| Research on erosion pattern evolution processes from sheet erosion to rill, and then to gully have great importance for understanding soil erosion mechanism and deepening soil erosion process, and provides valuable information for decision-making for arranging soil and water conservation measures. By using research methods of simulated rainfall and measurement of erosion topographic morphology, this thesis is used a dual-box system (one is test box located at down-slope and the other is feeder box located at upslope) and experimental equipment of a runoff input installation and a test box to study erosion pattern evolvement process from sheet erosion to rill erosion, and then to gully erosion. The research progresses on erosion evolvement process from rill erosion to gully erosion, role and contribution of erosion pattern evolvement to hillslope sediment yield, the influences of rainfall intensity, upslope runoff with variable sediment concentrations, and topographical factors on erosion pattern evolvement process, hydrodynamic mechanism of hillslope erosion process have gotten. The main research results are as follows:1. Erosion pattern evolvement processes from sheet erosion to rill, and then to gully as well as their contributions to hillslope sediment yield are qualified. Erosion pattern evolvement processes experience four stages of evolvement process from sheet erosion to rill erosion, rill erosion development, and evolvement process from rill erosion to gully erosion, and then gully erosion development process. Erosion pattern evolvement has great impacts on hillslope soil erosion process. The change processes of hillslope sediment yield are corresponding to erosion pattern evolvement processes. During different stages of erosion pattern evolvement processes, the contribution of gully erosion (rill and gully) to total sediment yield on hillslope is quite different. For example, at 50 mm/h of rainfall intensity and 15°of slope gradient, during initial stage of rill development, rill erosion occupies 58.7% of the total sediment delivery. During the middle stage of rill development and initial stage of gully development, rill and gully erosion accounts for 70% of the total sediment delivery. During the late stage of gully development, the sediment delivery of gully erosion occupies 53.7% of the total sediment delivery. 2. Rainfall intensity and upslope runoff rate have great influences on erosion pattern evolution process. An increase of rainfall intensity enhances erosion pattern evolution process of sheet erosion to rill erosion, and rill erosion to gully erosion, and reduces duration of erosion pattern evolution. Upslope runoff rates have important effects on erosion pattern evolution process and sediment yield too. Upslope runoff discharging into downslope section greatly quickens the speed of erosion pattern evolution and greatly increases hillslope sediment yield. At the same rainfall intensity and slope gradient, total sediment yield and the net sediment delivery caused by upslope runoff obviously increase as the upslope runoff rate increases. The net sediment deliveries caused by up-slope runoff account for 12.8% to 89.5% of the total sediment delivery, which is affected by rainfall intensity, slope gradient, erosion pattern evolution process. The relationship between hillslope sediment yield (S) and the upslope runoff rate(Q) is expressed as S = aQ b。3. The effects of slope gradient, upstream slope length and upslope source area on erosion pattern evolution process are discussed. Slope gradient has significant impacts on sediment delivery and erosion patter development. An increase of slope gradient quickens erosion patter evolution process and reduces duration of erosion patter evolution. Upstream slope length and upslope source area has large influences on erosion pattern evolution process and sediment delivery. At different rainfall intensities and slope gradients, a relationship between sediment yield (S) and upstream slope length (L) is descript as S = kL + b(k>0), and a relationship between sediment yield (S) and upslope source are (A) is expressed as S = kA + b(k>0).4. Erosion pattern evolution process and sediment yield at different runoff rates with variable sediment concentrations are analyzed. The change of hillslope sediment yield is virtual results of erosion pattern evolution. The sediment process was not equilibrium when upslope runoff with different sediment concentrations discharges into the downslope section, that is, the sediment delivery from the upslope section is completely transported, moreover upslope runoff causes additional (net) sediment delivery in downslope section, sediment regimes are always detachment-transport dominated. Under the experimental conditions, the net sediment delivery ( S ) caused by upslope runoff accounts for above 29.1~61.0% of the total sediment delivery ( S ft) at downslope section, which is affected by sediment concentrations in upslope runoff, slope steepness, rainfall intensity, and erosion pattern evolution.5. Hydrodynamic mechanisms during hillslope erosion process are presented. Under the experimental treatments, upslope runoff discharging into downslope section causes an increase of flow velocity, Reynold number, Froude number, and a decrease of Darcy-Weisbach coefficient, all those result in an increase of sediment delivery and quicken the speed of erosion pattern evolution. Meanwhile, upslope runoff discharging downslope section greatly increases flow shear stress, stream power, unit flow energy, and these hydrodynamic parematers also obviously increases with erosion pattern evolution. Sediment delivery has a positive linear correlation to flow shear stress, stream power, unit flow energy, respectively. The critical flow shear stress was 0.712 Pa, the critical stream power was 0.875 N/m·s, and the critical was 0.3294 cm.6. Erosive topographic morphology was measured with probe method in different experimental treatments and DEM was made by Surfer software. The effect of erosion process on surface topography morphology was analyzed. DEM in different periods of erosion pattern evolution directly reflected dynamic changes of surface erosion topographical characteristics and spatial distribution of soil erosion.
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