Influence of Longyangxia Reservoir operation on Yellow River runoff and its water storage
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Abstract:
Reservoir operation is an important cause of runoff process alteration.Quantifying the influence of reservoir on runoff [JP2]process is the basis of identifying the ecological impact of reservoir and improving the ecological protection function of a reservoir.In previous studies,the main method to evaluate the influence of reservoir on runoff process is to compare the key characteristics of the runoff process in the past and post operation period of a reservoir.However,the river runoff process is influenced by many factors,such as climate [JP]change,engineering regulation,water intake,water withdrawal,and so on.Thus,the difference in the runoff process before and after reservoir operation reflects the joint influence of multiple factors rather than only the reservoir. Longyangxia reservoir,located in the upper reaches of the Yellow River,is the only multi-year regulation reservoir on the mainstream of the Yellow River,having a strong regulation capacity for the runoff.Taking the Longyangxia Reservoir as an example,the difference indicators of hydrologic alteration and flow deviation indicator between the inflow runoff and the outflow runoff of Longyangxia Reservoir from 1986 to 2016 are compared.The influence of reservoir on runoff process was quantified based on the indicator value differences and the Mann-Kendall test.Then a significance test is used to reveal the relationship between reservoir water replenishment,the natural runoff,and the proportion of surface water consumption in natural runoff in the Yellow River Basin. Results showed that the outflow runoff of Longyangxia Reservoir changed significantly compared with the inflow runoff from 1986 to 2016.For mean annual flow,the extreme value ratio decreased by 24.84% and the coefficient of variation decreased by 26.92%.For mean monthly flow,it decreased from June to October while increased from November to May.For mean daily flow,the runoff process became smoother but small-scale fluctuation increased.The annual maxima 1,3,7,30,and 90 day mean decreased 34.74%~47.30%;the annual minima 1,3,7,30,and 90 day means increased 38.99%~144.43%;the means of all positive differences between consecutive daily values,the mean of all negative differences between consecutive daily values and the number of flow reversals increased significantly.Among the 33 indicators of hydrologic alteration,27 indicators were highly altered.The overall degree of hydrologic alteration was 95.04%,meaning the outflow runoff of Longyangxia Reservoir was highly altered compared with the inflow runoff.The operation of Longyangxia Reservoir led to a serious deviation of the runoff process from the natural situation.The result of the Mann-Kendall test showed that the value of the flow deviation indicator of reservoir outflow runoff mutated in 1986 when Longyangxia Reservoir firstly impounded water.The mean value of the flow deviation indicator after impoundment was 43 times that before impoundment.From 1980 to 1985,the mean value of flow deviation indicator was 0.05,while from 1986 to 2016,the mean value of flow deviation indicator increased to 2.15. Longyangxia Reservoir changed the runoff process of the Yellow River by impounding and replenishing water.It was revealed that there was a significant negative correlation between the annual replenishment of Longyangxia Reservoir and the natural runoff of the Yellow River,while there was a significant positive correlation between the annual replenishment of Longyangxia[JP] Reservoir and the proportion of surface water consumption in the natural runoff in the Yellow River basin.From 1986 to 2016,when the proportion of surface water consumption in natural runoff in the Yellow River basin exceeded 64%,the year that Longyangxia Reservoir replenished water took up 8462%.When the proportion of surface water consumption in natural runoff in the Yellow [JP2]River basin was below 64%,the year that Longyangxia Reservoir impounded water took up 75.00%.The Longyangxia [JP]Reservoir played an important role in ensuring the social water supply and river ecological water supply in dry years.
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