Abstract: Based on the data of the monthly precipitation in 1955-2007 in Rizhao City of Shandong Province,the grades of flood and drought were determined by using the anomalous percentage of precipitation and Z-index. The characteristics of the yearly and seasonal flood and drought and their influences on agriculture were analyzed. The result indicated that the Z-index could reflect the change features in Rizhao City well. Within the last 53 years there were fouteen years with occurrences of the drought,the occurrence probability of drought was 26.4%. While there were eleven years with the occurrence of the flood,the occurrence probability of flood was 22.6%. The yearly precipitation decreased at the rate of 41.1mm per decade. It showed a drier tendency. The variation trends of Z-index in every season were different. In winter and spring,the Z-index increased slightly,while the flood tend was not obviously. The Z-index in summer and autumn decreased,while the drought,tends in summer was clearer than that in autumn. The occurrence probability of drought was high in the last 53 years. The drought disaster was the major factor caused great loss of crops. This study provided a reference for the drought and flood forecast and disaster prevention and reduction.

Key words: Flood and drought index, Flood and drought index, Anomalous percentage of precipitation, Z-index