Estimating changes in effective values of surface detention, depression storage and friction factor at the interrill scale, using a cheap and fast method to mold the soil surface micro-topography

Surface detention and depression storage constitute the two conceptual storages of water at the soil surface and depend on the spatial configuration of the micro-topography. To be able to measure directly those storages and investigate how they evolve with time and precipitation, it is necessary to isolate the impact of the micro-topography from the infiltration. Therefore we developed a fast and cheap in-situ molding method (+/−85 €/m2) that combines alginate, plaster and lacquer. It allows creating stable and impermeable artificial micro-topographies that reproduce real field situations and that can be submitted to various laboratory runoff simulations. Both the surface of a specific soil and its artificial reproduction were measured with a laser scanner in order to assess the quality of the molding method. This method is shown to be precise with a standard deviation of 0.55 mm, which is also the spatial resolution of the laser scanner method.This novel molding method was applied to get ten footprints of the micro-topography of a loamy bare soil, at different levels of cumulative rainfall erosivity value (R), starting with a tilled soil, and letting it evolve with natural rain events. We studied the evolution of the bulk values of depression storage and surface detention at the interrill scale, considering that these bulk values may provide useful input to hillslope models for which the size of the single grid cell corresponds to the size of our entire molds. We related the hydraulic properties to R, assuming that R would help explain the change in hydraulic parameters of micro-topographies over time. We found a general decrease of maximum depression storage and steady state surface detention with R.We finally parametrized surface detention by an effective friction factor, using a classical equation for an equivalent laminar sheet flow. A simple model to compute the surface detention as a function of the discharge per unit width and the cumulative rainfall erosivity was proposed.

Research Highlights
► Soil surface may be reproduced with alginate and plaster, within 1 mm of precision.
► Surface detention and depression storage decrease with the cumulative rainfall erosivity.
► Random roughness is less sensitive to that evolution.
► Laminar sheetflow with an effective friction factor estimates well surface detention.