Variations in water diversion at three outlets to Lake Dongting from Jingjiang River based on the identification of multiple factor contributions since 1955

近70年来荆江三口分流持续减少,为分析各历史阶段三口分流变化的驱动因素,根据1955—2021年干支流水文、地形实测资料开展研究,并基于控制变量法识别各驱动因素贡献比例。结果表明:三口分流阶段性减少,且集中在5—11月。长江上游降雨减少导致的干流来流偏枯以及下荆江裁弯、葛洲坝拦沙造成的干流河道冲刷和三口口门淤积对三口分流减少促进作用明显,调弦口堵口对三口分流影响不大,三峡水库蓄水后干流河道与三口口门同步冲刷,三口分流变化不大。长江上游汛期降雨减少明显,对5—11月三口分流减少影响较大,且三口分流主要发生于汛期干流中洪水位时,导致在下荆江裁弯期干流河床大幅下切条件下5—11月分流显著减少;葛洲坝调度、三峡水库初期调度和试验性蓄水调度也集中于5—11月,使得5—11月三口分流量分别减小0.4×10⁸、12.5×10⁸和38.6×10⁸ m³。调弦口堵口对三口分流的增大作用远小于降雨的减小作用,贡献比例为负值;下荆江裁弯对三口分流减少的贡献比例达85%;水库运用期降雨对三口分流减少的贡献最大,贡献比例大于50%,葛洲坝调度几乎不产生贡献、下游河道调整贡献比例为21%,三峡水库初期调度贡献比例增大至7%、下游河道调整贡献比例为10%,试验性蓄水期则分别上升至23%和19%。长江上游降雨减少至少持续到本世纪末,三口分流年均值将长期呈减少趋势,而荆江干流与三口口门同步冲刷,干流同流量下三口分流仍将变化不大。;Over the past 70 years, the water diversion at the three outlets of the Jingjiang River has been continuously decreasing. In order to investigate the driving factors at different historical stages, this study used the measured hydrological data of the hydrology and topographic features of the mainstream and tributaries from 1955 to 2021, and identified the contributions of the factors based on the variable control approach. The results showed that the water diversion had gradually decreased, and the decrease had mainly occurred from May to November. The decrease in water diversion was greatly aggravated by the reduced mainstream inflow due to the decrease in precipitation in the upper Yangtze, and the erosion of the mainstream bed and deposition at the three outlets caused by the dam projects in the lower Jingjiang River and the sediment trap in the Gezhouba. However, the Tiaoxiankou dam had little effect on water diversion. Since the damming of the Three Gorges Reservoir (TGR), the mainstream and the three tributaries had experienced synchronous riverbed erosion, resulting in an imperceptible change in water diversion. The reduction in rainfall had been concentrated in the rainy season, which had had a major impact on the reduction in water diversion from May to November. Meanwhile, water diversion was mainly concentrated in the rainy season when the mainstream was at flood level, resulting in a large decrease in water diversion from May to November, while the Lower Jingjiang Cutoff Projects triggered a large lowering of the mainstream riverbed. In addition, the operation of Gezhouba, the initial damming of TGR and the experimental damming of TGR were mainly carried out from May to November, reducing water diversion by 0.4×10⁸ m³, 12.5×10⁸ m³ and 38.6×10⁸ m³, respectively. The enabling effect of the Tiaoxiaokou dam on water diversion was far less than the constraining effect of rainfall reduction, resulting in a negative contribution ratio for the dam. In comparison, the contribution ratio of the lower Jingjiang dam projects reached 85%. In the post-dam period, precipitation dominated water diversion reduction with a contribution ratio of more than 50%, while Gezhouba operation hardly played a role and its downstream riverbed adjustment produced a contribution ratio of 21%. During the initial and experimental damming of the TGR, the contribution ratios of its operation increased to 7% and 23%, respectively, while those of its downstream riverbed adjustment were 10% and 19%. As the precipitation decline in the upper Yangtze will continue at least until the end of this century, the annual water diversion will show a long-term downward trend, while the water diversion at a given mainstream flow will change little due to synchronous riverbed erosion between the mainstream and the three outlets.