Abstract: By studying the characteristic of dry season runoff and extreme low-flow in the downstream of Xijiang River in the presence of water quantity dispatching, we aim to provide a reference for the unified scheduling in dry season and the efficient use of water resources in the Xijiang River Basin. According to the daily flow data at Wuzhou Hydrological Station from 1950 to 2015, we compared the annual and monthly low flow series before and after the water quantity dispatching via Mann-Kendall method, sliding T-test method, and R/S method, and discussed the variability and future change of low flow. Results revealed that over the past six decades, the dry season flow in the lower reach of Xijiang River showed no significant upward trend with a positive persistence and small possibility of variation. After water quantity dispatching, monthly low flow witnessed a significant upward trend in November, January and February, but decreased distinctly in October, presenting the characteristics of impoundment at the end of flood season. In December, a critical low-flow period, there was a slight upward trend, which is particularly worthy of attention. The Hurst index and correlation coefficient C(t) of each month’s flow indicate that the future monthly flow during dry season in the lower reach of Xijiang River will be consistent with the past trend with no significant variation. Probability plot correlation coefficient (PPCC) test demonstrates that the LP-III frequency distribution best fits the annual and monthly flow during dry season in Xijiang River Basin. Taking P=90% in this distribution frequency as extreme low-flow standard, we found that extreme low-flow occurred more with the reduction of index flow scale in the early stage, but less in the later stage, implying that water quantity dispatching in dry season is conducive to reducing the occurrence of extreme low flow events.

Key words: dry season runoff, change characteristics, water quantity dispatching, Hurst index, Xijang River