The Redistribution Of ~7Be Deposition To The Land Surface And Its Application On The Soil Erosion Processes Of The Slopes

Atmospheric radionuclide ~7Be with short half-life, which has continuous deposition to the earth's surface, was used as a soil erosion tracer to trace event rainfall and short period (rainy season) soil erosion. At present, when use ~7Be as a tracer researching soil erosion, we all think that~7Be has the similar deposition - soil adsorption - migration process with 137Cs and 210Pbex. But in the worldwide, there are almost no researching the key basal problems which distributed in the soil particle, intercepted and adsorbed by the vegetation in the depositional process, and ~7Be content in runoff and sediment. Meanwhile, there are fewer reports about using ~7Be for studying the slope soil erosion process.Combined methods of sampling and experimenting in field with analysis in laboratory, this paper studies the characristics of ~7Be deposition, ~7Be adsorbed and interrupted by plants and its redistribution on the land surface. Then the dynamic change of soil erosion process under simulated rainfall is studied. The main conclusions as follow:1 ~7Be deposition characteristics and its redistribution to the land surface~7Be concentration in the rainwater had a large variation in loess plateau region. The ~7Be monthly deposition flux in the rainy season was larger than that in the non-rainy season. There had a significant linear correlation between ~7Be monthly deposition flux and monthly rainfall. The dry deposition flux of ~7Be was relatively larger in the winter and spring, while smaller in rainy season. ~7Be mainly existed as the residual state of soil. There was little changeable ~7Be. About 65% of ~7Be in the soil was adsorbed by the particle whose size was smaller than 0.001mm.~7Be was absorbed and intercepted by different level vegetation cover. ~7Be mass activity of different plant species was significantly different. ~7Be average mass activity for the different plant types was weeds> shrub> crop. ~7Be mass activity in the leaves was larger than that in the stem and the root. ~7Be mass activity was variable during growth time of plants and was obviously different among plant types, but ~7Be areal activity held by plants showed an increasing tendency during growth time. There was more and more influencing for ~7Be content in soil with plants growing.The rate of ~7Be distribution in soil, plant, runoff and sediment was estimated on the plots. The loss of ~7Be from the cultivated plot accounted for 14.53% of the total ~7Be activity in the entire plot. The loss of ~7Be activity with runoff was 3.21% of the total and with sediment was approximately 9.86%. The loss of ~7Be activity from the wasteland and the scrubland plots accounted for 33.32% and 33.54% of the total ~7Be activity in the entire plot, respectively. At the wasteland plot, the loss of ~7Be activity with runoff was 2.39% of the total ~7Be activity, with sediment was 0.97% and that held by vegetation was 29.96%. At the Scrubland plot, the loss of ~7Be activity with runoff was 1.18% of the total ~7Be activity, with sediment was 0.53% and held by vegetation was 31.83%. Runoff and vegetation as two factors were considered when ~7Be is used to trace soil ersion.2 Precision comparisons of the erosion rates derived from ~7Be measurement modelUsing Walling's model and Yang's model could accurately calculate the sheet erosion amount of the same size plot slope with simulated rainfall. Compared with the measured erosion amount, the absolute error was 6.67% and 1.9% on the 5°plot, respectively. Using Wilson's model could accurately calculate the total erosion amount of the plots slope under simulated rainfall. Compared with the measured erosion amount, the relative error was from3.59% to14.47% in the all plots, and the highest was the plot with no rill erosion. These showed that Wilson's model could make up the shortcoming of Walling's model and Yang's model which could only calculate the sheet erosion amount during event rainfall.3 The spatial distribution soil erosion intensity on the plot slopes derived from ~7Be measurementThe sheet erosion intensity calculated by ~7Be measurement was stronger on the middle-upper part of the plot slope and strongest in the part about 3.5m away from the foot-slope. Sediments appeared in the lower part of the plot slope. The rill erosion intensity by manual measurement was stronger on the lower-middle part of the plot slopes, and was strongest in the part about 1.5m away from the foot-slope. Under general rainfall intensity, the erosion intensity was strongest on the part about 1.5-3.5m away from the foot-slope, and slight erosion on the top and the foot- slope.The sediment caused by sheet erosion mainly derived from the part of 0-2m away from the slope top, accounting for over 60% of the total sheet erosion amount. The sediment caused by rill erosion mainly derived from the lower-middle part of the plot slope, accounting for over 75%of the total rill erosion amount. The sediment caused by erosion mostly derived from the middle-part of the plot slope.According to the results of ~7Be tracing soil erosion, the SDR of the each plot was calculated during event rainfall. The SDR of the all plots varied between 0.687 and 0.987, and was close to 1 excepted for the 5°plot.4 Partitioning the contributions of sheet and rill erosion using ~7Be on the plot slopesThe dynamic evolution processes which were from sheet erosion to rill erosion on the plot slopes were quantitatively analyzed by using Yang's model during simulated rainfall event. Meanwhile, it was found that the measured time when the rill occurred was close to the calculated time by model, and the maximum sheet erosion rate occurred was earlier than the maximum rill erosion rate. The strongest erosion intensity occurred in the early-middle period of the rainfall duration, and this time the sheet and the rill erosion were the strongest. The difference of erosion amount in each plot was mainly due to the rill erosion amount.Combining with the results obtained from ~7Be tracing method, it was found that the rill fractal dimension on the plot slope could not only represent the development process and the complex degree of rill, but also it had a better linear correlation with rill erosion amount, sheet erosion amount and total erosion amount. At a certain extent, it could also reflect dynamic change of erosion intensity on the plot slope during rainfall.5 The change of sediment particle size and organic carbon content on the soil erosion processes during the simulated rainfall eventsThe change of the volume fractal dimension of sediment particles could reflect the change of sediment particle composition during the erosion process. The sediment particle size of 0.001mm was a critical particle size which determined the volume fractal dimension of sediment particle. For the plots only sheet erosion occurred, the volume fractal dimension of sediment particle became relatively smaller compared with the volume fractal dimension of soil particle in the pre-erosion slope. For the plots with both the rill and sheet erosion occurred, they had complex changes compared with the volume fractal dimension of soil particle in the pre-erosion slope. It showed a tendency that the sheet erosion could increase the content of the particle size more than 0.001mm.during the erosion process.The sediment organic carbon content decreased and then tended to be stable as the rainfall continued in the below 25°plot and had no much change in the 25°and 30°plots. The sediment from the plots enriched the organic carbon and the sediment organic carbon content increased significantly compared with the soil organic carbon content of the pre-erosion slope. Sheet erosion in the below 25°plots was apt to enrich organic carbon. The organic carbon content of sediment from rill erosion was smaller than the organic carbon content of sediment from sheet erosion. Both the increase of sheet erosion amount and rill erosion amount could result in decrease of the organic carbon content of sediment. They had a significant linear correlation between the loss degree of soil organic carbon and erosion intensity during rainfall.