Paired watershed comparison of tillage effects on runoff, sediment, and pesticide losses

A paired watershed study was used to compare runoff, sediment, atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5 triazine), and cyanazine (2-[[4-chloro-6-(ethylamino)-S-triazin-2-yl]amino]-2-methyl propionitrile) losses from conventional-tilled (CT) and reduced-tilled (RT) corn (Zea mays L.) watersheds. During a 13-mo calibration period both watersheds were moldboard plowed and runoff was measured in H-flumes. Composite samples were analyzed for sediment and for atrazine and cyanazine in dissolved and sediment phases. During a 30-mo treatment period, one watershed was moldboard plowed while the other was disk harrowed. Reduced tillage decreased runoff by 64% and sediment losses by 99% but did not change dissolved atrazine or cyanazine concentrations in runoff. Concentrations of sediment-sorbed pesticides increased in runoff. The RT reduced mass losses of atrazine and cyanazine in runoff, primarily because of decreases in runoff volume and sediment mass. Most pesticide losses occurred within 3 wk of field application. Pesticide loss was primarily in the dissolved phase. The paired watershed approach was a useful technique for determining watershed treatment effects when differences existed between watersheds chosen for study.