Hydrological Response of Sloping Farmlands with Different Rock Fragment Covers in the Purple Soil Area of China

Abstract

In the mountainous area of China, the rock fragments in top soils are often removed by farmers because of their side effects on tillage. To understand hydrological processes and to assess the risk of soil erosion in purple soil areas with sloping farmlands, this study investigated the effects of rock fragment cover on surface runoff, infiltration, subsurface runoff, and erosion under field conditions using a portable rainfall simulator. Experimental twin plots (two subplots, 1 m wide by 2 m long, 32% slope) with different rock fragment covers ranging from 0 to 42% were exposed to four rainfall intensities: 45.5 � 1.9 mm=h (I1), 60.3 � 3.6 mm=h (I2), 92.0 � 4.1 mm=h (I3), and 123.9 � 9.4 mm=h (I4). Surface runoff, subsurface runoff, soil moisture, and sediment were measured. The results show that the hydrological response was influenced by the rock fragment cover and rainfall intensity. The rate of surface runoff in bare soils was affected by the rainfall intensity and increased with increasing rainfall intensity. The values of the surface runoff rates in bare soils were lower than 70 mm=h under the I1, I2, and I3 rainfall intensities. As the rock fragment cover increased, the surface runoff rate decreased while the infiltration rate, the subsurface runoff rate and the deep percolation coefficient increased. On the other hand, when the surface runoff rates in the bare soils were higher than 70 mm=h (in rainfall of I4), there were no significant differences in these hydrological variables among soils with different rock fragment covers. The differences in hydrological variables among soils with varying rock fragment covers decreased with increasing rainfall intensity. The rock fragment cover determined the erosive response. As the rock fragment cover increased, the sediment concentration de- creased. The presence of surface rock fragments significantly reduces soil erosion, and the relationship between the soil erosion rate and rock fragment cover can be expressed by an exponential function with a high degree of reliability for different rainfall intensities. However, the effectiveness of the rock fragment cover in reducing soil erosion decreased with increasing rainfall intensity. DOI: 10.1061/(ASCE)HE .1943-5584.0000576. © 2013 American Society of Civil Engineers. CE Database subject headings: Runoff; Rainfall; Soil erosion; China; Rocks; Slopes; Agriculture; Hydrologic aspects. Author keywords: Surface runoff; Subsurface runoff; Deep percolation; Rainwater distribution; Soil erosion rate; Simulated rainfall; Field condition.