燕溝流域土壤侵蝕強(qiáng)度演變特征

1、燕溝流域土壤侵蝕強(qiáng)度演變特征博士生：王曉燕學(xué)科專業(yè)名稱：土壤學(xué)研究方向：土壤侵蝕新技術(shù)新方法研究指導(dǎo)教師：田均良研究員本文以延安燕溝流域?yàn)檠芯繉ο螅院怂厥聚櫦夹g(shù)為新的研究手段，以研究小流域侵蝕強(qiáng)度演變特征為主要目的，分析了燕溝流域近幾十年來由于土地利用變化等因素的影響，坡面核素含量及土壤侵蝕強(qiáng)度隨時間變化的特征；對不同植被恢復(fù)方式的土壤保持效益進(jìn)行了評價，并根據(jù)侵蝕環(huán)境各因素分析，探討燕溝侵蝕環(huán)境與侵蝕強(qiáng)度的演變趨勢，為黃土高原生態(tài)環(huán)境建設(shè)和流域治理規(guī)劃提供科學(xué)依據(jù)。同時在研究方法上用空間序列代替時間序列和以線控面的方法，探索了利用核素示蹤方法研究流域尺度土壤侵蝕強(qiáng)度演變的可行性和可靠性，為

2、今后利用核素示蹤研究大尺度區(qū)域土壤侵蝕及侵蝕環(huán)境演變提供了研究基礎(chǔ)。闡明了 137 Cs 在流域土壤和侵蝕泥沙中的分布特點(diǎn)。研究了不同土地利用類型土壤剖面137Cs 的分布特征，為137Cs 示蹤侵蝕量的計(jì)算提供了基礎(chǔ)信息。分析了原狀土土壤顆粒組成及壩地泥沙與137Cs 含量的關(guān)系，137Cs 含量與土壤和沉積泥沙粒徑密切相關(guān)；通過對林地、坡耕地與溝口洪水泥沙的顆粒組成分析，發(fā)現(xiàn)其顆粒組成有明顯的差異，表明該研究區(qū)在侵蝕泥沙輸移過程中發(fā)生顆粒分選，應(yīng)用 137Cs 示蹤流域侵蝕速率或利用溝口泥沙推算流域侵蝕模數(shù)時應(yīng)適當(dāng)進(jìn)行顆粒校正。通過研究流域 137Cs 背景值與降水量的變異性， 發(fā)現(xiàn)兩者變

3、異系數(shù)分別為 11.14%和 20.11%，顯然降水的變異性對137Cs 的分布會產(chǎn)生重要的影響。2.提出了用坡長加權(quán)方法計(jì)算坡面平均坡度和坡面核素平均面積濃度、以線代面研究坡面核素含量和坡面侵蝕強(qiáng)度的方法。該方法通過相關(guān)研究方法的驗(yàn)證，表明其具有可行性。該方法為核素示蹤法用于流域尺度土壤侵蝕環(huán)境研究提供了新思路。3.分析了燕溝流域坡面上137Cs 含量的分布特征。研究表明， 坡面 137Cs 含量與坡度的相關(guān)程度遠(yuǎn)遠(yuǎn)大于坡長；流域不同時期開墾的典型坡耕地坡面137137Cs 含量分異非常明顯，不同時期開墾坡面上的Cs 含量與開墾時間、坡度呈線性關(guān)系，其回歸方程為： Cs=2356.79-22

4、.77t-35.53S ；不同時期的撂荒地坡面上137。Cs 含量的變異系數(shù)為 80.11%坡面137Cs 含量主要與土地利用歷史有關(guān)。研究了坡面土壤侵蝕與坡度、坡長、坡向等地形因子的關(guān)系。研究表明，土壤侵蝕強(qiáng)度與坡面形狀關(guān)系密切，坡面各點(diǎn)的侵蝕模數(shù)與坡度呈冪函數(shù)或者指數(shù)函數(shù)相關(guān)，順坡面向下侵蝕強(qiáng)度呈波狀變化。坡面土壤侵蝕模數(shù)與坡面平均坡度呈正相關(guān)，與坡長的相關(guān)性不顯著。侵蝕模數(shù)與坡度、坡長的復(fù)合相關(guān)關(guān)系式為：M104.18S1.241 L0 .07924 。谷坡陰坡刺槐林地的侵蝕程度遠(yuǎn)遠(yuǎn)小于峁坡陽坡刺槐林地的侵蝕程度，說明坡向通過影響地面植被蓋度，進(jìn)而影響土壤侵蝕的發(fā)生程度。首次利用核素示蹤

