Abstract: The ecosystems in the inland watersheds in arid and semiarid areas are consisted of the artificial oases, natural oases, ecotones between the oases and deserts, and deserts surrounding the oases. In northwest China it is arid, the ecological environment is vulnerable, the area of the oases is small, the sand deserts, Gobi deserts, and saline or alkaline land are distributed extensively. The social and economic development is mainly limited in the oases. In fact, the sustainable development in northwest China is the long-term development of the oases, and the protection of the ecological environment is that of the oases. In the recent decades, however, the artificial oases are rapidly expanded but the natural ones shrunk with the rapid social and economic development in these watersheds. In order to maintain the stability of the oases and achieve a sustainable development, a balance between the artificial and the natural oases should be kept. Based on the remote sensing information of the 1970' s and 1990's and a lot of ground investigation data and by using GIS spatial analysis, in this paper the general situation of the ecological environment in the inland watersheds in northwest China is assessed, and the evolution and main forced factors of the ecology are analyzed. The results show that some watersheds have been excessively developed and the desertification has been sped up since recent 20 years. It is considered that the area of the natural oases should be maintained for one third of the total area of the oases so as to ensure their general stability. In the construction of the ecological environment in future, it is suggested to regard the protection of the natural oases and the natural vegetation as the key, increase the dynamics in the exploitation, utilization and management of water resources while withdrawing from farming to planting trees and grasses and controlling excessive grazing, and restore and maintain the ecological groundwater level so as to provide the sufficient moisture conditions for the growth of the natural vegetation.

Key words: ecological zone, evolution, oasis, Northwest China