Hydrological responses to early-peak rainfall in unsaturated rooted soils

Vegetation has been commonly used in sponge city to remediate problems related to rainstorm events. Unlike uniform rainfall which has been widely studied, effects of early-peak rainfall on hydrological responses in vegetated soils are unclear. Besides, there is a lack of quantitative method of accurately measuring wetting front (WF). This study aims to propose a new WF tracing method, and explore the hydrological responses to early-peak rainfall in unsaturated soils vegetated with dwarf mondo grass. During soil column tests, WF position, matric suction, volumetric water content, surface ponding and overflow drainage were measured. The new WF tracing method works reasonably well for all cases. As compared to uniform rainfalls, early-peak rainfalls caused (1) earlier onsets of ponding (by 20 minutes for vegetation case and by 5 minutes for bare soil) and overflow (by 52 minutes for vegetation case and by 37 minutes for bare soil), (2) greater overflow velocity (by 28% for vegetation case and by 41% for bare soil), and (3) slightly more total overflow amount. Vegetation delayed the ponding/overflow generations, and decreased total overflow drainage, due to enhanced infiltration of surface soil. At 5 cm depth, high-density mixture of fine and coarse roots caused an increase in the saturated water content (θ_s) and a reduction in the residual water content (θ_r), because of root-induced changes in soil structure. At 10 cm depth, low-density fine roots caused reductions in both θ_s and θ_r, and increased air-entry value, as roots occupy the pores.