Hydrological processes on sloped farmland in purple soil regions with rock fragment cover

Purple soils with variable rock fragment cover are widespread and the effect of rock fragment cover on hydrological processes is important. We quantitatively study the effect of rock fragment cover on infiltration, surface runoff and subsurface runoff through in-situ artificial rainfall experiments. The experimental plots (1 m×2 m and 23°slope) with different rock fragment covers (0%, 11%, 20%, 33% and 42%)were exposed to three rainfall intensities ((53.9±2.8) mm/h, (90.8±6.1) mm/h and (134.3±14.9) mm/h). The results show that the hydrological differences in these experiments are significant and a positive relationship is found between the steady infiltration rate/coefficient and the rock fragment coverage. For the three experimental rainfall intensities, the coefficients of steady infiltration rate range respectively from 47.7% to 86.59% for the intensity of (53.9±2.8) mm/h, from 30.61% to 82.83% for the intensity of (90.8±6.1) mm/h, and from 17.76% to 77.44% for the intensity of (134.3±14.9) mm/h. The steady infiltration rate can be 1.95 to 4.94 times higher on the experimental plot with the most rock fragment cover of 42% compared to that with the least one of 0%. The surface rock fragment cover can retard surface runoff and reduce surface runoff generation. The larger the rock fragment cover, the longer the time for surface runoff generation. The surface runoff rate/ coefficient decreases with the increase of rock fragment coverage. The relationship between the relative surface runoff coefficient and the rock fragment coverage could be expressed by an exponential decay function. Surface rock fragment cover can enhance the generation of subsurface runoff and thus increase the amount of subsurface runoff. The larger the rock fragment cover, the shorter the time for surface runoff generation. Both flow rate of subsurface runoff and subsurface runoff coefficient are proportional to the rock fragment coverage. The relationship between the relative subsurface runoff coefficient and the rock fragment coverage could be expressed by an exponentially increasing function.