# An Estimation Method of River Dry Runoff Alteration after Upper New Reservoirs Storage
## 1 Introduction
Due to their importance as crucial waterways and their significant roles in the Chinese Southwest communication nets, the mainstream and certain tributaries of the upper Yangtze River have drawn increasing attention to their impact on channel hydraulic conditions. This attention has particularly focused on the cascade reservoirs of the Three Gorges, Xijiangba, and Xiluoudu in the mainstream, as well as Pubugou and Zipingpu in the Min River tributary. The accumulation of sand sediment in the upper reservoirs has resulted in the discharged flow carrying less sand than during natural stages, disrupting the original balance of erosion and deposition in the downstream riverbed. This disruption has led to intense scouring, water level decline, and insufficient depth in critical waterways. Conversely, in certain instances, the waterway depth has significantly improved in the upper reservoir regions, and there has been an increase in dry season runoff to some extent. In summary, the impact of cascade reservoir construction cannot be simply defined as good or bad; rather, it involves complex aspects, processes, and extents that continue to attract ongoing research and investigation. However, previous research has predominantly focused on the impact of downstream river hydrological changes after dam construction, while giving little attention to the prediction of river attributes. To date, nearly no theoretical method has been reported for accurately estimating future river runoff, despite its importance for river regulation and waterway construction. In the past, artificial values or additional designs were often employed to compensate for this lack of prediction, resulting in wasteful practices and potential engineering invalidation. For instance, if the previous runoff was underestimated, downstream waterway regulations would require deeper channel dredging, consuming additional resources and potentially causing excessive water level declines in the navigation locks of the dam. It is therefore crucial to develop a reliable method for predicting river runoff alterations to avoid unnecessary costs and ensure the effectiveness of engineering projects. This paper aims to develop an estimation method for calculating the alteration of river dry runoff after the construction of new upper reservoirs. The method is based on an analysis of the relationship between reservoir storage capacity, regulation schedule, and historical river runoff changes in the upper watershed of the Yangtze River. As a rank-based non-parametric test for detecting monotonic trends in time series, the Mann-Kendall (M-K) test is suitable for trend test of hydrological data [27; 28; 29; 30]. The long-term hydrological data from multiple key measurement stations were systematically analyzed using methods such as the Mann-Kendall test, dividing them into different periods corresponding to the construction of the dam. The hydrological data alterations were then resolved considering the regulated schedules of relevant cascade reservoirs, resulting in the development of a theoretical estimation method. The validity of the method was assessed using hydrological data from the Jinsha River and Min River basins over the past decade. Furthermore, the method was utilized to predict future river runoff based on the designed adjusive storage capacity of the reservoir.