A laboratory study on rill network development and morphological characteristics on loessial hillslope

Rills are basic pathways for runoff, sediment, and pollutant transport at hillslopes within agricultural watershed. The objectives of this study were to investigate the development processes of rill network and morphological characteristics and to examine their affecting factors. A soil box (10 m long, 1.5 m wide, and 0.5 m deep) was subjected to four successive simulated rains under rainfall intensity of 90 mm h(-1) with slope gradients of 15A degrees and 25A degrees. Digital elevation models (5 mm resolution) were created from the terrestrial laser scanning measurements. Total soil loss was 46.3 and 61.0 kg m(-2) at the 15A degrees and 25A degrees slope gradients, and rill erosion occupied over 75% of the total soil loss. Soil loss and rill erosion were expressed as power equations to the product of slope gradient and accumulated rainfall. Rill networks evolved in a converging way and reached maturity in the fourth rain. Main rill length and rill width, depth, and degree of contour line departure increased with increased rains, while rill width/depth ratio showed the opposite trend. Secondary rill length and rill density increased in the first two rains, and then both decreased in the latter two rains. Scour effect of lateral interfluve flow and meander cutoffs of rill flow were two sub-processes of rill piracy. Rill length and density decreased due to rill piracy specific in merging of secondary rills into main rills. Plow pan and secondary headcuts played key roles in main rill bed incision and sidewall expansion processes, while both had little impact on secondary rills. Results of this study can improve the understanding of how plow pan, rill piracy, and secondary headcut affect rill network and morphologies and provide fundamental knowledge for designing rill prevention practices.