Modeling Research On Estimating Soil Erosion Rates Using Lead-210, Beryllium-7, And Cesium-137 And Numerical Simulation

Soil erosion and its causing soil quality degradation are one of the critical environmental problems in the world?Not only does it result in the soil deposition,the declining productivity, and influence agriculture productivity and food safety,but also it can bring the serious influence to human survival and the development of social economy with the runoff and sediment transport of different pollutants. Soil erosion may not only cause onsite degradation of the natural resource base, but also offsite problems-downstream sediment deposition in fields, water pollution,eutrophication and reservoir siltation etc. Soil erosion could has potential risks of destroying ecosystem and bringing huge harm to economic benefits. Therefore, it is of vital importance for our country to have a correct understanding and handling of soil erosion problems, which concerns deeply the future and destiny of the nation.China is one of regions with the most serious soil erosion and water and soil losses all over the world. The land degradation is very serious in our country, which displays in the great areas of soil erosion, and it makes ecosystem productivity generally reduced. The soil erosion has constituted serous threat to food security,ecological security, social and economic sustainable development in our country.The study of soil erosion law has an important significance for ecology system construction and the recovery of ecological environment. However, there are some difficulties to study soil erosion processes in details by the traditional monitoring techniques because of itself limitation. The nuclear tracer technology applied in studies on soil erosion has been taken attention to studying some laws of soil erosion because of its particular advantage. Radionuclides tracing technique displays marked characteristics and advantages in soil erosion research. It was necessary to develop applying scope of the nuclides tracing technique with the research improvement, and to promote accuracy of tracing result, besides, it provides effective technical support for further understanding the soil erosion processes and establishing soil erosion quantitative model. At present,tracing technique using environmental radionuclides has been widely applied in the research of soil erosion and environmental evolution;it has been the major component of the research of catchments and environmental systems. Application of radionuclide as a tracer to study soil erosion is a hot spot in the world. The main conclusions as follow:1. Response of 210Pbex inventory to changes in erosion rates in soil.This analysis studies changes in the variation 210pbex inventory in soil in response to soil erosion on cultivated and uncultivated lands. The models were created to fit the response of 210Pbex inventory to variations in soil erosion rates on cultivated and uncultivated lands using the principle of mass balance. By numerical simulation of the variation in the soil erosion rate in soil of cultivated and uncultivated lands, we prove:(1) The view that fallout 210Pbex could be used to characterize average soil erosion rates over the long term (in the last 100 or 200 years) is not scientifically reasonable. One can conclude a minimum duration time (100 years) should be needed to obtain a steady state for the fallout 210Pbex concentration in the soil profiles after the changes of the erosion rate in the cultivated and uncultivated lands. The duration time (100 years) is not the time characterized by soil erosion rates by using 210Pbex measurement.(2) When the rate of erosion changed in the cultivated and uncultivated lands,210Pbex area activity in the soil profile responded rapidly and decreases or increases approximately exponentially with the passage of time. The area activity increases with the decreasing rates of soil erosion, and vice versa. The rate of soil erosion is inversely proportional to the 210Pbex concentration. Fallout 210Pbex area activity in the soils drops rapidly in the first several years after the erosion rate changes, then apparently changes slowly after 20 years, with very little change after 50 years, and reaches a state near equilibrium after 100 years.(3)In the cultivated and uncultivated lands, the changes in erosion rates are related to the relaxation mass depth and the time needed for a steady state of fallout 210Pbex to develop. The data illustrates the smaller the change of volume of erosion rate, the smaller the time needed for fallout 210Pbex to reach equilibrium, and vice versa. Changes in the relaxation mass depth can affect erosion rates in the cultivated and uncultivated lands. The smaller the relaxation mass depth, the greater the soil erosion rate change, and vice versa.(4) We make a comparison between the response model of fallout 210Pbex inventory to the changes of erosion rate in the uncultivated land and the response model in cultivated lands. The results show that no matter whether uncultivated or cultivated lands are being discussed, the tracing mechanism is almost uniform. The similar behavior of fallout 210Pbex in soils makes it a satisfied alternative to 131Cs for measuring soil erosion rates. Fallout 210Pbex can not only be used to quantify the rate of soil erosion for a past 100 years but also investigate the response of soil erosion to the recent land use change.2. Quantitative model to estimate soil erosion rates using Beryllium-7 for single-rainfall.A quantitative model was created to estimate the rates of soil erosion using Beryllium-7 for individual events based on mass models method. A method to simulate the relationship between the variations of inventory before and after single-rainfall and soil erosion depth can obtain the state of change of 7Be inventory.The results were compared with the model of Walling. The results prove:(1) Fallout 7Be area activity in the soils is inversely proportional to the rate of soil erosion after the rainfall. The area activity decreases with the increasing rates of soil erosion, and vice versa.(2) Comparing the results with model of Walling, the response of 7Be inventory to the rates of soil erosion in soil is identical. But, the rate of soil erosion corresponding to the same 7Be inventory is lower. This is because of selecting different background values. The calculated results of the rates of soil erosion using model established in this paper can improve the accuracy,which are more suitable to estimate the rates of soil erosion.3. Quantitative model to estimate soil erosion rates using Beryllium-7 for seasonal periods.Establishing the model with the relation between the soil erosion tracer and the rate of soil erosion was the key to estimating soil erosion. Used the mass balance model for creating the quantitative model to estimated soil erosion, which was based on the distribution characteristics of 7Be, the possibility of redistribution, the 7Be quantity before and after experiment and the coefficient of exponential. The relation between the 7Be quantity and the depth of soil erosion in the soil profile, sets up a quantitative model. According to the mass balance model, getting the quantitative model. With the depth of soil increasing by power function, the 7Be quantity in soil profile decreased. During constructing the model, the 7Be quantity around seasons,apparent activity,mass depth were considered. The result of model proves that these make a great difference to the rate of soil erosion or accumulation in soil profile.4. This article is presents a quantitative model that relates the amount of ceasium-137 lost from the cultivated soil profile to the rate of soil erosion. We have fully considered the following facts when we constructed the mentioned model According to mass balance model. They are the distribution of 137Cs in the surface enrichment layer, the thickness of the surface enrichment layer (Hs), the depth of plough layer (Hp), input fraction of the total ceasium-137 fallout deposition during a given year t (Ft),infiltration parameter (?) and sampling year (t).The simulation results show that the amounts of erosion rates estimated by using this model are very sensitive to the distribution of ceasium-137 in the surface enrichment layer and varying parameters Hs and Hp. We can also find that the relationship between the rate of soil loss and ceasium-137 depletion is neither linear nor logarithmic, it is a very complex one.