CRITICAL CONDITIONS FOR RILL INITIATION ON SANDY LOAM BRUNISOLS - LABORATORY AND FIELD EXPERIMENTS IN SOUTHERN ONTARIO, CANADA

In rainfall simulation experiments on 0.8 m x 10 m plots with agricult ural soils, knickpoint development and incipient rill erosion was asso ciated with distinct ranges of flow intensity, expressed by shear velo city, unit stream power, and unit discharge. Critical threshold shear velocity and unit stream power values were considerably higher than th ose identified in other studies, probably due to differences in experi mental methodology. Rill initiation was found to be a function of loca l flow hydraulic conditions and independent of total plot discharge. T his complicates prediction of this process for effective modelling as some index which links soil microtopography and flow concentration mus t also be incorporated in addition to runoff volume. Rill initiation c ould not be successfully linked to soil shear strength variations meas ured by a torsional shear vane. The vane shear strength values measure d cannot be linked sufficiently precisely to a thin surface soil layer to provide a reliable indicator of rill and interrill erodibility.