Interannual Variabilities in the Onset and Demise of Rainy Season: Implications for Water Management

This study examines potential changes in the summer rainy season of a watershed, using a cumulative rainy deficit curve to define the onset and demise of the rainy season. Historical daily rainfall data from 41 gauge stations within the Xiang River Basin in 1960–2017 were analyzed. Results indicate that the climatology of the onset date is Julian day 72 and the climatology of the demise date is Julian day 236. There is, however, a difference between the two segments, e.g., 1960–1989 and 1990–2017. The median rainy season duration in 1990–2017 is 138 days. This is larger than that of 1960–1989, which is 120 days. There is a statistically significant correlation between the onset date of the rainy season and the duration of the rainy season. Daily soil moisture estimates were derived from satellite remote sensing data. The averaged soil moisture during the rainy season is much higher than that of the non-rainy season. To examine the difference in quick flow between rainy and non-rainy season, baseflow separation was applied to daily streamflow time series. Result reveals the median of weekly quick flow in rainy season is 1316 cms, which is 5.7 times that of the weekly quick flow in non-rainy season. Different mechanisms that drive spring and summer rainfall, as well as potential use of the results in water management, are discussed.