Within-storm soil surface dynamics and erosive effects of rainstorms (vol 38, pg 131, 1999)

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 confir med that the characteristics of the infiltration curve are modified conside rably 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 du e to drop impact forces. The runoff coefficient is also influenced by the r aindrop impacting energy and it increases sharply with cumulate energy unti l a maximum value is reached. The surface micro-relief dynamics was also st udied. It was very clearly shown that impacting drop kinetic energy is the rainfall characteristic which is linked to random roughness decay. Cumulati ve rainfall was not able to align all the data in a single trend. The effec t of surface micro-relief decay on the rainstorm erosive power was examined using two equations, thus linking Manning's hydraulic roughness to random roughness. Using a simulated runoff over the field plots that were particul arly eroded by the rainstorm, it was possible to observe that the runoff dr ag forces reached values of between 3 and 100 times the ones which would ha ve been calculated if the random roughness had been constant during the sam e event, Many of the soil surface characteristics that are modified interac t with one another and with erosion. Examining each of them in isolation ca nnot explain the drastic increase in erosivity of a rainstorm, as the latte r is the result of the combined effects of all the surface modifications. ( C) 1999 Elsevier Science B.V. All rights reserved.