Statistical and geostatistical analysis of rainfall in central Japan

Abstract Precipitation data have been analyzed statistically and geostatistically in order to obtain fundamental information for assessing water resources and predicting natural hazards caused by heavy rains. The study area is the mountainous Chubu and plain Kanto districts, central Japan. For the statistical distribution of hourly, daily and annual precipitations, lognormal distributions were fitted well in both districts, but exponential distribution was more suited for monthly precipitations. Weibull distribution illustrates also hourly and monthly precipitations well. Spatial variograms of annual precipitations show clearly nuggets and sills as well as ranges. The range is about 130 km in both districts. This range value, which is about seven times of the average station distance, indicates that the station density is sufficient for assessing water resource. Temporal variograms of hourly precipitations through a year have ranges of 8 h. In the analysis of heavy rain on August 14, 1999, when severe floods attacked some areas in Kanto, variograms of hourly precipitations show clear ranges (50–70 km), if it rains heavy in a wide area on a series of rainfall. Ranges of variograms increase with increasing accumulation time, and become constant as 120–150 km over 3–5 h. This range value is two or three times of the average station distance, and the accumulation time is three to five times of the measuring intervals. It concluded, accordingly, that the station density and the measuring interval of AMeDAS are insufficient for predicting natural hazards.