燕沟流域土壤侵蚀强度演变特征

燕沟流域土壤侵蚀强度演变特征博 士 生： 王晓燕学科专业名称： 土壤学研 究 方 向： 土壤侵蚀新技术新方法研究指 导 教 师： 田均良研究员本文以延安燕沟流域为研究对象，以核素示踪技术为新的研究手段，以研究小流域侵蚀强度演变特征为主要目的，分析了燕沟流域近几十年来由于土地利用变化等因素的影响，坡面核素含量及土壤侵蚀强度随时间变化的特征；对不同植被恢复方式的土壤保持效益进行了评价，并根据侵蚀环境各因素分析，探讨燕沟侵蚀环境与侵蚀强度的演变趋势，为黄土高原生态环境建设和流域治理规划提供科学依据。

同时在研究方法上用空间序列代替时间序列和以线控面的方法，探索了利用核素示踪方法研究流域尺度土壤侵蚀强度演变的可行性和可靠性，为今后利用核素示踪研究大尺度区域土壤侵蚀及侵蚀环境演变提供了研究基础。

1. 阐明了137Cs 在流域土壤和侵蚀泥沙中的分布特点。

研究了不同土地利用类型土壤剖面137Cs 的分布特征，为137Cs 示踪侵蚀量的计算提供了基础信息。

分析了原状土土壤颗粒组成及坝地泥沙与137Cs 含量的关系，137Cs 含量与土壤和沉积泥沙粒径密切相关；通过对林地、坡耕地与沟口洪水泥沙的颗粒组成分析，发现其颗粒组成有明显的差异，表明该研究区在侵蚀泥沙输移过程中发生颗粒分选，应用137Cs 示踪流域侵蚀速率或利用沟口泥沙推算流域侵蚀模数时应适当进行颗粒校正。

通过研究流域137Cs 背景值与降水量的变异性，发现两者变异系数分别为11.14%和20.11%，显然降水的变异性对137Cs 的分布会产生重要的影响。

2. 提出了用坡长加权方法计算坡面平均坡度和坡面核素平均面积浓度、以线代面研究坡面核素含量和坡面侵蚀强度的方法。

该方法通过相关研究方法的验证，表明其具有可行性。

该方法为核素示踪法用于流域尺度土壤侵蚀环境研究提供了新思路。

3. 分析了燕沟流域坡面上137Cs 含量的分布特征。

研究表明，坡面137Cs 含量与坡度的相关程度远远大于坡长；流域不同时期开垦的典型坡耕地坡面137Cs 含量分异非常明显，不同时期开垦坡面上的137Cs 含量与开垦时间、坡度呈线性关系，其回归方程为：Cs=2356.79-22.77t-35.53S ；不同时期的撂荒地坡面上137Cs 含量的变异系数为80.11%。

坡面137Cs 含量主要与土地利用历史有关。

4. 研究了坡面土壤侵蚀与坡度、坡长、坡向等地形因子的关系。

研究表明，土壤侵蚀强度与坡面形状关系密切，坡面各点的侵蚀模数与坡度呈幂函数或者指数函数相关，顺坡面向下侵蚀强度呈波状变化。

坡面土壤侵蚀模数与坡面平均坡度呈正相关，与坡长的相关性不显著。

侵蚀模数与坡度、坡长的复合相关关系式为：07924.0241.118.104L SM 。

谷坡阴坡刺槐林地的侵蚀程度远远小于峁坡阳坡刺槐林地的侵蚀程度，说明坡向通过影响地面植被盖度，进而影响土壤侵蚀的发生程度。

5. 首次利用核素示踪法研究不同耕垦历史的坡面土壤侵蚀特征。

不同开垦历史的坡面侵蚀模数与开垦时间、坡度、坡长的偏相关分析结果表明，侵蚀模数与开垦时间的相关程度最大，其次是坡度，与坡长的相关性不明显。

坡面土壤侵蚀强度随开垦年限的增长呈增加趋势。

根据137Cs和210Pb ex比较示踪，探索了近100年土壤侵蚀强度的变化趋势。

初步结果表明，开垦60的农耕地坡面近50年来平均侵蚀速率明显大于近100年来平均侵蚀速率，表明近100年中，后50年来的土壤侵蚀强度比前50年的侵蚀强度剧烈，说明流域内随着林(草)地的毁坏、开垦加强，人为活动产生的加速侵蚀明显。

