Human-induced spatio-temporal changes of oasis through landscape pattern analysis:a case study of oasis in the Sangong River

Among landscape changes, a particular type is the landscape change in an oasis that is isolated in desert surroundings. Due to the available groundwater and runoff from nearby high mountains, oasis areas have higher productivity than the surrounding desert. The landscape changes in oases have been a major concern in Xinjiang province, China. In the past 50 years, large-scale land reclamation and cultivation of water resources in Xinjiang have resulted in tremendous changes in the oases. We have investigated the spatio-temporal change of oasis through landscape pattern analysis including the landscape diversity, patch shape and patch fragmentation which reveal complexity of human activity, oasis expansion and intensity of activity in an oasis. Sangong River watershed, the study area, is located in the eastern central section on the northern slope of the Tianshan Mountains and in the southern Junggar Basin, Xinjiang, China. Data collected for this study include 65 aerial photographic images at the scale of 1∶35 000 taken in the Sangong River watershed in August 1978, 26 color-infrared aerial photo images at the scale of 1∶70 000 taken in the watershed in July 1987, and a Landsat TM image (synthesized from the wavelength bands 2, 3, and 4) of the watershed in August 1998. Classification of landscape patch type is needed to study spatio-temporal changes of oasis. 16 landscape patch types in oasis are classified according to the classification system of China State Bureau of Land Administration. These 16 types include irrigated lands, woodlands, shrubbery lands, shelter-forest lands, natural grasslands, cities, towns, villages and industrial and mining lands, highways, reservoirs and ponds, rivers and flood lands, desert grasslands, saline or alkaline lands, swamps, deserts, and gravel lands. The classification accuracy of types for 1978, 1987 and 1998 are estimated at 93%, 96%, and 94%, respectively. The results show that (1) Shannon's diversity index and Shannon's evenness index across the oasis have declined. Their regional differences are significant but temporal differences are not. Human activities may evidently change oasis landscape diversity; (2) As oasis landscape patches are developed they generally, but slowly, become more anomalous and complex. In the development of oasis, humans endeavor to expand the periphery of oasis, which will increase anomaly and complexity of oasis landscape patch. On the other hand, humans endeavor to arrange the interior of oasis, which will decrease anomaly and complexity of oasis landscape patch. In the last 20 years, the former has always been predominant; (3) The oasis regions with the more intensive human activity have the higher fragmentation of landscape. The human activity is most intensive in the central part of oasis, whose intensity of landscape fragmentation is higher than in the upper and lower part of oasis, where human activity is relative weak compared with the central part. The patch density and fragmentation of oasis have declined as a whole. The extent of fragmentation of human-introduced patches generally is higher than that of environmental resources patches.