陕北安塞县马家沟流域近20 a来土地利用变化分析

Land Use Change of Majiagou Catchment in Ansai County,Northern Shaanxi Province in Recent 20 Years

黄土高原土地不合理利用是土壤侵蚀和水土流失的根本原因。以安塞县马家沟流域为例,采用1990年第一次全国土地利用调查图件,经过实地考查,获取2008年土地利用数据,分析了近20a来流域土地利用的变化趋势及类型间的转化情况。结果表明,马家沟流域1990—2008年农耕地类面积呈显著减少趋势,变化面积占流域面积比例为11.88%,同时草地类面积也减少了1.58%。林地类面积呈显著增加趋势,增加面积比例为11.61%,园地、居民地及工矿用地类面积也增加了1.73%和0.01%。流域农林草结构由1990年的3∶1∶6变为2008年的2∶2∶6。变化流向以草地变为林地,坡耕地类变为草地,坡耕地类变为林地,农耕地变为园地为主要特征。近20a间变化速率最快的地类主要是林地类、园地类和农耕地类,分别是8.06%,2.41%,-2.43%。近20a来流域林地类面积虽有增加,但人工生态系统的特征十分突出,良好自然生态系统的稳定性和持续性比较欠缺。1999年以来国家退耕还林还草政策的实施,以及农民经营方式的转变是林地和园地面积增加的主要原因。

Irrational land use is the major force of the severe soil erosion and loss occurred in the Loess Plateau.Taking Majiagou catchment in Ansai county as an example,the characteristics and trends of land use/cover change were analyzed based on the first national land use survey maps in 1990（obtained from the Bureau of Landuse Survery of Ansai County） and the 2008 land use map（mapped by field survey）.In ArcGIS,two types of land use maps were overlaid and the dynamic changes were analyzed.The results show that,from 1990 to 2008,farmland and grassland decreased by 11.88% and 1.58% of the total area of the catchment,respectively.At the same time,forest,orchard and residential lands increased by 11.61%,1.73% and 0.01% of the catchment area,respectively.The areal ratio of farmland,forest and grassland was changed from 3∶1∶6 to 2∶2∶6 during this period.The major transferring patterns among the land use system are grassland to forest,farmland to grassland,farmland to forest,and farmland to orchard.Forest,orchard and farmland have relatively high changing rates of 8.06%,2.41%,-2.43%,respectively.Although the forestry area has increased over the past 20 years,it remains as an artificial ecosystem with the nature of instability and non-sustainability.The deployment of the national policy of ＂Grain for Green＂ since 1999 and the substitution of farmers＇ operating strategies can be contributed to the increase of forest and orchard areas in the studied region.