Soil surface densification under simulated high intensity rainfall

The formation of surface crusts in agricultural soils has adverse effects on seedling emergence and infiltration and causes excessive water loss by runoff. Changes in soil surface characteristics of Zamora soil were examined under high intensity (76.2 mm/hr) simulated rainstorms A hard, dense, several-cm-thick, consolidated layer developed on the soil surface upon drying. The thickness and bulk density of this layer were strongly related to rainfall duration. A strong exponential function was found between the thickness of the consolidated layer and the soil wetted depth. Soil surface strength was strongly correlated with rainfall duration and with the initial bulk density of the soil despite the marked change in the density of the consolidated layer. The largest changes in both strength and bulk density occurred within the first 10 min of the rainstorm. Mechanisms known to be responsible for crusting appear to partly hold for the densification process, but they do not adequately explain the deeper consolidation. Particle reorganization and close-packing associated with settling under saturated or near-saturated conditions appear to provide more logical mechanisms. The relative change in bulk density was inversely correlated with the initial soil bulk density. Measurements of soil strength with a small penetrometer probe indicated an agreement between surface strength and the cohesion of the consolidated layer as determined by a triaxial shear test. Temperature of drying had slight, but mostly inconsistent effects on the strength and density of the consolidated layer. Soil densification was also found to markedly decrease the infiltration rate. The physical properties of both the consolidated layer and the subsoil, however, appeared to control the infiltration process.