历史时期董志塬地貌演变过程及其成因

【作者】 姚文波；

【导师】 侯甬坚；

【作者基本信息】 陕西师范大学 ， 历史地理学， 2009， 博士

【摘要】 作为中国黄土高原最大的残留黄土塬董志塬,其地貌演变和土壤侵蚀情况历来为学术界所关注。这是因为:1)历史时期黄土侵蚀地貌的侵蚀过程是单向的,不会出现反复,通过董志塬地貌演变的研究,可以进而了解整个黄土高原的侵蚀状况;2)黄土高原的侵蚀量、侵蚀速率等问题,涉及黄土高原水土流失综合治理目标的设定、黄河下游水患的治理等国计民生大事;3)进入历史时期以后,因人类活动的干预,黄土高原的土壤侵蚀和地貌演变出现了新特点,其具体表现是什么?从土壤侵蚀和地貌演变角度,人为因素和自然因素的贡献分别有多大?如何更好地防、治土壤侵蚀?研究这些问题不仅具有较高的学术价值,现实意义更重大。通过董志塬历史地貌演变的研究,有助于了解黄土高原沟壑区在历史时期的土壤侵蚀情况的特殊性,进而为研究黄土高原沟壑区与丘陵沟壑区不同地貌类型的形成原因提供帮助;有助于人们进一步了解黄土高原沟壑区历史时期土壤侵蚀的特点,同时为有关部门制定相关地区水土流失综合治理的目标、退耕还林(草)等相关政策提供参考依据。选择董志塬作为研究对象,还因为近数十年来大量的实地观测和试验研究虽然成果斐然,但因资料的时间序列太短,无法对其地貌演变和土壤侵蚀的情况达成整体性、宏观性的共识,借助历史地理学方法,可对其在历史时期的地貌演变情况、演变规律和驱动因素作进一步探讨分析,使人们对黄土高原沟壑区的水土流失及其成因形成更加明确的认识。具体的研究步骤为:通过大量野外考察——摄影地形、采集标本、鉴定遗址及文物、观察地理形势等,以获取第一手资料。借助文献记载、古地图、文化遗址等来初步确定样点地貌年龄;以当代地貌发育理论为依据,构建黄土高原现代侵蚀沟地貌年龄的计算公式,进一步确定样点的地貌年龄。根据得到的地貌年龄,测算其侵蚀速率;再以样点流域的平均侵蚀速率,计算董志塬现代侵蚀沟的侵蚀速率和塬面的变化速度,总结董志塬地貌的演变规律。在此基础上分析董志塬地貌演变的成因。通过对董志塬9个完整样本流域和沟头调查所获取的77个沟头样本资料的研究发现,古代侵蚀沟形成于全新世以前,距今有数万至数十万年;历史时期董志塬塬面的萎缩主要是现代侵蚀沟发育造成的,侵蚀活动主要体现为现代侵蚀沟的发育和发生在塬面、坡面上的浅沟、细沟、面状侵蚀。先秦时期董志塬塬面变化不大,最多只是面状侵蚀带来的微量萎缩。战国以后,塬面开始出现明显变化,但直到隋唐时期,也只有大约7条现代侵蚀沟伸入塬面,其造成的影响仍有限。到了隋唐至元明时期,就先后有多达1789条沟谷对塬面造成了影响,塬面开始迅速破碎。明清以后至今,蚕食董志塬塬面的长度在500m以上沟谷的数量又增加了2409条沟谷,因此造成董志塬的地貌演变越来越剧烈,水土流失量也越来越大。历史时期,在现代侵蚀沟发育的地方,董志塬塬面的平均萎缩幅度介于942～6242m之间,最大的萎缩幅度不能超过9000m。没有现代侵蚀沟发育的地区,塬面后退幅度不超过2m,变化十分微小。活跃于董志塬塬面的现代侵蚀沟沟头平均延伸速度介于0.445～4.068m/a之间;最大值一般不会超过6.56m/a;最小值0.25m/a,但这一数值可能会更小。沟头延伸速度的大小,一般情况下与沟头距离流域源头的距离有关:沟头越接近流域上游,延伸速度越慢;受人类干预较明显的沟谷,上述规律会弱化。综合分析地理环境各要素在历史时期的变化,进一步探讨了董志塬地貌演变的成因。自然因素主要表现为地震和降雨径流的变化。在黄土高原地区,当地震烈度达到Ⅳ度时,就会造成黄土崩塌和滑坡;这与之前最小烈度为Ⅴ度或稍低时,才能引起崩塌和滑坡的认识略有不同;说明黄土高原特殊的环境特征,对地震的敏感度很高:地震的影响虽仅限于重力侵蚀,其作用力不容忽视。近2000a来,陇东黄土高原的气候以魏晋南北朝时期(200～589’sA.D.)最为湿润,之前与之后的气候都相对干旱;经历了由相对干旱到湿润,再由相对湿润到干旱的变化过程;董志塬现代侵蚀沟的迅速发育,大约自500’sA.D.前后进入了高峰期,这与同时期的湿润气候有关。坡面上细沟、冲沟的发育,与30min最大雨强呈较好的正比关系;古代侵蚀沟中现代侵蚀沟的发育,主要受降雨量、径流量大小所控制。对于现代侵蚀沟发育较快的沟谷来说,降雨径流的变化只是其中的一个原因,道路、城镇等因素也在起作用,而且后者的影响更为明显;对于更大数量的现代侵蚀沟发育较为缓慢的沟谷而言,降雨径流的影响就比较突出。植被覆盖度与现代侵蚀沟的发育呈良好的正相关,但两者的因果关系尚需进一步研究。植被保持水土的功能,虽已被无数次的观测和试验所证明,但不能过分夸大其作用,一般在塬面、坡面、川台地等处,可以起到良好的减水减沙作用;但在土壤侵蚀最严重的沟道部位,其作用有限。因此,植被对土壤侵蚀和地貌演变,既有正的作用,也有负的作用。间歇性的洪水侵蚀,与经常性的流水侵蚀,孰轻孰重,还难以区分。植被覆盖度越大土壤侵蚀越轻微的规律,只适合面状侵蚀,对于沟状侵蚀,还需要慎重对待。尽管实测数据中,林牧业区的侵蚀模数远大于农耕区,但由于没有将硬化地面和土地利用的影响从中分离,因此,还是不能确定植被覆盖度的影响力。人类活动对黄土高原地貌发育的影响,以硬化地面最为明显。特别是围绕古城镇、古村落、古道路形成了董志塬发育最快捷的的现代侵蚀沟,就这类具体沟谷而言,无论其沟头延伸速度,还是年均侵蚀量,都远大于自然沟谷,是自然沟谷的2～15倍,甚至更大;但是,长度较大的、受人类活动干预较多的沟谷的数量及其在沟谷总量中所占比例有限,对董志塬地貌发育的影响,远没有以往研究所认为的那么严重。自秦汉时期大规模移民垦殖,董志塬塬面地区成为农耕区以后,塬面上的土地利用类型基本以耕地为主:而周围山地,则随着人口数量的起伏变化而有所变化。不同土地利用对黄土地貌发育的影响比较明显;一般说来,在农耕区,因为人口密度大,人类活动频繁,土地利用以农耕地为主,会造成土壤侵蚀较为严重,地貌演变速度较快;林牧业区则相反。但土地利用方式的转变,与土壤侵蚀和地貌演变,并不是简单的正比关系;时地不同,其关系会有所不同。更多还原

