Infiltration, runoff and erosion characteristics of agricultural land in extreme storm events, SE France

Abstract

Following the extreme rainfall event of 22 September 1992 in Southeastern France, an attempt was made to characterize the response of areas cultivated under vines to heavy precipitation. This response was determined in three ways by means of rainfall simulation experiments. First, the infiltration rate is calculated as the difference between rainfall and runoff rates. Secondly, the flow depths are measured to allow the flow hydraulics to be calculated. Thirdly, sediment transport, both by splash and overland flow, is sampled. These measurements were carried out as near simultaneously as possible, in order to define any possible interactions. Surface characteristics of the experimental plots were measured to define possible controls on individual responses. Buffering of response of the agricultural areas is seen both in the relatively high infiltration rates, and in the high Darcy-Weisbach friction factors observed. However, a problem is noted in the case of the latter, in that the small plot size used probably leads to the over-estimation of the friction factor. The rainfall rate and the surface slope mainly characterize the infiltration rate, whereas the friction factor is controlled by the flow Reynolds' number and the percentage of the surface covered by stones ( > 2 mm). The erosion rates measured are comparable to those found on vineyards elsewhere in the Mediterranean. Extrapolation from the measured values indicates that up to 3.4 kg m −2 of soil may have been lost in interrill flow from these areas in the event of 22 September 1992. Splash is the dominant control of sediment production in the interrill zone. The splash rates are a function of the rainfall rate, surface slope, water depth, soil shear strength, and the percentage of the surface covered either by stones or by vegetation. The sediment transport rate in overland flow can be shown to follow the characteristic equation of Julien and Simons, as a function of flow discharge and surface slope, as well particle size characteristics of the soil. Alternatively, it can be described using hydraulic parameters such as excess shear velocity, effective stream power or unit stream power, as demonstrated experimentally by Everaert.