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题名: 黄土丘陵区坝地沉积序列重构及来源定量示踪
作者: 唐强
学位类别: 博士后
答辩日期: 2016-05
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 傅伯杰
关键词: 黄土高原 ; Loess Plateau ; 土壤侵蚀 ; soil erosion ; 泥沙来源 ; sediment source ; 断代 ; dating ; 生态修复 ; ecological restoration ; 淤地坝 ; check dam
学位专业: 生态学
中文摘要:       黄土高原位于我国北方干旱半干旱区,生态环境脆弱,人类活动强烈,生产力低下,土地利用粗放,是我国典型的水土流失区和黄河泥沙主要来源区。过去几十年来,黄土高原经历了复杂的人类活动过程,实施了一系列水土保持和生态修复措施,导致区域土地利用和覆被格局发生重大变化,驱动流域侵蚀产沙过程持续演变。黄土高原生态治理突出表现在治理模式简单粗放,措施实施范围局限在小区域尺度,治理效益缺乏稳定性和可持续性,治理成果可推广性差等方面。措施的设计和实施缺乏充分的理论认识支撑。因此,开展黄土高原侵蚀产沙过程对人类活动的响应研究,探明各项水土保持和生态修复措施的效益及潜在机理,为当前措施的优化调整提供理论支撑。本研究选取黄土丘陵区典型恢复小流域,通过坝地沉积序列重构及来源复合指纹示踪,揭示淤地坝泥沙沉积过程及基于土地利用类型的来源构成。取得的主要研究结论如下:
     (1)降雨是半干旱黄土丘陵区侵蚀产沙的主要驱动力,降雨特征(强度和频率)决定侵蚀产沙的时间动态过程和强度。侵蚀性降雨主要发生在雨季6~9月,侵蚀产沙和坝地淤积主要发生于频次低、历时短、强度大的次暴雨过程。因此,历史降雨资料能有效指示流域侵蚀产沙和坝地沉积事件,为坝地沉积反演提供重要前提。分析研究区临近的延安气象站 1951~2014年日降雨资料得出,年均降雨量为 520.1 mm,年均降雨日数为 78.2天。其中雨季6~9月,降雨日数占全年总降雨日数 58%,贡献了年总降雨量的 73%;>10 mm降雨日数占全年总降雨日数 15%,贡献了年总降雨量的 56%;>20  mm降雨日数占全年总降雨日数7%,贡献了年总降雨量的 38%。研究区所在的延河流域出口甘谷驿水文站 1953~2012年,年均径流量 1.98亿  m3,雨季 6~9月径流量占年总径流量  65%,47%的年径流量发生在 7~8月;年均输沙量为   0.37亿  t,雨季 6~9月输沙量占年输沙量98.7%,86%的年输沙量发生在 7~8  月。径流和泥沙输出更加集中,主要发生在高强度降雨集中的 7~8月。
    (2)坝地沉积物和流域内来源土壤的粒度、有机质和 137Cs  比活度存在差异,反映了侵蚀产沙过程的差异和土地利用/覆被类型的影响。坝地沉积物颗粒组成以砂粒为主。颗粒组成较流域内表土(沟间地)颗粒细,比冲沟土壤(沟谷地)的颗粒粗,反映侵蚀类型差异对颗粒的分选机制,即沟间地以雨滴溅蚀、面蚀和细沟侵蚀为主,地表土壤中细颗粒组分优先被剥蚀和搬运,而沟谷地以崩塌等重力侵蚀为主,在一次侵蚀事件中,所有颗粒组分都脱离原位而运移。沉积物137Cs 浓度明显高于流域内耕地和冲沟土壤的 137Cs  浓度,一方面可能是由于颗粒差异的影响,即  137Cs在表面积较大的细颗粒上的富集特性决定了经历颗粒筛选后的沉积物 137Cs浓度高于耕地;另一方面,冲沟侵蚀速率快,采集到的沟壁土壤为亚表层土壤137Cs  浓度低。沉积物有机质含量低于流域内沟间地坡面表层土壤,与沟壁土壤有机质含量接近。林地和灌丛有机质含量明显高于草地和耕地,表明植被恢复有利于黄土高原土壤有机碳库的积累。
    (3)通过沉积剖面粒度和   137Cs  的分布,辨识出沉积旋回层结构,结合建坝历史和降雨资料,筛选出有效的暴雨事件,作为反演沉积旋回的年代标记,重构出坝地泥沙淤积时间序列过程。坝地沉积序列未受到溃坝和人为干扰,旋回层结构得到了较好地保存。根据沉积剖面的粒径变化,辨识出了 11个明显的旋回结构,代表次暴雨过程侵蚀产沙的产物。结合坝地建设和历史次暴雨资料,将辨识出的坝地沉积物旋回层与历史侵蚀产沙事件相对应,甄别出 1981~1983年间11次侵蚀沉积事件,估算出次暴雨过程淤地坝沉积速率介于 20~135cm。
    (4)基于泥沙来源复合示踪框架,根据地形分异和土地利用划分出五类来源类型,筛选出 9种具有诊断识别能力的指纹因子,估算了各来源对坝地沉积物的相对贡献。流域泥沙来源按地形分异划分为沟间地和沟谷地(冲沟),再根据土地利用将沟间地进一步划分为林地、灌丛、草地、耕地 4大类。基于坝地沉积物旋回分层,将沉积剖面分层样品合并为 12个混合泥沙样,代表流域产沙。利用统计分析筛选出具有诊断能力的九种指纹因子( Na,Fe,Sr,
Co,Cu,Li,V,Be,C/N),运用混合模型估算了各物源单元对坝地沉积物的相对贡献。结果表明,耕地和冲沟为坝地沉积物的主要贡献来源,但在 12个旋回层中其相对贡献大小存在波动,反映次暴雨过程侵蚀产沙过程的差异。
英文摘要:     The Loess Plateau  in the arid  and semiarid region of  northern China is  a fragile landscape   with  occurrence   of   intense  human   activities,   irrational  land   use.   It represents a typical soil erosion region in China and constitutes a principal source area for sediment yield in the Yellow River. Over the past several decades, complex human activities have  taken place in  this region,  as well as  numerous soil conservation  and ecological restoration  measures  have been  implemented, which  consequently  led to dramatic changes in  regional land use  and land cover pattern,  and constant evolution of  catchment soil  erosion  and sediment  yield  processes.  