5、法研究不同耕墾歷史的坡面土壤侵蝕特征。不同開墾歷史的坡面侵蝕模數(shù)與開墾時間、坡度、坡長的偏相關(guān)分析結(jié)果表明，侵蝕模數(shù)與開墾時間的相關(guān)程度最大，其次是坡度，與坡長的相關(guān)性不明顯。坡面土壤侵蝕強(qiáng)度隨開墾年限的增長呈增加趨勢。 根據(jù) 137Cs 和 210Pbex 比較示蹤， 探索了近 100 年土壤侵蝕強(qiáng)度的變化趨勢。初步結(jié)果表明，開墾 60 的農(nóng)耕地坡面近 50 年來平均侵蝕速率明顯大于近100 年來平均侵蝕速率， 表明近 100年中，后 50 年來的土壤侵蝕強(qiáng)度比前50 年的侵蝕強(qiáng)度劇烈， 說明流域內(nèi)隨著林 (草 )地的毀壞、 開墾加強(qiáng)，人為活動產(chǎn)生的加速侵蝕明顯。探索利用核素示蹤法研究不同植

6、被恢復(fù)方式和恢復(fù)時間的退耕地的土壤侵蝕特征。研究結(jié)果表明，退耕地不同植被恢復(fù)方式影響土壤保持效益。在燕溝流域，土壤保持效益從總體上來說， 自然恢復(fù) 直接還生態(tài)林 直接還經(jīng)濟(jì)林。 利用137Cs 和7Be 示蹤法分別探討了不同時期的撂荒地坡面土壤侵蝕強(qiáng)度特征和面蝕量大小。根據(jù)研究結(jié)果，提出了由于短時間尺度的環(huán)境變化引起的侵蝕強(qiáng)度變化， 利用核素示蹤法區(qū)分短期土壤侵蝕強(qiáng)度和長期土壤侵蝕強(qiáng)度的研究尚有待深化。7. 利用核素示蹤法研究小流域近幾十年來侵蝕強(qiáng)度演變特征。分析了流域侵蝕環(huán)境及侵蝕強(qiáng)度的發(fā)展趨勢。過去 50 年來燕溝流域土壤侵蝕經(jīng)歷了較弱強(qiáng)弱的演變過程。根據(jù)前面的研究結(jié)果，計(jì)算各種土地利用類

7、型的土壤侵蝕模數(shù)，利用面積加權(quán)平均方法，計(jì)算各個時期燕溝流域的平均侵蝕模數(shù)分別為： 1975 年 3409 t/km 2. a、 1997 年 4647 t/km 2. a、 2000 年為 1588 t/km 2 . a，反演了過去50 年里人為活動引起土地利用方式的改變使燕溝人為加速侵蝕發(fā)生的變化。研究得到流域1997 年、 2000年的侵蝕強(qiáng)度數(shù)據(jù)與流域所處的侵蝕分區(qū)的侵蝕強(qiáng)度劃分和監(jiān)測趨勢比較符合，初步表明應(yīng)用核素示蹤流域侵蝕環(huán)境變化的方法基本可行。由現(xiàn)在至以后的一段時間內(nèi)，延安地區(qū)的氣候會繼續(xù)干旱化，燕溝流域的農(nóng)業(yè)產(chǎn)業(yè)結(jié)構(gòu)調(diào)整逐步合理、人口增長得到有效控制、人口素質(zhì)和人均收入逐漸提高

8、、各項(xiàng)溝谷治理工程日益發(fā)揮作用，人為加速侵蝕將可得到基本控制。關(guān)鍵詞： 土地利用侵蝕強(qiáng)度演變核素示蹤小流域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 trace

9、r 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 c

10、onservation 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 radio

11、nuclide tracer method to study the erosion intensity evolvement in a watershed scale were explored.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

12、 forusing137tracer to estimatesoil erosion. The relationship between theCsconcentration of137Cs and the grain size of soil was analyzed.137Cs concentrations in clay layer and sandlayer in a dam land were also compared. The results show the higher concentration of137Cs the smaller thesoil grain. Them

13、echanicalcompositionof the sediment at the outlet of the watershed has significantdifference 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

14、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 variabilit

15、y of precipitation would haveimportant effect on the distribution of137Cs.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 areaconcentration of radionuclide on hillslo

16、pe 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.The characters of 137Cs distributions on hillslopes in Yangou

17、watershed were analyzed.137The result shows that Cs 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

18、 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 utilizati

19、on is considered to be the main cause that affects 137 Cs area concentrations on hillslopes.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

20、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

21、than that between erosion modulus and slope lengths, which indicates that hillslope with higher gradient suffers more serious soil erosion. Therelationshipamongerosionmodulus,slopegradient,andslopelengthcanbeexpressed as:104.18S1.241 L0.07924 . Slope direction also plays a significant role on soil e

22、rosion. For example, soil erosion intensity of hillslope south in direction is lower than that of north slope .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

23、 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 betweenerosion modulus and slope leng

24、th is not significant. Soil erosion intensity on hillslopes becomes severer with the increase of cultivation age. According to the results determined from 137Cs and 210Pbex tracer, theaverage erosion rates in the last 50 years are much larger than that in the last 100 years on the hillslope lands cu

25、ltivated 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 des

26、troyed and cultivated gradually.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

27、 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

28、 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.It is the first time to elucidate the characters of soil erosion intensity evolvement in the past several decade

29、s in the small watershed by using tracing method. The evolvement trends of theerosion environment and soil erosion intensity in Yangou 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/km 2. a in 1975, 4647 t/km 2. a in 1997 and 1588 t/km 2. a in 2001 respectively. The data r