Preferential flow is expected to provide an important pathway to replenish soil water at deep soil layers in arid or semiarid areas; however, few studies have addressed this topic, especially in semiarid Loess hillslopes. This study aimed to quantify the effect of stand origin and slope position on the contribution of preferential flow to total infiltration and spatial variations in water flow. A blue dye tracer experiment was conducted to visualize water flow in Robinia pseudoacacia plantation (PL), natural forestland (NF), and natural grassland (NG) at the upslope, midslope, and downslope, and semivariance analysis was used to determine spatial variability at the centimeter scale. The results showed that role of macropore flow was dominant in upslope and midslope, and larger in NF than that in PL and NG, due to presence of abundant root systems in the upper soil of NF. Moreover, contribution of preferential flow at the upper slope in NF was larger than that in PL. At the downslope, the role of rock fragments coverage at the downslope was emphasized, leading lateral flow dominant at the downslope in PL and NG. In addition, contribution of preferential flow and vertical variability of infiltration at the downslope in PL was higher than that in NF. The findings demonstrate that compared with PL, NF has a more positive impact on increasing infiltration and preferential flow that can replenish deep soil water, and reducing surface runoff and soil erosion. The presence of rock fragments coverage can make lateral flow dominant at the downslope on the Loess hillslope, related to water movement along the slope toward streams and catchment outlets.

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Abstract:As a basal measure of soil bioengineering, the living brush mattress has been widely applied in riparian ecological protection forest construction. The living brush mattress shows favorable protective effects on riverbanks. However, there are few reports on the root structure and the soil strengthening benefit of the living brush mattress. The present work reports a series of experiments on root morphology and soil shear strength enhancement at the temporal and spatial scales. The object of the study is 24 living brush mattress trees constructed with Salix alba L. 'Tristis' (LBS hereafter). Traditional root morphology and mechanical measurement methods were used to collect the parameters. The results showed that the root systems of LBS had the characteristics of symmetry and upslope growth. The roots were mainly distributed in a cylindrical region of the soil (radius × thickness: 0.4 m × 0.5 m) and their biomass increased with different growth rates for the periods from 1 to 5 and from 5 to 7 years. Both age and slope position were factors that influence root growth. The root diameter falls within 0-5 mm, has a significant effect on the soil shear strength and provides a conical-shape potentiation zone to ensure the efficient protection of a riverbank. The results of this study demonstrate that LBS is an efficient and feasible engineering measure in the field of riverbank protection.

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