6. 探索利用核素示踪法研究不同植被恢复方式和恢复时间的退耕地的土壤侵蚀特征。

研究结果表明，退耕地不同植被恢复方式影响土壤保持效益。

在燕沟流域，土壤保持效益从总体上来说，自然恢复>直接还生态林>直接还经济林。

利用137Cs和7Be示踪法分别探讨了不同时期的撂荒地坡面土壤侵蚀强度特征和面蚀量大小。

根据研究结果，提出了由于短时间尺度的环境变化引起的侵蚀强度变化，利用核素示踪法区分短期土壤侵蚀强度和长期土壤侵蚀强度的研究尚有待深化。

7. 利用核素示踪法研究小流域近几十年来侵蚀强度演变特征。

分析了流域侵蚀环境及侵蚀强度的发展趋势。

过去50年来燕沟流域土壤侵蚀经历了较弱－强－弱的演变过程。

根据前面的研究结果，计算各种土地利用类型的土壤侵蚀模数，利用面积加权平均方法，计算各个时期燕沟流域的平均侵蚀模数分别为：1975年3409 t/km2.a、1997年4647 t/km2.a、2000年为1588 t/km2.a，反演了过去50年里人为活动引起土地利用方式的改变使燕沟人为加速侵蚀发生的变化。

研究得到流域1997年、2000年的侵蚀强度数据与流域所处的侵蚀分区的侵蚀强度划分和监测趋势比较符合，初步表明应用核素示踪流域侵蚀环境变化的方法基本可行。

由现在至以后的一段时间内，延安地区的气候会继续干旱化，燕沟流域的农业产业结构调整逐步合理、人口增长得到有效控制、人口素质和人均收入逐渐提高、各项沟谷治理工程日益发挥作用，人为加速侵蚀将可得到基本控制。

