Application of Box-Cox transformation in the determination of extreme precipitation climate events in the Yangtze sub-basins

According to the geographical boundaries, we divided the Yangtze river basin into nine sub-basins, and calculated areal rainfall in each sub-basin based on monthly precipitation data at 700 meteorological stations in the basin from 1961 to 2017. We find threshold values of extreme precipitation climate events in each sub-basin by percentile method after applying Box-Cox transformation. The results show that compared with year number ranking method, the threshold value of extreme precipitation climate events in the Yangtze sub-basins defined by these normalized areal rainfall series have obvious advantage. It presents a more stable characteristic when the length of the areal rainfall series changes. The threshold value changes are reduced, and the threshold changes for vast majority of the sub-basins are within 10 mm. Thus the extreme years are stable. Using the threshold values by this method, we examined whether extreme precipitation climate events have occurred during flood season in 2018. As a result, extreme rainy climate event happens in the Mintuo river basin and extreme less rain climate events happen both in the stem stream from Chongqing to Yichang and in the basin that from Hanjiang river to the middle reaches of the Yangtze river.