Seasonal Evolution of Surface Detention and Retention Properties with Rain Erosivity, at the Interill Scale

Micro-topography, in interaction with the global slope, triggers and directs surface runoff. By concentrating the overland flow, it can promote the development of eroded pathways, while, by delimiting depressions where water accumulates, it can favor sedimentation. These erosion-deposition processes will in turn modify the micro-topography. The erosion-deposition processes depend on the runoff velocity field. Locally, velocity is a function of the water depth, of the local slope, of the friction of the bed and of backwater effects due to constrictions by obstacles. All those factors will evolve with the history of a particular site, favoring the connectivity of the runoff. According to the spatial patterns of the micro-topography, the runoff may conceptually be distributed among two compartments, each influencing the runoff connectivity: the surface retention and the surface detention. The surface retention (also called depression storage or dead storage) is the amount of water stored in surface pits and depressions. This water will subsequently infiltrate or evaporate. On the contrary, the surface detention corresponds to the water storage in excess of depression storage. It is due to the presence of flowing water and is proportional to the discharge itself. This amount of water will deplete as discharge decreases, and flow away at the end of the rain event. The velocity of the runoff is highly variable in space and particularly between the surface retention zones and the surface detention zones. In order to understand the connectivity evolution of a soil surface subjected to rainfall and runoff, we studied the seasonal evolution of the surface detention and retention hydrologic properties, for a bare soil just after tillage. Since surface detention and retention are not easily measured in situ due to the perturbing effect of the infiltration that occurs simultaneously during a rain event, we developed a fast and cheap in situ molding method (+/- 80 euros/m2) that combines alginic acid, plaster and a lacquer. It creates a stable, and almost impermeable artificial reproduction (to within 1 mm) of the in situ soil micro-topography, preserving the small scale overhangs. Ten molds (0.5 m2 each) were thus made, at 5 different stages during a 3 month period. Rainfall and runoff experiments with a dye tracer have been made on the artificial micro-topographies, under laboratory conditions. For each micro-topography, the volume of the depression storage and its relative surface connection function is computed, as well as the volume of the surface detention and its tortuosity as a function of the rain intensity. From this data set, we propose simple models for the evolution, at the early stage, of the surface detention and retention properties as a function of cumulative rainfall erosivivity.