# An Estimation Method of River Dry Runoff Alteration after Upper New Reservoirs Storage
## 1 Introduction  
Over the past decades, numerous reservoirs have been constructed in the southwest region of China, leading to significant alterations in the hydraulic characteristics of upstream rivers in China; correspondingly, the upstream river hydraulic character has been seriously altered [1; 2; 3; 4]. However, considering the critical importance of water supply, shipping development, power generation, and ecological conservation, there are plans to construct reservoirs of varying sizes along the mainstream and tributaries of the upper basin of the Yangtze River in the future. The extent of the impact on river hydrology following the construction of reservoirs has consistently been a popular and widely discussed topic, attracting considerable attention from researchers in the field. It is widely acknowledged that the river flow undergoes alterations on an annual, monthly, and daily basis after upper cascade reservoir storage. The quantitative variations are generally close to the storage capacity but may differ slightly [5; 6; 7; 8; 9]. In general, reservoirs can be classified into three types based on their regulating characteristics: multi-annual regulating, annual regulating, and non-annual regulating. Multi-annual regulating reservoirs, such as the Xiluoud-Xiangjiaba reservoirs in the upper Yangtze River, have the capacity to modify water resources on an interannual basis, partially mitigating the impact of basin meteorological variations. Annual regulating reservoirs primarily bring about seasonal changes in river flow, typically resulting in reduced downstream discharge during the flood season and increased discharge during the dry season. The specific alterations depend on the reservoir control mode [10; 11]. In recent years, the alteration of river hydrological characteristics has been associated with another significant factor, namely climate variation [12; 13]. With the gradual rise in temperature, precipitation and evaporation within larger spatial watersheds occur simultaneously, influencing the processes of runoff yield and concentration. Consequently, researchers have made efforts to isolate the individual impacts of climate variation and reservoir storage. Various hydrological datasets spanning several decades have been utilized, employing different regression models, wavelet methods, and other techniques to analyze the aforementioned impacts [14]. Downstream runoff exhibits close variability with meteorological variables and is significantly influenced by dam construction [15]. However, the long-term average runoff over multiple decades can encompass the combined effects of climate change. Ultimately, the impact of climate change becomes evident over an extended period of time. Regarding the hydrological factors influenced by reservoir operations, considerable attention has been focused on downstream river discharge and sediment load [16; 17; 18; 19]. Regardless of whether the reservoir operates on a multi-annual, annual, or daily basis, the total runoff volume in a year is relatively less adjusted, except for climate variations. However, the construction of dams has led to a significant reduction in sediment load by intercepting the entire river flow [20; 21; 22; 23]. As a result of these pronounced alterations, various aspects of river functioning have been extensively studied, including riverbed regime, riverbank stability, waterway hydraulic conditions, and ecosystem health [24]. Consequently, to mitigate these impacts, different recommendations for adjusting reservoir operation schedules have been proposed. For example, downstream river flood processes have become more subdued, with fewer multi-peak natural floods occurring after reservoir storage, and the seasonal water temperature cycle has experienced slight delays [25; 26]. These new variations have directly impacted the reproductive process of Chinese carp following the construction of the Three Gorges Dam, prompting the development and practical utilization of optimized scheduling to simulate a more natural flood pattern since 2011.