Within-storm soil surface dynamics and erosive effects of rainstorms

This study deals with the characteristics which make a rainstorm an event t hat can produce intense erosion and even trigger the formation of a badland s site. In order to keep the presentation closely Linked to a real situatio n, a rainstorm which took place on an experimental farm equipped for soil e rosion studies was selected, The effects of the erosive rainstorm, which fe ll on dry antecedent moisture conditions, are given in terms of total rill erosion and rill cross-section along the slope. Unfortunately, the data col lected did not answer the basic question, i.e., what combination of factors makes a rainstorm critical? A set of rainfall simulation experiments was t herefore carried out, in the field and in the laboratory, in order to evalu ate the soil surface variations caused by the rainstorm. All the experiment s were performed on dry antecedent soil moisture conditions. It was confirm ed that the characteristics of the infiltration curve are modified consider ably during such rain events. The saturated conductivity of the first thin top-layer is also modified and it can easily decrease by a factor of 10 due to drop impact forces. The runoff coefficient is also influenced by the ra indrop impacting energy and it increases sharply with cumulate energy until a maximum value is reached. The surface micro-relief dynamics was also stu died. It was very clearly shown that impacting drop kinetic energy is the r ainfall characteristic which is linked to random roughness decay. Cumulativ e rainfall was not able to align all the data in a single trend, The effect of surface micro-relief decay on the rainstorm erosive power was examined using two equations, thus linking Manning's hydraulic roughness to random r oughness. Using a simulated runoff over the field plots that were particula rly eroded by the rainstorm, it was possible to observe that the runoff dra g forces reached values of between 3 and 100 times the ones which would hav e been calculated if the random roughness had been constant during the same event. Many of the soil surface characteristics that are modified interact with one another and with erosion. Examining each of them in isolation can not explain the drastic increase in erosivity of a rainstorm, as the latter is the result of the combined effects of all the surface modifications. (C ) 1999 Elsevier Science B.V. All rights reserved.