Dynamics of water resources and land use in oases in middle and lower reaches of Shiyang River watershed,Northwest China

Based on the TM image of the oases in Liangzhou District and Minqin county in middle and lower reaches of Shiyang River in 1986 and 2000,this paper calculates the transformation matrix of the landscape mosaics,and analyzes the relationship between the land use changes and the dynamics of runoff and groundwater resource and their exploitation in the two oases. The results show that:(1)Farmland area expanded distinctly by 6302 hm² in Liangzhou and 36434 hm² in Minqin oasis. And the transition of landscape elements in Minqin oasis is developing toward desertification,with the area of 23349 hm² turned into desert land,however it is also developing toward the other way-the oasis landscape expansion by 81581 hm² in Liangzhou oasis.(2)The ratio of the utilized water quantity in the upper and middle reaches to that in lower reaches increases great from less than 2 before 1970 to more than 8 since 1995;the annual recharge quantity to the Hongyashan reservoir decreases by 63.1% in recent 40 years.(3)Groundwater exploitation develops progressively,and the density of water-supply wells increases remarkably,up to 5.7 and 1.8 wells/km² in Liangzhou and Minqin oases respectively in 2000;and the groundwater table drops obviously,the lowering rates reach up to 0.6-0.8m/year in some place.(4)the cropping patterns in the study area are distributed irrationally,too.If the planting percentage of water-wasting crops drops to 50% in both oases, it can save irrigating water by 1.2×10⁸m³ in Liangzhou oasis and 0.2×10⁸m³ in Minqin oasis one year.