Contour and conservation tillage for corn and soybeans in the Tama silt loam soil. Hydraulics and sediment concentration

Simulated rainfall was applied to a Tama silt loam soil under six tillage treatments, two crops (corn and soybeans), two crop stages (at planting and one month later), and two row orientations (on the contour and up-and-down the slope, UD). Runoff was greatest from UD moldboard plowed plots and least from plots which had been chisel plowed immediately prior to planting and from no-till plots following soybeans. Non-linear regression models, using plot slope, percent residue and canopy cover as independent variables, accounted for 84% of the variation in average sediment concentration from the UD treatments, but only 62% of the variation in average sediment concentration from the contour treatments. Under steady runoff conditions, 76% of the log of the sediment concentration from either row orientation was accounted for by equations which included either measured runoff velocity or unit stream power calculated from measured runoff velocity.