唐以来方家沟流域地貌的演变与复原

Geomorphologic changes and restoration of Fangjiagou Drainage Area from early Tang Dynasty

以方家沟沟掌台地上的十六国末期赫连伦墓地和沟口汉墓群为依据,确定出方家沟流域现代侵蚀沟的地貌年龄上限为北魏太武帝始光二年(425 A D)。结合现代侵蚀沟地貌年龄计算公式,最终确定其现代侵蚀沟初始侵蚀年代为唐贞元五年(789 A D)前后。根据洪河流域20多年来的侵蚀模数值,计算方家沟流域1 129 a来的侵蚀总量。在1∶10 000地形图上测得方家沟流域现代侵蚀沟的空腔体积。在此基础上,以今天地貌为基准,复原其地貌至唐初,并虚拟绘制出地貌复原图。结果显示,至少到唐初,方家沟流域还是低丘宽谷地貌,不存在深陷的V型谷,V型谷形成的时间要稍晚一些。这种现象普遍存在于陇东盆地泾河的三、四级支流和部分一、二级支流流域。复原结果很好地支持了历史时期黄土高原水土流失日趋严重这一结论。

This paper focuses on the research of landform restoration of the entire watershed - Fangjiagou drainage area, with some breakthrough in research methods and contents. The upper limit of the initial development period of the contemporary erosion ditch is obtained by using the tomb relies＇ sites, and then its initial developmental period is gotten by using geomorphic age formula of the contemporary erosion ditch of the loess plateau. The final result of the contemporary erosion ditch＇s geomorphic ages is got by making the comparative analysis of the two results. The historical landform recovery of the entire watershed is achieved by integrating the landform age, erosion modulus of the Honghe River basin for nearly 20 years, and the data of cavity of the contemporary erosion ditch on 1 ： 10 000 landform map. It is ascertained that the geomorphologic age of the contemporary erosion ditch of Fangjiagou drainage area was 425 A. D. , the 2nd year of Shiguang, Taiwu Emperor, Northern-Wei Dynasty. This is the result based on the information obtained from the Cemetery of Lun Helian in the Sixteen Countries Period （317 - 420 A. D. ） and the Tombs of the Han Dynasty in Fangjiagou Mizoguchi. With the formula of geomorphologic ages of the contemporary erosion ditch, the age in this area was finally defined as around 789 A. D. , the 5th year of Zhenyuan, Tang Dynasty. The total amount of erosion in this area since 1 129 has been calculated based on the da- ta in the recent 20 years. The volume of the contemporary erosion ditch in this area was measured on a topographic map （ 1 ： 10 000）. The landforms were restored to early Tang dynasty, and a geomorphologic map was virtually de- signed in accordance with the data obtained and with the present landforms in this area. The results show that at least in early Tang Dynasty, the landform was a low-lying hill and roomy valley, and there was no chasmal V-model erosion ditch. It was estimated that the V-model erosion ditch was shaped later. It was also found that this phenom- enon is universal in the 3rd and 4th, and some of 1st and 2nd grade＇s drainage areas of the branches of the Jinghe River, Longdong Basin. It is supported by the results of restoration of landforms that soil and water loss was in- creasingly severe in historical period in Loess Plateau. Although it is only one sample, evolutional law of the loess plateau landform in history period can be discovered through the research of a number of samples of the watershed. Therefore the research can provide control in the Loess Plateau.