Jinsong Wang Feng Wei Jianying Feng The climatic characteristics of length of extreme drought period and its relation with NDVI in Northwest China. 3353-3356 2007 IGARSS https://doi.org/10.1109/IGARSS.2007.4423563 conf/igarss/2007 db/conf/igarss/igarss2007.html#WangWF07