Some shortages  regarding these measures  has emerged,  including  relatively simple  and crude  measures, small scale  of  implementation,  instability  and unsustainability  of  practices  efficacy,  and unfeasibility of large scale promotion. The design and implementation of conservation measures lack  sufficient theoretical  support. Therefore,  it is  necessary to  realize the response  of soil  erosion  and sediment  yield  to  human intervention,  and  reveal  the potential   effectiveness   and   underlying   mechanisms   of   numerous   conservation practices,   which    finally   will   support    contemporary   strategy   adjustment   and optimization.  In this  study, a  typical  small catchment  in  the loess  hilly region  was selected.   Historical  sedimentation   processes   in   a  controlling   check   dam  were interpreted  and reconstructed  by  stratigraphy  differentiation and  its  land  use-based source    compositions   were    quantified   using    fingerprinting    technique.   Major conclusions obtained are as follows:
    (1) Rainfall  is a major  driving force for  soil erosion and  sediment yield  in the semiarid loess  hilly region,  rainfall pattern  (intensity and  frequency) determines  the timing and  magnitude of  sediment yield. Erosive  rainfalls mainly  occur in  the rainy season  from June  to  September,  while  sediment yield  and  sedimentation  in check dams  mains   occur   in  association   with  low   frequency,  low   duration  and   large magnitude   summer  storm   events.  Therefore,   historical  precipitation   data   is  an effective indicator of  the occurrence of sediment  yield and sediment accumulation  in check dams, which affords an important premise for sedimentary reinterpretation. Analyzing daily rainfall data of 1951~2014 observed at a nearby meteorological station  (Yan’an), it  was found  that  mean annual  rainfall is  520.1  mm, and  average annual  rainfall days  are  78.2  d. During  the  rainy  season from  June  to  September, cumulative rainfall  days  account for  58% of  annual total,  but contribute  to 73%  of annual precipitation. Rainfall  days with intensity larger than  10 mm account for  15% of  annual total,  and  contribute  to  56% of  annual  precipitation.  Rainfall  days with magnitude larger than 20 mm account for 7% of annual total, but contribute to 38% of annual  precipitation.  Mean  annual runoff  observed  at  the  controlling  hydrological station  (Ganguyi) of  Yanhe during  1953~2012  is 0.198  billion  m3, 65%  of which occurs during  June to September  and 47%  of which occurs  during July  and August. Mean annual  sediment load  is 0.037  billion t,  98.7% of  which occurs  from June  to September and 86% occurs  in July and August. Runoff and  sediment yield behaved a much  more  concentrated  pattern  than  regional  rainfall,  which  mainly  takes  place during July and August.
