MATHEMATIC-STATISTICAL SIMULATION OF TOPSOIL PARTICLE LOSSES DURING HEAVY RAINFALL

Knowledge of clay, silt and sand losses during heavy rainfall is still imperfect. The study area on soils on Astian/Quaternary sediments of Roussillon (SE-France) is characterised by a high regularity of betwee n one and two heavy rainfalls per year. Sediment transportation during such singular events represents about 90% of yearly total transportat ion. An increasing number of flood disasters during the last two centu ries has been registered. Therefore, heavy rainfalls of between 25 and 100 mm/h have been simulated on small plots and the transported mater ial analysed. Models are verified during natural rainfalls on differen t field plots. Findings: clay concentrations in surface runoff on the catchment area in nearly-natural environments can be forecast by veget ation cover in %, by slope in degrees and by clay contents in the tops oil in weight %. In nearly all natural and human influenced environmen ts, the best predictor to explain loss and concentration (as total loa d) of silt, fine sand, medium sand and coarse sand is the concentratio n of clay. Model simulations allow distinction between particle size t ransportation on the topsoil caused relatively more by water runoff an d by splash/whirling. Models and analyses are illustrated by graphics.