Cropland Change In Region Of Brahmaputra River And Its Two Tributaries In Tibet During Over 300 Years

Human activities especially reclaiming land that lead to the Land Use / Land Cover Change(LUCC) has being one of the main driving force for global environmental change. LUCC is the important content of global change research. The study in reconstructing historical land use / land cover change not only can reveal the evolution of the relationship between human and land of historical periods very well, but also can provide a theoretical basis for simulating past climate change process and diagnosing climate formation mechanism. And LUCC in the past 300 years has become one of the core area in study of global change. Cropland is one of the most common form of land use on earth, it is a land type that most affected by human activities. Since the Neolithic age, human began the long process of land reclamation and ecological fragile area of cultivated land reclamation’s response to the global change is particularly prominent.Tibetan Plateau with high altitude, cold climate, poor natural conditions and fragile ecological environment become the sensitive and promoter region of global climate change. Studying for Land reclamation of historical period in Qinghai-Tibet Plateau is not only the specific way to participate in the global environmental change, but also can provide the comprehensive research of land use change with abundant regional information, there is important significance for studying history in our country even the whole world of land use/cover change research. The region of Brahmaputra River and its two tributaries in Tibetan Plateau pastoral transitional zone is one of the important typical agricultural area, and is the area with the most intense land reclamation activities and the fastest population growing. Proceeding deep historical data mining in the study area to reconstruct the cropland spatial patterns over the past 300 years has important significance to study the human land use activities under the background of global climate change. Therefore, this article extracted the useful information from the historical documents(such as local Chronicles, cadaster and the survey data, etc.), and correction processing to obtain the cultivated land quantity over the past 300 years in region of Brahmaputra river and its two tributaries. Then we used the grid distribution model of cropland which suitable for valley in Tibetan Plateau, and the cultivated land quantity allocated into 500 x 500 m grid to establish spatial properties of cultivated land data. The results are as follows:(1)Using the grid model designed for Tibetan plateau valley to allot cropland, combining with the actual situation of region of Brahmaputra River and its two tributaries to modify the model. In terms of choice of factor more consideration to the characteristics of the cold in the study area, the maximum altitude distribution determined for 4700 m, slope up to 15 °. While the heat is the key factor in region of Brahmaputra River and its two tributaries, therefore we choose 0 ℃ accumulated temperature instead of climate production potential. In the process of reconstruction, we found the spatial distribution of cropland land space is wider than it really is. In combination with field investigation and historical documents records, in this paper, the degree of appropriate reclamation of cropland level should be set a threshold value, when the threshold is 0.065, the results tend to be more practical.(2) The cropland area of the past 300 years was reconstructed by using the cadaster, land survey data, local chronicles and population data. The results indicated that the cropland area in the region of Brahmaputra River and its two tributaries over the past 300 years has increased and the increased area was 29430hm2 from 1730 s to 1950 s. Because of the malpractice of feudal serf system emerged and the influence of factors such as war, the magnitude of cropland expansion in 19 th century is less than that in 18 th century.(3)Spatially, cropland concentration distribution in Lhasa, Shigatse, Dagzê, Qüxü, Gyangzê, Gyangzê, Lhatse, Gonggar and zhanang country, which located in broad valley with warm climate, abundant sunshine and fertile soil. Cropland distribution has the characteristics of vertical in different elevation and slope, and the characteristics of the level differences in the different physiognomy types. The reclamation index of the whole study area has increased. The average reclamation index increased from 0.56% to 1.01% and the maximal reclamation index increased from 23.37% to 49.97% over past 300 years. But the overall reclamation index is too low.