    (2) Difference  in particle  size, organic  matter and   137Cs content  exists between check dam infilling sediments and upland  source materials, reflecting the influence of contrasting sediment yielding processes and land use and vegetation cover. Check dam  infilling sediments is dominated  by sandy fractions.  Its particle size is  relatively finer  than upland  source  materials (inter-gully  slope), but  coarser  than soils collected from gully wall, respecting particle selectivity associated  with different erosion  processes.  Soil erosion  on  inter-gully  slopes  is  mainly  conducted through raindrop detachment, sheetwashing, and rilling. Fine fractions of bulk surface soils are preferentially eroded and moved away,  while all fractions of gull soils are transported during a  signal event  such as mass  failure.  137Cs content  of check  dam sediments is obvious higher  than that  of source materials  from arable land  and gully  wall, which may  be attributed  to  particle  difference between  sediments  and  farmland  soils, on account of  that 137Cs  is preferentially adsorbed  on fine particles with  relatively large specific  surface area.  It may  also  be ascribed  to that  gully  wall soils  is  subsurface material  with  low  level  of    137Cs  content,  due  to  atmospheric-input   fallout   137 Cs content is enriched  in surface land. Organic  matter of sediments is  lower than source materials from inter-gully slopes, but get close to that of gully wall soils. Surface soils from forestland  and shrubs  have higher  levels of  organic matter  than grassland  and arable land, reflecting the effect of vegetation restoration on soil carbon stock.
      (3) According to  depth distribution of particle  size and  137Cs content of the  core profile,  sedimentary  stratigraphic  units  were  differentiated. Effective  storm  events were selected  based  on the  operation history  of  the check  dam and  historical daily rainfall data, which were used as time marker  for the identified stratigraphy units, and finally, sedimentary processes were reconstructed. The sedimentary archives  of the check  dam has not been  disturbed by past  dam failure and  human interruption,  and the  original sedimentary  laminations have  been well   preserved.  According   to   depth   distribution   of   absolute  particle   size,   11 stratigraphic units  were obviously  differentiated. Each one  represents the  product of one storm  event. Based on  operation history of the  check dam and  historical rainfall data, the  identified units  were ascribed  to storm  events, and  the sedimentation  time was marked during 1981~1983. The sedimentation rate ranges from 20 to 135 cm.
      (4) Applying the sediment source fingerprinting technique,  five potential sources were  identified based  on landform  and  land use.  Nine  diagnostic fingerprints  were selected and were used in mixing model to calculate the relative importance. Within  the  source  classification  framework,  the  catchment  was  preliminarily divided as gully  area and inter-gully  area according to landform  property. Inter-gully slopes were further classified  into four categories of forestland,  shrubs, grassland and arable land based on land use. The sectioned sediment samples were mixed to form 12 composite  samples  according  to   the  stratigraphic  differentiation.  Nine  diagnostic properties were selected  using statistical analysis, and  the relative contributions  were estimated  using  mixing  model.  Result   indicated  that  of  the  12  mixing  sediment samples, arable  land and gully  are the two  most important sources,  but their relative importance differs among the 12 events, reflecting effect of varying rainfall property.
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/36962
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唐强. 黄土丘陵区坝地沉积序列重构及来源定量示踪[D]. 北京. 中国科学院研究生院. 2016.
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