Regional ecological risk assessment is characterized by multiple risk stressors

receptors and endpoints with uncertain factors and spatial heterogeneity.By taking the vegetation ecosystem of the ShiyangRiver basin in Gansu Province for an example

the ecological indices of fragility degree

ecological loss andrisk source level of different vegetation ecosystems are analyzed for different risk patches by overlaying analysis with the aid of ArcGIS 9.3.Results show that the spatial variability of vegetation ecological risk in theShiyang River basin is more obvious.The first level risk area

accounting for 3.87% of the total area

ismainly located in the upper reaches of the Shiyang River basin

with Sunan and Tianzhu Counties being mostconcentrated and vegetation type being mostly woodland.The second level risk area

accounting for 4.38%of the total area

is connected with the first level risk area and vegetation type is mostly shrubbery.The thirdlevel and low level risk areas are mainly located in the downstream and midstream and vegetation types aregrassland and farmland.Finally

the large-scale land development and utilization of Shiyang River basinshould be avoided in the first and second level risk areas

and land use in the third level and low level risk areas should be protected.