Comprehensive evaluation of ecological risk of land during 1990-2012 in Yulin City

Land use/cover change is a determinant factor in the analysis of ecosystem services for regional ecological security and maintenance. Ecological risks of land serves as a vital index in measuring sustainable utilization of land resources. The research area used in this study was Yulin City, a city in northern Shaanxi Province located in the Loess Plateau and Maowusu Desert border where the climate is arid and the land use structure is irrational. This region is one of the areas with the most severe water and soil erosion on the Loess Plateau, with ecologically-fragile environment, damage susceptibility and recovery difficulty. Thus based on the natural geographical conditions, development history of the city, basic land system characteristics and removed single factor evaluation uncertainty, this study established an evaluation index system of various sources risk in Yulin City. Percent precipitation anomaly index was used as indicator for the risk of drought, the risk of land use structure as the indicator for the effect of human activity on the ecological system, and wind/water erosion as the indicators of soil erosion. Then the four evaluation indicators were used to construct a comprehensive evaluation model for the qualitative and quantitative evaluation and analysis of the factors affecting ecological risk of land in Yulin City for the period 1990-2010. The main conclusions of the study were as follows: (1) the distribution of precipitation was uneven, with percent precipitation anomaly index initially decreasing and later increasing in the period from 1990 to 2012 for Yulin City. The area with lower value of the index mainly covered the eastern part of the city. And the index increased from the south to the north. (2) There were large and significant changes in the differences in regional risks of land use structure. The land use structure risk first increased and then decreased in the course of the investigated 21-year period. (3) More severe soil water erosion occurred mainly in the eastern and the southern regions of the study area (driven mainly by topographic factors) with an initial mean increase and then a decrease over the 1990-2012 period. (4) There was more severe wind erosion mainly in the northern region, with Yuyang District having the largest erosion and Suide and Mizhi Counties the smallest erosion, and with a decreasing trend in mean erosion for the investigated period. (5) The trend in ecological risk of land was decreased, and significant differences in ecological risk of land were observed among counties. The area of Yulin City with severe ecological risk in 1990 was mainly in the northwest, area with high ecological risk in 2001 was mainly in the central region and west of Yulin City (e.g., Dingbian County), area with moderate ecological risk was mainly in the southeast. The ecological security status in the study area was improved overall, although the situation remained grim. This research put forth a theoretical proposal for optimizing the allocation of land resources and protecting the ecological environment in order to realize a harmonious development of regional economy and ecology.