The Relationship between NDVI Change and Precipitation in Guyuan City
泾河上游固原地区的NDVI变化与降水的相关性研究

The relationship between precipitation and vegetation is an important part in the field of earth surface processes.Here MODIS(Moderate-resolution Imaging Spectroradiometer) NDVI time series data with 250m resolution was used to study the NDVI change from 2000 to 2004 in Guyuan city which lies in west semiarid China.Grain for green policy was carried out in this area from 2000 to reduce the severe soil erosion and improve the ecological environment.The dada showed that NDVI between 0.2 and 0.4 decreased 31 percent and NDVI between 0.4 and 0.6 increased 67 percent.This change indicated an improvement of vegetation cover.Rainfall data derived from six weather stations in Guyuan City was used to calculate rainfall use efficiency and corelation coefficient between NDVI and rainfall.The result showed that the average rainfall use efficiency increased from 0.53 to 0.65.The corelation coefficient between NDVI and rainfall increased from 0.27,0.58,0.50 to 0.83,0.92,0.95 in Yuanzhou,Pengyang and Xiji county respectively,while Jingyuan and Longde county showed a small change.It was considered that this increase was mainly due to the accordance between rainfall and NDVI,not grain for green policy.This implied that the variance of annual rainfall is bigger in semiarid areas than that in semiwet areas and the vegetation in semiwet areas has a higher stability than that in semiarid areas.Here it is also found that when the rainfall in growing season is less than 450mm,NDVI increased slowly with rainfall,while when the rainfall is greater than 450mm it is observed that NDVI increased quickly.The main vegetation is grass in areas with less than 450mm rainfall,and forest in areas with bigger than 450mm rainfall.So 450mm annual rainfall can be considered as a threshold to decide the suitability of forest and grass plantation in this area.The nonlinearity between forest cover and rainfall is based on NDVI,if a new vegetation index such as NDWI or EVI is used,maybe this nonlinearity will changed.So more research is still needed to verify.