Effect of vegetation on the flow pathways of steep hillslopes: Overland flow plot-scale experiments and their implications

Abstract

Overland flow pathways probably play a key role in linking vegetation to its effects on runoff generation and soil loss on the hillslope. However, there is little empirical work testing the links. This study aims to address how flow pathway characteristics vary with plant spacing and determine their relationships with infiltration and soil loss. Upslope inflow experiments were carried out on 10 vegetated plots under a small inflow rate. Total flow path width (RW), mean width per flow path (WPP), and total flow path length (FL), were used to quantify the flow pathway characteristics by analyzing photographs of the plot surface taken during the experiment. The results showed that RW and WPP decreased (R2 = 0.86 and R2 = 0.90), while the FL increased (R2 = 0.67) as alfalfa spacing increased. The RW was positively related to the infiltration rate in the early period of the experiment, indicating that runoff loss via infiltration as affected by the flow paths needs to be considered in hydrological models. Runoff velocity was detected to be significantly related to the flow pathway characteristics. The relationship can partially explain the variances of runoff hydraulics on the vegetated hillslopes in previous studies, in which the variation of flow pathways was not taken into account. The soil loss decreased with the RW and WPP while increased with the FL. As the flow pathway characteristics expressed real-time and actual movement of overland flow, they represented the functional connectivity of sediment on the hillslope. The relationships between soil loss and flow pathway became insignificant as the upslope inflow progressed despite no marked change in the flow pathway. Our results imply that both functional connectivity and the soil's resistance to erosion should be considered to obtain a good prediction of the soil loss from hillslopes.