关键词：土地利用侵蚀强度演变核素示踪小流域Study on the Characters of Erosion IntensityEvolvement in Yangou WatershedPh.D. Student: Wang XiaoyanSupervisor: Prof. Tian JunliangIn this paper, Yangou watershed in the Loess Plateau was selected as a case study site. By using radionuclide tracer technique as a new method, the characters of erosion intensity evolvement in Yangou watershed were studied. It’s analyzed that the characters of soil erosion intensity changed with time in the past several decades resulted from the influences of soil utilization type changes. The effects of soil conservation in different vegetation restoration manners were evaluated, and the evolvement trends of the erosion environment and erosion intensity in Yangou watershed were explored according to the analysis of erosion environment factors. Meanwhile, the feasibility and reliability of utilizing radionuclide tracer method to study the erosion intensity evolvement in a watershed scale were explored.1.The characters of 137Cs distribution in the soil and sediment were elucidated.The characters of 137Cs distributions in soil profiles in different land using patterns were studied toprovide a foundation for using 137Cs tracer to estimate soil erosion. The relationship between the concentration of 137Cs and the grain size of soil was analyzed. 137Cs concentrations in clay layer and sand layer in a dam land were also compared. The results show the higher concentration of 137Cs the smaller the soil grain. The mechanical composition of the sediment at the outlet of the watershed has significant difference compared with that of soil in woodland and cropping slope fields, which indicates that there is soil particle selectivity occurred in the course of eroded soil transmitting. This means that the results need to be revised if 137Cs in sediment of the outlet is used to estimate erosion modulus of the watershed. Variation coefficients of 137Cs background value and the amounts of precipitation among different sampling sites of the watershed are 0.11 and 0.20 respectively. . It is obvious that the variability of precipitation would have important effect on the distribution of 137Cs.2.Develop a method to calculate average area concentrations of radionuclides and soil erosion intensity of a whole hillslope by using the average gradient of the hillslope and average area concentration of radionuclide on hillslope with the weighting method of slope length.These methods are proved to be feasible by other relative studies. They may offer new thoughts for studying erosion environment on a watershed-scale by using the radionuclides tracer technique.3.The characters of 137Cs distributions on hillslopes in Y angou watershed were analyzed.The result shows that 137Cs area concentrations on hillslopes are correlated with slope gradients more closely than that with slope lengths. It is evident that there exits great differences among 137Cs areal concentrations on hillslopes of different cultivation ages. There is a linear relationship among 137Cs areal concentrations, cultivation ages and slope gradients. The regressive equation is as follows: Cs=2356.79-22.77t-35.53S. Variation coefficient of 137Cs area concentration is 80.11% among hillslopes with different abandonment ages. The history of land utilization is considered to be the main cause that affects 137Cs area concentrations on hillslopes.4.It was studied about the relationships among soil erosion and terrain factors such as slope gradient, slope length and slope direction.The results show that erosion intensity is related closely to the shapes of hillslope. There exits a power function relationship or exponent function relationship between erosion modulus and gradients at sampling sites on hillslopes. Erosion modulus changes wavily along hillslope. The relativity between erosion modulus and average slope gradients on hillslopes is higher than that between erosion modulus and slope lengths, which indicates that hillslope with higher gradient suffers more serious soil erosion. The relationship among erosion modulus, slope gradient, and slope length can be expressed as: .124107924.0104LM . Slope direction also plays a significant role on soil erosion. For example, soil S18.erosion intensity of hillslope south in direction is lower than that of north slope .5. It is the first time to study characters of soil erosion on cultivated hillslopes with different cultivation ages by using radionuclides as tracer.The result of partial correlation analyses among soil erosion modulus, cultivation age, slope gradient and slope length shows that the correlation coefficient between erosion modulus and cultivation age is the largest, the second one is that between erosion modulus and slope gradient, and the relativity between erosion modulus and slope length is not significant. Soil erosion intensity on hillslopes becomes severer with the increase of cultivation age. According to the results determined from 137Cs and 210Pb ex tracer, the average erosion rates in the last 50 years are much larger than that in the last 100 years on the hillslope lands cultivated for 60 years. So it is suggested that soil erosion is severer in the later 50 years than that in the former 50 years during the last 100 years. It can be explained that soil erosion (especially that caused byhuman activities) became more serious because of forestland and grassland being destroyed and cultivated gradually.6. The research explored erosion characters of abandoned fields with different vegetation restoration manners and ages by using radionuclides as tracers.The effects of soil conservation of different vegetation restoration manners show significant differences on abandoned fields, as a whole, autogenic restoration >constructing ecological forest >constructing economic forest in Yangou watershed. Soil erosion intensity and the amounts of sheet erosion on hillslopes with different abandoned ages are discussed by using 137Cs and 7Be measurements respectively. According to the results, there is a clear need for further studies on distinguishing short-time soil erosion intensity and long-time soil erosion intensity by using the radionuclide tracer technique.7.It is the first time to elucidate the characters of soil erosion intensity evolvement in the past several decades in the small watershed by using tracing method. The evolvement trends of the erosion environment and soil erosion intensity in Y angou watershed were analyzed.In the past 50 years, soil erosion intensity in Yangou watershed undergoes with a erosion change series of weaker-strong-weak. By using the results studied in this work, soil erosion modulus on different land utilization types are evaluated. By mean of the method of area weighting, soil erosion modulus in Yangou watershed are 3409 t/km2.a in 1975, 4647 t/km2.a in 1997 and 1588 t/km2.a in 2001 respectively. The data reappear that acceleration erosion rate changes with the change of land using manners in the last 50 years. Compared on the soil erosion modulus of Yangou watershed in 1997 and 2000 obtained from this study with those determined from the others, it is suggested that the method of using radionuclides as indicators of erosion environment changes in a small watershed is basically feasible. From now on, the climate in Yan’an area may be more droughty, agriculture industry structure will be adjusted more reasonably, the population increases will be controlled effectively, the people stuff and the average income of people will enhance greatly, and all gully control projects will play more and more important roles. All of these would play positive role on reducing sediment production, and the acceleration erosion caused by human unreasonable activity would be controlled in the near future in Yangou watershed.Key Words: Land utilization, Erosion intensity evolvement, Radionuclide tracer, Small watershed。