| Soil erosion is the result of the interaction of erosive rainfall and soil, rainfall is the driving force of erosion, and soil erosion is the result. In purple soil region of Sichuan province, rainfall is the main driving force of soil erosion. Rainfall erosion force reflects this role, it refers to the potential of soil erosion due to rainfall,it is a function of rainfall characteristics. Hilly area of purple soil due to fragile ecological environment and high planting rate of sloping land, soil erosion is very serious. Study to erosive rainfall and erosion characteristics of purple soil is the basic work of master the basic laws of soil erosion,and also provide a theoretical basis and technical support for comprehensive management of soil and water conservation.In this paper, based on rainfall data and 5°,10°,15°,20°and 25°five different slope runoff plot observational data, Characteristics of the rainfall, rainfall erosivity, soil erosion characteristics and erosion factors was studied. The results show that:(1) In Soil and Water Conservation Experiment Station of Sichuan Suining, multi-year average rainfall is 908.78mm, the average number of rainy days os 111.29 days,30% of the number of days throughout the year is rain days. According to Pearson type III curve analysis,1% of annual rainfall for the year is 1389.70mm, annual maximum daily rainfall is 281.20mm; 5% of annual rainfall is 1231.63mm, annual maximum daily rainfall is 204.45mm; 50% of annual rainfall is 897.17mm, annual maximum daily rainfall is 88.73mm; 95% of annual rainfall is 625.52mm, annual maximum daily rainfall is 49.37mm. Throughout the year, June, July, August are the months of concentration of rainfall, accounted for 18.23%,17.79%,17.34% of annual rainfall, total 53.37%, rainfall is about 45.1mm in 12 month and January, February of the following year, only accounted for 4.96% of annual year. The moderate rain is 263mm, light rain is 258.74mm, together accounted for 60% of annual rainfall, heavy rain, torrential rain rainfall ois 162.03mm, accounting for 18% of annual rainfall. Rain events have occurred each month, heavy rain occurred mainly in april to november.(2) In the study area,the average rainfall erosivity values is 4436.29MJ·mm/(hm2·h), rainfall erosivity standard deviation is 2206.51, deviation coefficient Cv is 0.50. According to Pearson III curve analysis,1% of annual rainfall erosion is 12276.08 MJ·mm/(hm2·h),5% of annual rainfall erosivity is 8822.83 MJ·mm/(hm2·h),75% of annual rainfall erosion is 2853.40 MJ·mm/(hm2·h). During the year the value distribution of rainfall erosivity showed single peak, the maximize value of rainfall erosivity is 1213.18 MJ·mm/(hm2·h) in August. June, July, August the three-month rainfall erosivity values accounted for 73.63% of annual. Correlation analysis of annual rainfall and the R values reflects that the correlation coefficient r value reaches to 0.928, significance level is very significant. Through regression analysis, we can use empirical formula to calculate annual rainfall erosivity. Correlation analysis also found that the correlation coefficient r of month rainfall erosivity with the amount of heavy rain is 0.985, then the correlation coefficient of heavy rain with moderate rain is 0.976.(3) Based on the observation data of slop cropland runoff plots, it's found that slop gradient have great significant impact on the number of erosion in the same time series(Correlation Coefficient r=0.968),and the Standard of Erosive Rainfall of Purple Soil in Mid-Sichuan is drafted. The precipitation Standard of Erosive Rainfall of 5°,10°,15°,20°and 25°are 48mm,37.6mm,33.2mm,32.4mm,24.2mm;the average rainfall intensity Standard are 5.26mm/h,5.20 mm/h,4.86 mm/h,4.51mm/h,4.63mm/h; the PI Standard are 292.6mm2/h,262.5 mm2/h,179.45mm2/h,132.8mm2/h,126.5 mm2/h,And get the three relationship of standards and slope gradient.(4)Soil erosion features were analyzed and found that the the relationship between amount of annual soil erosion and annual rainfall is not obvious. Through analysis of different types of rainfall within a month and erosion,5°plot observational data shows that the correlation between erosion and moderate rain, heavy rain gradually increased, the correlation coefficient r are 0.787,0.842,0.912; 10°plot observational data shows that the correlation between erosion and moderate rain, heavy rain,increased firstly and then decreased, the correlation coefficient r are 0.706,0.776,0.773; 15°plot observational data shows that the correlation between erosion and moderate rain, heavy rain increased,the correlation coefficient r are 0.775,0.821,0.801; 20°plot observational data shows that the correlation between erosion and moderate rain, heavy rain gradually increased, the correlation coefficient r are 0.779,0.791,0.853; 25°plot observational data shows that the correlation between erosion and moderate rain, heavy rain weakened firstly then increased, the correlation coefficient r are 0.757,0.754,0.825. Overall, the relationship between heavy rain and erosion is closest.(5)Through correlation analysis we found that slope gradient and annual erosion coefficient r is 0.749, and then analysis of erosion and slope gradient shows that in April, May, June, September the correlation coefficient r reaches 0.93, July the correlation coefficient is only 0.242,August correlation coefficient is 0.738. Through monthly erosion and R value scatter found that only monthly R value reached 900 MJ·mm/(hm2·h) or above then greater erosion will happen. Through correlation analysis,the correlation of rainfall erosivity and erosion is high, the correlation coefficients r values are larger than 0.8, especially in the 5°,10°,15°plot the correlation coefficients reach to 0.9. Soil erosion and rainfall erosivity and slope have relatively close relationship. empirical formula was regressde to calculate the amount of soil erosion.
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