【Abstract】 Dongzhi Yuan has been recognized as the largest broken tableland on the Loess Plateau of China, whose geomorphic evolution and soil erosion have been always concerning scientists in related research areas for following reasons. 1) Erosion processes on the eroded landform of the loess in history period occurred in a definite direction and were not repeated. Accordingly, status of soil erosion on the Loess Plateau can be understood through the study of geomorphic evolution of the tableland. 2) The issues such as erosion amount and erosion rate dealt with the national welfare and people’s livelihood, like setting the targets of comprehensive control of soil and water loss on the Loess Plateau and solving the troubles created in the lower reaches of the Yellow River. 3) Some new features of soil erosion and geomorphic evolution have occurred on the Loess Plateau. What are their specific forms? How can we prevent and control soil erosion on the Loess Plateau? Studies of the problems are important not only to academic values, but also to practical meanings.Study of geomorphic evolution of Dongzhi Yuan helps understand the specific characteristics of soil erosion in different historic periods of the gully area on the Loess Plateau and further finds out the causes of geomorphic types in the gully region and the hilly and gully region of loess plateau. Results from the study may be used as a scientific base for some institutions to set the goals of comprehensive soil and water loss control and make policies on converting farmland to forest/grassland in related regions. Selecting Dongzhi Yuan as a study object was also because of the following considerations. In recent decades, tremendous in-situ experiments and observations have been made on the tableland. However, a holistic and macroscopic consensus on geomorphic evolution and soil erosion in it has not yet been reached due to a short time series of observed data. Using the methodologies of historical geography, we can further analyze the geomorphic evolution, evolution processes, and driving factors in various historical periods of the tableland. The analysis may give us a clearer understanding to soil and water loss and its causes in the gully area of the Loess Plateau..Three specific procedures were taken to complete the study. First, basic data were obtained by conducting a large amount of field investigation works on landscape images, sample collection, and ancient site identification, as well as cultural relics and geographical observation. Geomorphic age of a sample site was preliminarily determined by historical documents, ancient maps, and cultural relics. Second, a formula for computing geomorphic age of present eroded gully on the Loess Plateau was constructed based on the theory of geomorphic evolution and then, geomorphic ages of various sampling sites were determined. Erosion rates were estimated according to the computed geomorphic ages. Third, by taking the averaged erosion rate over sampling watersheds, erosion rate of present eroded gully and change rate of upland in Dongzhi Yuan were calculate and its evolution processes were understood. Accordingly, the causes of geomorphic evolution on the tableland were identified.9 typical watersheds and 77 gully heads on the tableland were investigated. Data obtained from the sampled gully heads indicate that ancient eroded gullies formed before the Holocene, more than 10-100 thousands years ago. Historically, the upland of Dongzhi Yuan shrank mainly due to the development of present eroded gullies. Erosion history was mainly presented as the development of present eroded gullies and the occurrence of rill, ephemeral gully, and surface erosion. In the Pre Chin Period, the upland was subject to a slight change and maximally, the slight shrinkage of the upland was caused only by surface erosion. After the Warring States Period, the upland started to change significantly. About 7 large present eroded gullies and valleys did not penetrate into the upland until the Sui and Tang Dynasties, but the influence of the penetration was not considerable. From the Sui and Tang Dynasties to the Yuan and Ming Dynasties, more than 1789 large gullies and valleys developed and the upland started to break. After the Ming and Qing Dynasties, the 2409 large gullies and valleys of more than 500 m which cut into the upland were developed additionally. It caused more and more rapid geomorphic evolution and more and more serious soil and water loss on the tableland.In the historic periods, the average shrinkage of the upland in the positions where present eroded gullies developed was between 942 and 6242 m, with the maximum shrinkage being less than 9000 m. However, the shrinkage of the upland in the area without developed present eroded gully was less than 2 