紫色土坡地侵蚀产沙的7Be示踪研究
文献类型：学位论文
| 作者 | 史忠林
|
| 学位类别 | 博士 |
| 答辩日期 | 2012-05-21 |
| 授予单位 | 中国科学院研究生院 |
| 授予地点 | 北京 |
| 导师 | 文安邦 |
| 关键词 | 紫色土 7Be 侵蚀 堆积 |
| 其他题名 | Using 7Be Measurements to Estimate Rates of Erosion and Sedimentation on Purple Soil Slopes |
| 学位专业 | 自然地理学 |
| 中文摘要 | 环境散落核素137Cs、210Pbex和7Be被越来越多地应用于不同时空尺度的土壤侵蚀堆积速率研究，该技术被认为是传统监测方法的有益补充。天然放射性核素7Be来源单一，半衰期短，是定量研究单次或短期侵蚀性降雨引起的侵蚀堆积空间分布和描述坡面侵蚀过程演变的重要手段。论文采用室外试验和室内分析相结合的方法，研究三峡库区7Be沉降通量，分析其季节性变化的主导因素，查明7Be在土壤中的剖面分布特征及季节性本底值；通过核示踪技术和传统监测技术对比的方法验证7Be法应用于紫色土裸坡次降雨侵蚀堆积空间分布研究的可靠性；引入植被截留因子，修正现有的7Be次降雨侵蚀模型，并与无修正模型对比，验证修正模型应用于有植被覆盖坡面侵蚀堆积空间分布的准确性，扩展7Be示踪技术的应用范围；以模拟降雨为手段，采用7Be示踪技术，结合泥沙颗粒分析，定量判读紫色土坡面侵蚀过程演变。主要结果和结论如下：（1）采样期间（2009-5-31~2010-5-31）研究区7Be沉降通量介于0.4 ~ 5.2 Bq m-2 d-1，平均为2.6 Bq m-2 d-1。该值低于同纬度等雨量其他地区的监测结果。库区7Be沉降通量较低的原因可能主要有：①四川盆地云层较厚，携带7Be的气溶胶在厚厚的云层中停留时间较长而衰变损耗；②冬季富含7Be的高纬度气流受盆地北缘山脉阻挡而无法到达，导致7Be输入量小。库区7Be沉降呈现“春峰冬谷”特征，可能是由于春季对流层折叠变窄7Be“泄露”以及冬季降雨量小，湿清除效率低所致。同时，7Be沉降量与降雨量成正相关。紫色土中7Be集中分布在质量深度0～20.0 kg m-2的表土层，其中0～3.0 kg m-2 土层含量最高，向下随深度增加以指数形式迅速减少，20.0 kg m-2以下基本不含7Be。2008-12-1～2010-1-1期间土壤7Be面积活度介于172.4～328.6 Bq m-2，平均为246.1 Bq m-2，呈现“春最高、秋最低，春夏高、秋冬低”的季节性变化特征。春季土壤中7Be含量高可能是由于研究区地处中纬度，春季对流层和平流层之间气体交换加强，使得平流层中富含的7Be通过气体交换进入对流层，此后随降水输入土壤所致；土壤7Be含量呈现夏季高冬季低，这显然是由于夏季降水量大于冬季的结果；秋季土壤7Be含量最低，可能主要是受植被的影响。植被对7Be的截留明显，特别是当覆盖度较高时可能成为影响地表7Be再分配的主要因素。（2）通过对比试验研究了三峡库区裸坡土壤侵蚀沉积的空间分布，结果表明7Be示踪法可以用于次降雨侵蚀沉积速率估算，并用侵蚀针法和径流小区法分别从侵蚀空间分布和产沙总量的不同角度验证了7Be技术的可靠性。然而本研究选择的土壤为中性土壤，由于7Be在酸性环境中可能发生解吸，因此该技术在其他土壤环境条件下特别是对于酸性土壤的适用性及精度尚有待检验。（3）应用7Be示踪法研究了三峡库区紫色土坡面短期土壤侵蚀堆积速率。结果表明，对于有植被覆盖的坡面，受植被叶面对大气沉降7Be的截留作用影响，现有模型的计算结果会明显高估侵蚀速率，而考虑植被截留因子的修正模型可显著提高坡面侵蚀堆积速率计算准确度，结果更为可靠。（4）放射性核素7Be在土壤剖面中分布较浅，且随深度增加呈指数减少，对坡面侵蚀非常敏感。通过模拟降雨试验，以7Be单核素示踪为主要手段，结合侵蚀过程中泥沙颗粒变化，定量判读紫色土坡面不同侵蚀方式的动态演变过程。结果表明，7Be指示的坡面侵蚀方式转折点与侵蚀泥沙颗粒变化得到的结果基本一致。7Be在表征侵蚀过程演变时具有独特的优势，可以将坡面片蚀发育过程随降雨时间的变化明显地识别出来。这一成果对防治紫色土坡面侵蚀，特别是细沟侵蚀的发生具有重要意义。 |
| 英文摘要 | The fallout radionuclides 137Cs, 210Pbex and 7Be are increasingly being used as a means of obtaining quantitative information on soil erosion and sediment redistribution rates over a range of different timescales, and they are frequently seen to represent a valuable complement to conventional measurement techniques. Due to its steady source and short half-life, the cosmogenic radionuclide 7Be serves as a useful tool for quantifying soil redistribution over shorter timescales, particularly individual storm events or short periods of heavy rainfall, and describing erosion processes. Combining the methods of field investigation and laboratory analysis, this paper aims to: ⅰ) investigate the 7Be deposition flux and the dominant factor for its seasonal variation, as well as the depth distribution of 7Be in soil profiles and its seasonal reference inventory, ⅱ) validate the reliability of using 7Be measurements to document soil redistribution associated with a heavy rainfall on bare purple soil slopes, ⅲ) modify the current 7Be model by taking account of the vegetation interception factor, and test the new model by comparing the results of soil erosion and deposition estimated from both models, and ⅳ) quantify the erosion processes on purple soil slopes by combining the 7Be technique and grain-size analysis, based on the simulating rainfall method. The results and conclusions are as follows: （1）The deposition flux of 7Be was measured in the range from 0.4 to 5.2 Bq m-2 d-1, with a mean value of 2.6 Bq m-2 d-1, during the period of May 31, 2009 ~ May 31, 2010 in the Three Gorges Reservoir region, China. The low annual deposition flux of 7Be was probably due to the adsorption of 7Be within