Runoff initiation from raindrops falling onto planar inclined surface

Runoff initiation from raindrops falling onto an inclined planar impervious surface is investigated in a laboratory rainulator experiment. Visual observations, Forward Looking Infrared imagery and direct cluster statistics measurements indicate that runoff initiation is associated with complex changes in connectivity of drop clusters on the surface. Depending on the surface inclination several different regimes of flow initiation have been observed. For very small inclinations flow initiation is governed by critical clusters merging, a phenomenon known from the mathematical percolation theory. For intermediate inclinations the rivulets regime takes place resulting from clusters non-critical coalescence and sliding. Finally, for high inclinations flow in the form of threads is generated as a result of almost immediate sliding of small clusters and individual drops. The role of several effects is emphasized, such as critical clusters merging, coalescence shrink, and the presence of wetted trails.In particular, critical merging of drop clusters on the surface leads to the abrupt emergence of surface runoff on impervious surfaces with low inclination, e.g. such as most paved surfaces in urban areas.To our knowledge, this is the first study considering the details of flow initiation from raindrops falling onto impervious surfaces.