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Event Based Analysis on Spatial Distribution of Heavy Downpour Using Radar Rainfall Data in Yoneshiro River Basin, Japan

Author(s): So Kazama; Yuta Sugawara; Yoshiya Touge

Linked Author(s): So Kazama, Yoshiya Touge

Keywords: DA analysis; Return period; Flood; Spatial analysis

Abstract: The purpose of this study is to understand the characteristics of spatial distribution on heavy rainfall using analyzed radar rainfall data. The study area is Yoneshiro River Basin in the northern Japan, which has 136km length and 4,100km2 catchment area, and has 11 rainfall gauges. Rainfall data at these gauges were used to detect the heavy rainfall events regionally. JMA (Japan meteorological agency) analyzes and provides hourly radar rainfall data rescaled with spatial resolution with 250m by 250m from 1988 to 2015. Then, the return period was calculated at each grid cell using GEV (Generalized extreme value) distribution by PWM (Probability weight moment) method to estimate the parameters for the radar data after testing several distribution functions. Regarding the rainfall height, the DA (Depth - Area) relationship was analyzed for two types, a heavy rain event without a flood and a heavy rain event when a flood occurred. Several former formulas for DA relationship were tested for the agreement to actual rainfall obtained by radar rainfall data. Arakawa’s formula, which was based on Horton’s formula and was commonly used in Japanese sewage design, was the best fitted for the relationship of DA in the case of maximum hourly rainfall. After making a spatial distribution of stochastic rainfall, the spatial distributions for each probabilistic year (return period) were compared in Yoneshiro River Basin. We call RA (Return period - Area) relationship to this comparison. As the results, it was greatly different between the point where extreme heavy rainfall is likely to occur and the point where annual average rainfall is large. The relationship between the probability rainfall and the rainfall area was analyzed in the cases of heavy downpour generating a flood. We obtained 28 heavy downpour events and obtained RA relationships in the upper, middle and downstream regions. Then the 2013 event had the largest area with heavy downpour in the upper and middle regions and the 2007 one had the largest area in the downstream region. As a result of comparing the heavy rainfall events with 50 years return period of 2013 and 2007 when floods occurred in the upper, middle and lower regions, the area in the 2013 was about 14 times that in 2007. Additionally, in the area where heavy rainfall occurs in the 100 years return period, the heavy rainfall area in 2013 was about 50 times that in 2007.There was a large difference in the area of heavy downpour depending on the event.

DOI: https://doi.org/10.3850/IAHR-39WC25217119202230

Year: 2022

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