Field experiments on quantifying the contributions of Coreopsis canopies and roots to controlling runoff and erosion on steep loess slopes

Grass recovery is often implemented in the loess area of China to control erosion. However, the effect mechanisms of grass cover on runoff erosion dynamics on steep loess hillslopes is still not clear. Taking the typical forage species (Coreopsis) in semiarid areas as subject, this study quantified the effects of canopies and roots on controlling slope runoff and erosion. A series of field experiments were conducted in a loess hilly region of China. Field plots (5 m length, 2 m width, 25 degrees slope gradient) constructed with three ground covers (bare soil; Coreopsis with intact grass; only roots of Coreopsis), were applied with simultaneous simulated rainfall (60 mm h(-1)) and upslope inflow (10, 30, 50, 70, 90 L min(-1)). The results showed that compared with bare soil, intact grass significantly reduced runoff and soil loss rates by 16.6% and 62.4% on average, and decreased soil erodibility parameter by 66.3%. As inflow rate increased, the reductions in runoff and soil loss rates increased from 2.93 to 14.00 L min(-1) and 35.11 to 121.96 g m(-2) min(-1), respectively. Canopies relatively contributed 66.7% to lowering flow velocity, turbulence, weakening erosive force and increasing hydraulic resistance. Roots played a predominant role in reducing soil loss and enhancing soil anti-erodibility, with relative contributions of 78.8% and 73.8%. Furthermore, the maximum erosion depth reduced by Coreopsis was at the upper slope section which was previously eroded the most. These results demonstrated the efficiency of Coreopsis cover in controlling runoff and erosion on steep loess slopes, especially under large inflow rates and at upper slope sections. We suggest protecting Coreopsis with intact grass at upper slope sections, while the aboveground grass biomass can be used for grazing or harvesting at middle and lower slope sections, with roots reserved.