Critical conditions for rill erosion are determined through laboratory experiments and one-dimensional numerical simulations. Relationships between rill flow characteristics and slope angle $theta$ or plot size are studied both experimentally and numerically for realistic rainfall intensities. The experimental results show that maximum rill flow velocities occur within a specific range of slope angles, $theta$ = 30¿--40¿, which is independent of plot size and degree of overland flow concentration. For a given plot size and rainfall intensity the threshold slope angle for rill erosion depends on a number of factors, including soil erodibility, the degree of overland flow concentration, and the effect of gravity on soil erosion. The effect of plot size on the stable rill width generated by rill erosion is also discussed along with its implications for soil loss predictions.