| In recently decades, along with the dual role of the global climate changes and human activities, oasis changed rapidly at different temporal and spatial scales. Researches on oasis change has attracted widespread concern and attention of scholars at home and abroad, because it is the most directly reflects of environmental evolution and human activities in arid areas. Along with the implementation of comprehensive research covered multi-disciplinarity, mechanism of oasis change is an increasing important research field of arid zones. The Heihe River basin is the oldest area with large sacle irrigated agriculture in arid zone of northwest China. The human activity changed drastically during the long human history, so the Heihe River basin became one of the most active region in studying environmental evolution and regional sustainable development. Oasis in the middle and lower reaches of Heihe River basin is the best witness of large-scale development of irrigated agriculture in arid zone of northwest China. The studies on oasis change process and its driving mechanisms in the middle and down of Heihe River basin is not only an important way to understand the problem of land and water resources utilization, but also the ideal case of oasis change research in arid areas.Based on the reconstruction of spatial-temporal change by using the long-term remote sensing images in the recent nearly 50 years (after liberation) and the materials of maps, meteorological records, social-economical data and surveying data, the driving mechanisms of oasis evolution were analyzed by using the method conjoining qualitative and quantitative ways. The results were of great importance in the study of environment change and promoting the rational utilization of water-land resources in arid regions further.We got 14 phases effective remote sensing images from 1963 to 2013 in 3 to 5 years intervals. Using the object-oriented extraction method and combined with the visual interpretation, we got the oasis distribution for each phares. By adopting the method of area of statistics and change detection, we analyzed the characteristics and regularity of oasis spatial change. The results shows that the spatial pattern of oasis keep the basic trend of maintaining the stable and subject local expansion in recently 50 years; And the oasis area has expanded generally, inceased from 4448.64 km" to 7191.92 km2, increasing 2743.28 km2 with the average annual growth rate of 1.03%; Oasis changed areas mainly distributed in the main body of oasis peripheries, covering the pluvial fan, low-lying areas along the canals or rivers, and the transition zones between desert and oasis. Simulating the productivity of oasis by using CAS A model, the results shows that the biggest monthly NPP level in 58.5～124.0 g · m-2 month-1, which is generally higher in middle plain oasis but lower in Ejina oasis of downstream, and the overall level raised in 1986-2013. The mean NPP per unit area is higher in Minle, Ganzhou, Shandan. Linze counties, while the lowest one located in Ejina.On the basis of in-depth analysis of the driving forces and mechanism of the oasis change, the conceptual model of driving mechanism of the oasis was established by logical reasoning. Oasis’expansion and contraction, and productivity change are the results of the interaction together with man and nature, but the cultural factors play the main role. The driving force includes both sides of natural factors and social-economical factors. As respect to natural factors, climate, soils, and hydrology are considered to be the main ones. Meanwhile, population, policy, economic development, and technological progress are considered to be the main driving forces of the social-economical aspect. These driving forces can be divided into direct and indirect factors according to their mechanism of action. In the driving force system, each factor’s impact is not independent, but influenced and restricted by many other factors.Based on clear cognition that the oasis change are mainly dominated by human factors, almost a hundred investigating sampling areas, located in the regions where the oasis changed intensely, were selected for field investigation in the form of semi-structured interviews. The results showed that:the increasing population is the "rigid factor" that promotes the oasis expansion; increasing agricultural inputs and rapid socio-economic development provides a certain material base for the development of oasis; technological progress has increased the production capacity gradually, making the increase in economic benefits of per unit area of arable land. Therefore, population, fixed investment, technical factors, and policies determined the direction, structure and size of the oasis development. That is to say human activity is the main cause of the oasis change.Above all, driving factors for oasis were screened and quantified based on logic inference and correlation analysis. Then taking the selected indicators as explanatory variables in Panel Data model, oasis area as explained variables, the relative contribution of natural and human factors were calculated by the regression coefficient of each variable, and the main driving factors were identified. The results show that:the relative contribution of human factors in countries were:91.82% in Suzhou,89.81% in Ganzhou,87.53% in Linze,83.99% in Minle,83.58% in Jinta, and 78.65% in Gaotai, respectively; The relative contribution of human factors in the entire oasis is 86.76% obtained by area-weighted method; human factors affected the oasis even greater in Ganzhou and Suzhou compare with other countries, indicating that the stronger the oasis disturbed by human activity, the more stable the oasis is. Taking NPP as the indicator of vegetation change, potential NPP as a measure of natural factors contribution, the difference value between actual NPP and potential NPP as a measure of human factors’contribution, the method to evaluate the relative contribution of natural factors and human factors in oasis’NPP change was constructed. The results showed that human factors play a leading role in NPP change among the five sites within the scope of the Heihe River Oasis.Comparing the model results with field interview results, we can get the consistent conclusion:human activity is the main drivers of oasis change in Heihe River basin. Natural factors, such as soil, hydrology, climate, and etc., provide the basic conditions for oasis’development, although the water resources quantity restricts the size of the oasis. Human is the dominant power of the oasis change. Essentially, the basic driving force of oasis change in Heihe River basin is the peoples’pursuit of affluence and better life by cultivating arable lands to increase revenue. |
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