Study On The Characteristics Of Rill Erosion From Contour Ridging Systems In The Purple Soil Area During The Maize Growing Season

Rill erosion is one of the most serious problems in cultivated slope land of purple soil, and it also seriously hinders the sustainable development of agriculture and the sustainable utilization of the soil. With the consistently development of rills'forms the hydrodynamic characteristics of flows are affected which will accordingly cause the evolution of rill erosion. Therefore, an experiment about field artificial rainfall was conducted under different rainfall intensity and the slopes, and determinated the characteristics of rill erosion, like cultivated slope land at different maize growing stages, and revealed the relationship between the sediment yield and dynamics parameters. This research provides a theory basis for the mechanism of water erosion and the prediction of rill erosion. These research results are as followed:1) With the promotion of growing period of maize in the field, the overall variation of rill formation showed a rising-and-falling trend. Rill erosion occurred after jumping ridge or breaking ridge of purple soil during all growing stages. The average interval between drop-pit occurrence and rill formation was 10'59",15'30" at the seedling and jointing stages, respectively, which indicated that rills formed much more easily during the seedling stages. Rills developed completely in morphology at the seedling stage, with the longest rill reaching 77.62 cm in length,6.25 cm in width and 4.04 cm in depth respectively. The average amount of rainfall in rill formation reached 3.17 mm,48.91 mm,81.63 mm and 65.82 mm at seedling, jointing, tasseling and maturing stages, respectively. The developmental process of rill was drop-pit, intermittent rill (breaking ridge), rill evolution, shaping rill in seedling and jointing stages. However, the developmental process of rill was splash pit, splash pit connection (jumping ridge), rill evolution, shaping rill at tasseling and maturing stages.2) There was no significant difference in the sediment yield of rill between 15°and 20° in all stages. With the increasing of rainfall intensity, runoff and sediment yield in different growth periods presented an increasing trend. In general, surface roughness before rainfall (R1) was declined as maize growing in the field. Maximum amplitude of surface roughness after rainfall (R2) appeared at tasseling stage. In the 15 degree slope, the average roughness amplitude in tasseling stage was higher than seedling stage, jointing stage and maturing stage, and increased 2.63 times,1.74 times and 3.21 times respectively, while it increased 2.85 times, 6.54 times,37.74 times in the 20 degree slope. Leaf area index of maize showed a rising-and-falling trend with maize growing in the field. The leaf area index of tasseling stage was 2.34. The grey correlation between various factors and runoff was presented in the following order:rainfall intensity>>leaf area index>vegetation coverage>slope>surface roughness before rainfall>plant height, while the sediment yield was in the order of vegetation coverage>leaf area index>plant height>surface roughness before rainfall>rain intensity>>slope.3) As the growth period of maize promoting, the variation of runoff velocity, its shear stress and flow power temporally showed a rising-and-fall ing trend, while the average depth of runoff generally decreased. The rill flow pattern was "subcritical transition flow" mainly in maize season of purple soil. The rill flow pattern at tasseling stage had a extending trend to "subcritical laminar", while seedling and maturing stages accessed "subcritical turbulent" locally. Manning roughness rate coefficient n was small at tasseling stage, while it was large in seedling stage. The manning roughness coefficient n ranged for 0.088-0.16 in 15 degree slope under 1.0 mm min-1 rainfall intensity. The Darcy-Weisbach resistance coefficient f range for 83.67?140.69 in 15 degree slope under 1.0 mm min-1 rainfall intensity. Variation characteristics of manning roughness coefficient n and Darcy-Weisbach resistance coefficient f was same basically.4) Coupling of runoff rate and unit stream power was the highest at all stages. It was a quadratic function at seedling stage. Fitting degree sediment yield and unit stream power was high, while it was a quadratic function in the seedling stage. The coupling relationship between sediment yield rate and hydrodynamic parameters was worse at the jointing stage. The coupling relationship between sediment yield rate and unit energy was better at the tasseling stage and maturing stages, which was a quadratic function relation. A significantly relationship was observed between volume of runoff and rainfall intensity, leaf area index, velocity during the rill erosion stage in slope maize fields. The regression equations of sediment yield rate and velocity, Froude number, Darcy-Weisbach resistance coefficient f were rated at an extremely significant level.