The development and utilization of water and land resources in the Shiyang river basin is the highest among the 3 inland rivers in the Hexi Corridor of Gansu province, northwestern China. Based on field investigation and social-economical data, in combination with TM images of the study area in 1986 and 2000, this paper analyzed the changes of farmland and their driving forces in Liangzhouqu and Minqin oases in the middle and lower reaches of Shiyang river basin in the last 20 years.
Farmland increased in both oases, while Minqin oasis increased more quickly. Most of the increased farmland was distributed on desert-oasis ecotones. But it was mainly converted from desert in Liangzhou oasis, and from woodland and grassland in Minqin oasis. In view of driving forces, the population increase was one of the most important factors, and improvement of agricultural technique and economic development provided conditions for farmland expansion in both oases. In addition, the expansion of farmland was closely related to the amount of surface water in Liangzhou oasis, but was effected by ecological environment such as gale and dust storm, and weather conditions such as rainfall and evaporation in Minqin oasis. With the development of human activities and expansion of farmland in Liangzhou oasis, the need for surface water is ever-growing from year to year, resulting in a decrease of annual discharge to the Hongyashan reservoir from 5×10⁸ m³ during the 1950s and 1960s to only 1.0×10⁸ m³ or so at present. Therefore overuse of groundwater in these regions, especially in the downstream Minqin oasis is very notable. All these indicated that the development of oases in the middle and lower reaches of Shiyang River had already lost the balance to a great extent, and the economic increase of Liangzhouqu oasis is at the cost of environment deterioration of Minqin oasis.In order to realize rational resource utilization and sustainable development of Shiyang river basin, the surface water should be allotted at a reasonable proportion in the whole watershed, recovering the runoff in the lower reaches to 18% of the total runoff (about 2.5×10⁸m³/a) of Shiyang river, which used to be the level in early 1980s. In addition, the charge for transfer of water from other watersheds should be allocated according to the proportion of runoff utilization, and thus reduce the cost of water use in the lower reaches such as Minqin Oasis.