high altitude clouds and the Qinling-Daba Mountain’s barrier and blocking effects on the cold air during winter months. There was a seasonal trend for 7Be deposition, showing a spring maximum and winter minimum, which may be attributed to the seasonal folding of tropopause in spring and little rainfall amount in winter. A strong positive correlation between 7Be deposition flux and rainfall was observed. The depth distribution of 7Be in undisturbed purple soil profiles suggested that the nuclide was mainly distributed within the top 20.0 kg m-2 (~1.5 cm) of soil in depth and the maximum concentration occurred at a depth of 3.0 kg m-2 (~0.2 cm), then the concentration declined quickly and exponentially with depth. The soil horizon below 20.0 kg m-2 mass depth contained little 7Be. The local 7Be reference inventories over the sampling period were 172.4 ~ 328.6 Bq m-2 with a mean of 246.1 Bq m-2. The reference inventories were high in spring and summer and low in autumn and winter, which were highest in summer and lowest in autumn. The interception o f vegetation may be an important factor in reducing 7Be flux to soils. （2）A combination study of 7Be measurements and conventional monitoring methods of erosion pins and runoff plots was carried out on a bare slope. The results clearly confirm the potential for using 7Be measurements to quantify soil redistribution associated with a single rainfall. The information on soil erosion provided by 7Be measurements is comparable with that generated by traditional methods. This study also demonstrated the potential for using 7Be to document soil erosion for neutral purple soil areas. However, further work is required to explore the potential under other soil conditions, especially for acidic purple soil areas. （3）A modified model which takes into consideration the impact factor of vegetation was reported in this paper and the estimation of soil redistribution was compared by using both the conventional and the modified models. Field experiment on 7Be distribution in above-ground grasses and soils was carried out on a 100 m2 grass-covered slope. The vegetation interception factor was determined and the soil redistribution rates were calculated by using the previous model and the modified model. The result shows that nearly one quarter of the atmospherically deposited 7Be will be sequestered by leaf surfaces of herbaceous plants. Soil loss rates on grassland will be remarkably overestimated by using the previous model. The net soil loss estimated from the modified model is more accurate than that derived from the conventional model and the modified model will be more appropriate to estimate soil redistribution rates on soils with significant vegetation cover by using 7Be technique. （4）The erosion processes on purple soil slopes were quantified by combining the 7Be technique and grain-size analysis, based on the simulating rainfall method. The results show that the indicated turning point of slope erosion shape by using 7Be method was in agreement with that derived from sediment grain-size analysis. The development of sheet erosion could be identified by using 7Be tracing technique. The result has great significance in preventing soil erosion, especially rill erosion. |
| 语种 | 中文 |
| 公开日期 | 2013-01-14 |
| 源URL | [http://192.168.143.20:8080/handle/131551/4803]  |
| 专题 | 成都山地灾害与环境研究所_山地表生过程与生态调控重点实验室 |
| 推荐引用方式 GB/T 7714 | 史忠林. 紫色土坡地侵蚀产沙的7Be示踪研究[D]. 北京. 中国科学院研究生院. 2012. |
入库方式： OAI收割
来源：成都山地灾害与环境研究所
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