Water table control on rill initiation and implications for erosional response

Abstract

On sloping sandy agricultural soil sites near Toronto, Canada, summer storms adequate to generate runoff occur frequently, but rill development occurs mainly in spring when snowmelt or rainfall-induced runoff occurs above frozen subsoil. This suggests that on low and moderate slopes on these soils rill initiation is controlled primarily by hydraulic impedance close to the surface, rather than critical hydraulic conditions in runoff. Laboratory flume experiments were carried out on 10 m slopes at 1.5°, 5° and 9° with loamy sand/clay composite soil sample to test this hypothesis. Runoff with hydraulic conditions adequate for rill initiation occurred rapidly in most tests, but on 1.5° and 5° slopes little knickpoint scour or sediment transport occurred before water table development. This coincided with reduced surface soil strength, knickpoint scour and marked increase in sediment discharge, particularly on 5° slopes where increase was 20- to 30-fold. Further increase in sediment discharge occurred when water tables reached the surface. On 9° slopes runoff occurred more quickly, with higher hydraulic values. Significant rill incision and sediment discharge occurred well before water table development, and ultimately reached much higher values than on lower slopes. Results show that soil erodibility can change dramatically over short time periods during storms due to soil moisture conditions, and that the presence of a hydraulic impedance close to the surface which causes a perched water table to develop can strongly influence rill incision and sediment transport. The influence is unlikely to be marked on soils which are very erodible regardless of moisture conditions, or on extremely resistant soils. It will also be limited on very gentle or steep sites, but can be a significant factor in rill development on intermediate slopes.