrogen, pesticides, andmicro-organisms are decreased with subsurface drainage. However, nitrate-nitrogen leaching isincreased with subsurface drainage water, and has been implicated as a major factor relative tohypoxia in the Gulf of Mexico. Research has identified several factors relative to soils, weather,and management (cropping, tillage, chemical application practices, and drainage parameters) thatinfluence the nitrate-nitrogen leaching problem. This will be discussed along with implications forpossible changes in the drainage systems and land management that may be needed to sustainproduction while reducing nitrate-nitrogen losses.