| In view of the serious problems of soil erosion on sloping farmland in purple soil area,we took the sloping farmland in the purple hilly area of Sichuan basin as the research object,combined theories and techniques such as laser scanning,Arc GIS,multifractal,wavelet analysis and Hilbert-Huang transform method through simulated rainfall experiments to study the occurrence and development of rill erosion under different slope gradients and rainfall intensities,to clarify the influence of microtopography on slope confluence and hydrological-sediment connectivity,to investigate the hydrodynamic mechanism of rill erosion processes,and to reveal the coupling mechanism between microtopography change and spatiotemporal variations of runoff and sediment yield in rill evolution process on tilled slopes.The main results are as follows:（1）The surface roughness values ranged from 173-202 mm for the linear slope（CK）,187-219 mm for the artificial digging slope（AD）,and 198-246 mm for the ridge tillage slope（RT）under different slope gradients and rainfall intensities.In general,the surface roughness values showed that RT>AD>CK.The microtopography of different tilled slopes had strong multifractal characteristics and spatial autocorrelations,and the spatial heterogeneity of microtopography increased and the spatial autocorrelation range decreased with the evolution of rill erosion.The spatial autocorrelation range of tilled slopes ranged from 0.7-3.3 m,which provided a theoretical basis for the selection of sampling spacing in the later microtopographic scale study.The rill development intensified the degree of microtopographic undulation,enhanced the spatial heterogeneity of microtopography,and sharpened the microtopographic morphology.（2）The length of 4th order stream on different tilled slopes gradually increased with the evolution of rill erosion,and the length of 4th order stream showed that the CK>RT>AD.With the increase in microrelief,the runoff distribution was more concentrated.The flow length（l F）values on different tilled slopes were mainly concentrated in 0-500 mm,and with the evolution of rill erosion,the l F values all shifted to longer flow lengths.The hydrological connectivity ratio on each tilled slope increased rapidly at the early stage of rainfall,and then gradually stabilized,which was less than 1.The hydrological connectivity of tilled slopes showed that the CK>RT>AD.The sediment connectivity of tilled slopes showed that AD（-2.14）>RT（-2.18）>CK（-2.59）.The formation of rill significantly enhanced the sediment connectivity on the tilled slopes.The slope and rainfall intensity factors promoted the development of hydrological-sediment connectivity on tilled slopes.（3）The Reynolds number and Froude number of rill flow on different tilled slopes gradually increased with the increase of slope gradients and rainfall intensities,and the flow regimes on different tilled slopes changed from subcritical transition flow to subcritical turbulent flow with the evolution of rill erosion.The critical rill flow shear stress was 1.214,2.505and 3.323 Pa for the CK slope,AD slope and RT slope,respectively,the critical stream power was 0.151,0.205 and 0.267 N m-1 s-1,respectively,and the critical unit stream power was 0.002,0.003 and 0.005 m s-1,respectively.The flow velocity was the optimal hydraulic parameter to characterize the rill detachment rates on different tilled slopes.The stream power was the optimal hydrodynamic parameter to characterize the rill detachment rates of the CK and AD slopes,and the flow shear stress was the optimal hydrodynamic parameter to characterize the rill detachment rate of RT slope.The flow shear stress was the best hydrodynamic parameter to characterize the spatial heterogeneity of microtopography,while the Froude number and the unit stream power were the best hydrodynamic indicators to evaluate the rill morphological characteristics.（4）Compared with the CK slope,the average runoff generation time of the AD and RT slopes was delayed by 15.7 min and 24.3 min,and the runoff yield was reduced by 8.4-50.2%and 17.3-75.0%,respectively.The runoff and sediment yield control benefits on different tilled slopes gradually decreased with the increase of slope gradients and rainfall intensities.The effect of microtopography of AD and RT slopes on sediment yield changed from inhibition to promotion under the rainfall intensity of 2.0 mm min-1 on 20°slope.The proportion of rill erosion rate to the slope soil erosion rate was 72-87%,and rill erosion rate on different tilled slopes increased as a power function with the increase of slope gradients and rainfall intensities.The effect of slope gradient on soil erosion on RT slope was greater than those of CK and AD slopes.（5）The Hurst indexes of runoff and sediment yield on different tilled slopes ranged from0.708 to 0.899,which exhibited positive long-range correlation behavior,and the persistence of this state was generally shown as CK>AD>RT.The time series of runoff and sediment yield on different tilled slopes varied periodically every 3-32 min during rill erosion processes,and the fluctuations and the dominant periods of runoff and sediment yield on different tilled slopes tended to increase and decrease respectively with the increase in microrelief.Based on the Hilbert-Huang transform method,the runoff and sediment yields on different tilled slopes were all decomposed into five intrinsic mode functions（IMF）and one residual value.For the high-frequency components（IMF1-IMF3）,rougher microrelief led to increase in the periods of runoff and sediment yield,while the trend of the low-frequency components（IMF4-IMF5）was opposite to that of the high-frequency components.Tillage-induced microtopography altered time-dependent intrinsic correlation of runoff and sediment yield during rill erosion processes. |
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