Abstract:

Based on the daily precipitation data from 172 national meteorological observation stations in the Huaihe river basin and the 500hPa ridge position data of the northwest Pacific subtropical high from 1960 to 2022, this study used the Empirical orthogonal function (EOF) to delineate the boundary between the northern and southern subregions of the basin. By using the sliding average and statistical test methods, a set of key indicators for the monitoring of the rainy season in the Huaihe river basin was established, including the determination of the number of rainy days in the region, the start and end dates of the rainy season in the region, and the rainy season in the whole Huaihe river basin. Based on the key indicator system, this study analyzed the rainy season characteristics of the entire Huaihe river basin from 1960 to 2022, and selected typical drought and flood years to verify the start and end dates of the rainy season, aiming to establish a scientific and practical monitoring method for the rainy season in Huaihe river basin. The results showed that the southern and northern subregions of the Huaihe river basin during the rainy season were bounded by the zero line of the spatial loading coefficient of the second EOF mode. Regional rainy days were identified when more than one-third of the meteorological observation stations in the region record at least 0.1mm of daily precipitation, and the average daily precipitation in the region reached or exceeded 1.0mm. From 1960 to 2022, the average onset date of the rainy season in the southern subregion was June 20, the average end date was July 20, and the average duration was 31d. In the northern subregion, the average onset date was June 25, the average end date was July 25, and the average duration was 29d. For the entire Huaihe river basin, the average onset date was June 19, the average end date was July 26, and the average duration was 32d. To verify the reliability of the indicator system, a typical drought year (1988) and a typical flood year (1991) were selected to conduct a comparative analysis between their rainy season start−end characteristics and the actual meteorological disaster features. In 1988, the Huaihe river basin was identified as having no rainy season, in line with the characteristics of the basinwide drought disaster that year. In 1991, the rainy season in the Huaihe river basin began on May 29 and ended on August 7. Compared to the average rainy season (June 19–July 26) during 1960–2022, it started earlier and lasted longer. This aligns with the severe, widespread and persistent basin−wide flooding of that year, thereby confirming the reliability of the rainy season monitoring indicators for the Huaihe river basin established in this study. The monitoring indicators for the rainy season established in this study provide a theoretical basis and technical support for meteorological services related to agricultural production, flood control and drought mitigation in the Huaihe river basin. They are also highly significant for improving the early warning of meteorological disasters and the scheduling capabilities of resources within the basin.

Key words: Huaihe river basin, EOF, Meiyu in the Yangtze?Huaihe region, Regional rainy day, Regional rainy season