7RICHARD R, HEIM J R. A review of twentieth century drought indices used in the United States [ J ]. Bull Am Meteor Soc,2002,83(8):1149-1165.
8QUIRING S M, PAPDRYIAKOU T N. An evaluation of agricultural drought indices for the Canadian prairies [ J ].Agricultural and Forest Meteorology, 2003,118:49-62.
9JI Lei, PETERS A J. Assessing vegetation response to drought in the northern great plains using vegetation and drought indices [ J ]. Remote Sensing of Environment,2003(87) : 85-98.