m and apparently, the change was not considerable. The averaged penetration rate of present eroded gully heads was 0.445～4.068 m/a, with the maximum rate being less than 6.56m/a and the minimum rate being 0.25m/a (possibly, the minimum rate even had a lower value). In general, penetration rate of a gully is related to the distance from the headstream of watershed and becomes slow as it approaches the upstream gully head. The feature is not evident for those gulliessignificantly interfered by human activities.Changes of the factors responsible for geographical environment in historical periods are analyzed comprehensively and the causes of geomorphic evolution in Dongzhi Yuan are discussed.Influences of natural factors mainly include the changes by earthquake and rainfall-runoff. On the Loess Plateau, collapses and landslides may occur if seismic intensity reaches IV degree. The finding differs from the point of view that collapse and landslides can occur only if seismic intensity is V degree or less, which indicates that environmental features of the Loess Plateau are highly sensitive to earthquake events. Although gravity erosion is the only consequence of earthquake, but impacts from earthquake are not negligible. In the past 2000 years, the climate of the Loess Plateau in East Gansu was the moistest in Wei, Jin, Southern and Northern Dynasties (200～589’sA.D.). Before or after the period, the climate was in relative drought and changed from relative drought to humid and again from relative humid to drought. The development of present eroded gullies in Dongzhi Yuan reached the highest stage in about 500’s A.D., in response to the humid climate in the same period. The developments of rills and large gullies are proportional to the maximum 30 min rainfall intensity. The development of present eroded gullies within ancient eroded gullies is mainly governed by rainfall amount and rainfall intensity. For the rapider development of valleys with present eroded gullies, rainfall-runoff is one of the factors, whereas the factors like roads and towns also play important roles and impact of the latter is even more evident. However, influences of rainfall and runoff are more significant for those valleys where present eroded gullies develop in a slower rate.Vegetation coverage is positively correlated with the development of present eroded gully, but the relation between the two deserves to be further studied. The roles of vegetation in conserving soil and water should not be over-exaggerated, though they have been proved by tremendous observations and experiments. Generally speaking, vegetation can play sound roles in reducing water loss and sediment only in the places like upland, slope, and flat valley land, whereas its roles are limited in the gullies subject to severe soil erosion. Thus, influences of vegetation on soil erosion and geomorphic evolution can be positive or negative. It is hard to distinguish between the intermittent erosion by flooding and the constant erosion by water. The conclusion that greater coverage corresponds to lower soil erosion is only applied to surface erosion and however, may not be applied to channel erosion. Effects of vegetation coverage are difficult to determine because the effects of harden land surface and landuse are not able to be separated in the analysis, though observed data indicate a greater erosion modulus in forest-pastoral area than cultivated land.Effects of human activities on geomorphic development on the Loess Plateau are the most evident for harden land surface. Particularly, the most rapidly developed present gullies occur in ancient towns, villages, and roads. Both the penetration speed and the averaged erosion amount of the present eroded gullies are greater than those of natural gullies and valleys, being 2-15 times or even more. However, the percentage of the gullies and valleys with longer length and severer interference by human is low and their influences on geomorphic development of Dongzhi Yuan are not so great as estimated by some institutions before. Since the Chin and Han Dynasties, large scale immigrations have made the upland of Dongzhi Yuan become an agricultural area and landuse type has been dominated by cultivated land. Landuse in the surrounding mountainous area has varied somewhat with change in population. Generally speaking, soil erosion in the agricultural area is severe and geomorphic evolution is rapid, due to high density of population, frequent human activities, and dominated cultivated land. The situation in the forest-pastoral area is contrary to that in the agricultural area. However, change in landuse manner is not a simple positive relation with soil erosion and geomorphic evolution and the relation between them is somewhat different, depending on time and location.更多还原