Re-Saturating Riparian Buffers In Tile Drained Landscapes.

Riparian buffers are effective in removing nitrate moving to streams in shallow groundwater by a combination of plant uptake and denitrification.  However, in the 3.15 million ha of tile drained land in Iowa and the 17.4 million ha of drained land in the Midwest, much of the nitrate laden water leaching from row crop fields is routed directly through the buffers in drainge pipe, bypassing the buffer and emptying directly into surface waters.  To more fully exploit the riparian buffers installed in tile-drained landscapes for removing nitrate, the hydrology between the tile drains and the buffer must to be reconnected.  By intercepting tiles where they cross riparian buffers and diverting a fraction of the flow through lateral distributary tiles into the buffer, we propose to increase the buffer’s capacity for removing nitrate.  An initial trial of this concept is being conducted in the Bear Creek watershed in central Iowa.  Flow from a single field tile has been diverted into the shallow groundwater along 320 m of an existing 27-m wide riparian buffer.  Initial measurements indicate that approximately 0.4 m³ of tile flow can be absorbed per meter of the buffer per day.  Given the average nitrate concentration of tile drainage of 9 mg L^-1 this is equivalent to diverting about 3.5 g of nitrate-N per m of buffer per day.  Shallow ground water observations indicate that at this site all of the nitrate that enters the shallow groundwater of the buffer is denitrified and does not enter Bear Creek.  Thus, re-saturating 320 m of riparian buffer prevents about 410 kg of nitrate-N per year from entering Bear Creek greatly improving the performance of the buffer for removing nitrate in this tile-drained landscape.