Abstract:

Remote sensing has the advantage of high temporal resolution and monitoring large scale environment,thus it has been widely used to monitor the dynamic status of desertification.The vegetation index extracted from remotely sensed image can be used for regional desertification assessment.The normalized difference vegetation index (NDVI),as a common vegetation index,has been widely used to quantitatively and qualitatively assess the desertification during the past 20 years.However,the soil background and atmospheric noise influenced the application of NDVI.The enhanced vegetation index (EVI) is developed to avoid the interference from soil backgrounds and atmosphere,thus may show efficiency in monitoring the desertification dynamism.The plain area in the middle reaches of the Hanjiang River,especially for the valley,is characterized by high coverage.Monitoring the desertification status with regular remote sensing data is difficult,although the desertification of this region is ubiquitous.In this study,the vegetation coverage changes of this region during 2003—2011 were investigated with the time series of MODIS EVI data.The dynamics of desertification decreased during this period,as the EVI increased gradually.The SouthtoNorth Water Diversion Project triggered the decline of groundwater level for middle reaches of the Hanjiang River,and this might facilitate the desertification.Furthermore,the impairment of the decline in groundwater on EVI was negatively correlated to the distance from river.In response,a large area of land afforesting has been initiated to prevent the land from deserting in Lao Hekou,Xiangfan and Zhongxiang since 1990.However,the improvement of this project on vegetable coverage has not been accessed yet.According to the distance from the river,the region investigated was assigned as 8 buffer zones with equal distance of 1 km and the associated characteristics were analyzed.There were pronounced spatial fluctuations for EVI in buffer regions of different distances.In 1—3 km buffer region,the mean value of annual EVI increased with the distance from the river,and reached the maximum value at 3 km.Whereas in 3—8 km buffer zone,the EVI value decreased gradually.Additionally,these changes could not be well explained by synchronized climate factors,such as mean monthly temperature or/and precipitations.However,the variations of EVI in temporal and spatial scale may be attributed to the innate physiological,chemical and biological differences in soil.In conclusion,EVI values of the plain area in the middle reaches of the Hanjiang River valley showed a conspicuous ascendant trend during the last nine years,suggesting the efficiency of afforestation against desertification.While the discrepancy of EVI in different spatial scales suggested that attentions should be paid to the innate features of soil and proceed from local reality conditions