Modeling wash load transport of soil in a headwater catchment cultivated by non-tillage methods

Abstract

Records of suspended sediment load in a headwater catchment (10.4 ha) cultivated with maize and pasture using non-tillage methods, showed that suspended sediment load decreased over successive floods to an extent independent of rainfall intensity. This trend was also observed in a new flood sequence following a dry period and is consistent with exhaustion of erodible soil. The sources of the loose soil particles is due to the activity of soil fauna and livestock footfalls, which cause soil particles to detach and accumulate on the surface during dry periods. This behaviour was modeled using a distributed model of sediment routing that simulates the transport of suspended solids as wash load. Particle motion is not limited by the energy of raindrop impact, but rather from the sediment lift and transport by surface runoff. Once a particle is moved, it is transported as wash load. The transport rate by wash load is governed by the runoff tractive strength and velocity, as well as by the particle density, diameter and shape. A model of wash load transport based on sediment storage depletion accounts reasonably well for the suspended load concentrations. Transport is influenced by the space distribution of loose particles: simulations show that the total mass of erodible soil, its particle size distribution and space distribution are significant factors affecting the concentrations of suspended load. The transport efficiency ratios varied over the range 0.003–0.247 of total mass of erodible soil.