Subsurface Drains on Agricultural Land in the Conterminous United States, 1992: National Resource Inventory Conservation Practice 606
This data set represents the estimated percentage of the 1-km grid cell that is covered by or subject 
to the agricultural conservation practice (CP606), Subsurface Drains (SD) on agricultural land by county.  
Subsurface Drains are described as "a conduit, such as corrugated plastic tubing, tile, or pipe, installed …