| Basing on the theory of social division, considering the main characteristics of regional nature, social economy and the local soil erosion, this paper studied the spatiotemporal dynamic monitoring and evaluation of land resources at county scale, aiming at the key points of ecological environmental degradation and the targets of ecological construction in Ansai county, using remote sensing and GIS technology, combined with ground surveys. By doing this, it can provide scientific basis for government to make proper strategic decision-making and regional programming. The main conclusions were listed below:1) The environmental characteristics of land resources in Ansai countyThe environmental characteristics such as types of Ansai County landscape, climate resources, soil vegetation and socio-economic etc were studied in this paper. The results showed that the main types of Ansai's landform are loess beam and flume,loess hilly and gully region, loess hilly patches and valley terrace; the climate is dry and sandy in spring, tepid and thunder stormy in summer, fair and cooling rapidly in autumn and dry-cold and less sleet in winter; the cultivated soil is loess soil, with area of 18,100hm2 of 77.06% of the total land area; the vegetation is forest steppe-typical steppe transition zone, where lives forests in south, psammophytes in north and shrubby grassland in the middle. The coverage rate of forestland is about 27.23%. And the area with a slope gradient more than 25。is 81.85% of total forestland .Because of the restriction of the natural conditions, the agricultural production pattern of this region mainly to dry farming, both flood and drought; facing the double pressure of population and the environment ,this region must be reasonable, sustainable use of land resources.2) The land type and its distribution characteristics of AnsaiAccording to the distribution and assemblage of the landform, soil and vegetation, Ansai County's land types can be divided into five levels of land types, 10 kinds of secondary land unit types referring to the Loess Plateau land classification system. Its distribution is: Hilly sideline for the hilly slopes above; hilly sideline below, above the edge of the ditch to-hilly slopes; ditch margin line, above the foot of the slope for the ditch slope; foot line to the ground category. Because of the secondary forest in the distribution and concentration of special features, grass hilly is divided into one type separately. On this basis, the distributions of 10 categories of land type were elucidated in detail.3) Mapping the resources and environment in AnsaiUsing ARC / INFO and ARCVIEW for technical support platform to process and pre-treat the data and topographic maps, the land use maps of Ansai in different periods and different scales have been digitized, edited and validated completely; and the attribute database has also been established for these maps. Meanwhile, the related themes have been produced by the latest land resource information from remotely sensed imagery interpretation. And these maps will provide the data support for studying the spatial and temporal dynamic changes and evaluation of this county's land resources and evaluation.4) The evaluation of Ansai's land resourceThis paper analyzed and evaluated land resources in Ansai County from the land type structure, dynamics and land resources, and the quality of land resources, etc. The results showed that the structure of Ansai County's land types have the following trends and features: more steep sloping land but less flat land; the greater slope, the greater the frequency of land type and the more complex shape coefficient, while its diversity is also increasing trend. Dynamic monitoring of land resources indicated that slope cultivated land gradually reduced, basic farmland area is expanding, secondary forest degradation is serious, orchard area is expanding, and grass-building situation is grim. The quality evaluation of land resource showed that at Ansai county, the area of the land, which is suitable for crop is 883.62km2, accounted for 29.95% of total area; the land suitable for forestry is 844.42km2, accounted for 28.62% and suitable for shrub and herb is 918.90km2 which is 31.14% of total area. The total area's 10.28% which should be hillclosing afforested is 303.39km2.
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