In order to improve the accuracy of runoff prediction, based on the control variable method and the daily runoff data of Lanzhou hydrometric station from August 2001 to December 2019, the models of the LSTM, ARIMA, SVR and XGBoost were used to establish 12 model schemes, including single model, EMD decomposition and reconstruction, EMD decomposition and reconstruction after removing noise components, and evaluation indicators of the 12 schemes were compared. The results show that the EMD sequence decomposition and reconstruction technology and noise component elimination based on Hurst exponent are helpful to improve the prediction accuracy. Compared with the single model, the R_RMSE of the model constructed by the former decreased by 15.16% on average, and that of the latter decreased by 28.49% on average. Among the 12 schemes, EMD-SVR-ARIMA with noise components removed is the best model.

The change of channel storage capacity is an important content of river evolution analysis. In the past, the section method was mostly used for simplified calculation. However, in the complex river pattern with multistage branching or tributaries, the section method often needs to be dealt with separately, and the calculation efficiency is low. By introducing the water surface width ratio of the branching channel, optimizing the distribution of the calculated area of the section at the branching point, adding the section, estimating the storage capacity of the channel in the missing section near the tributary estuary, and using the topographic method to restore the section spacing under different water levels, the calculation model of the section method under the complex river pattern is improved. The comparison results show that the calculation accuracy of tank storage capacity is greatly improved by using the above method, which can provide a reference for batch calculation of tank storage capacity under various complex river types.

Flow and sediment are the driving force and material basis for shaping the riverbed. The process of flow and sediment changed by the operation of Three Gorges Reservoir leads to severe riverbed adjustment downstream of the dam. This paper selects the straight-braided channel-Bailuoji as typical which locates in the middle reaches of the Yangtze River, and analyzes the channel development characteristics from perspectives of channel storage capacity, beach-trough scouring and silting and branch channel diversion capacity variation after the impoundment of the Three Gorges Reservoir. Combined with the change in the duration of floods above the channel forming discharge, the impact of the flow process change on the development of the Bailuoji Reach is quantitatively calculated. The results show that in recent years, the channel storage capacity of the Bailuoji reach has increased and scouring mainly locates in the low-flow channel. The right branch is the main branch and the diversion ratio has increased under the low-discharge. The flow and sediment conditions after the impoundment of the Three Gorges Reservoir can maintain the development of the main branch of the Bailuoji reach. Compared with the flow process without reservoir regulation and storage, the duration of high flow above the channel forming discharge is relatively shortened under the current dispatching mode, the total amount of erosion in the Bailuoji reach is relatively small and the diversion ratio of the right branch is slightly reduced. The research results can provide reference for middle-small flood dispatch of Three Gorges Reservoir.

In order to understand the impact of vegetation groups on the hydraulic characteristics of natural meandering river, this paper designed two kinds of layout forms in which the cluster layout simulated the natural cluster vegetation group, and the uniform layout simulated the artificial regular planting vegetation group. Through the flume experiment, the impact of different layout forms of vegetation on the distribution of flow field, hydrodynamic axis, turbulent kinetic energy and transverse circulation structure in continuous bends was explored. The results show that the flow field in the bend appears obvious velocity partition in which the high velocity area is close to the convex bank, and the low velocity area is attached to the concave bank. Vegetation layout makes the distribution range of high and low flow rate areas have the opposite change rule: the high flow rate area increases, while the low flow rate area decreases. Under the action of the bend, the hydrodynamic axis 'gradually swings to the convex bank on the upstream side of the bend top, close to the convex bank near the bend top and swings to the concave bank on the downstream side of the bend top, completing a cycle of motion changes. Vegetation layout makes it swing to the middle of the flume and the deviation degree of cluster layout is greater than that of uniform layout. Under the action of the bend, the distribution of turbulent kinetic energy of water flow is small on both sides and large in the middle. On the whole, the layout of vegetation increases the turbulent kinetic energy on the downstream side of the bent top, and the impact of cluster layout is greater than the uniform. The vegetation layout changes the circulation structure of the section, which is manifested by the swing of the vortex core position and the change of the distribution range.

Aiming at the impact of large-scale ecological water replenishment (four times of ecological water replenishment in Beijing section of Yongding River) on the long-term cut-off river, a MIKE 11 hydrodynamic model was established. The variation characteristics of runoff and infiltration capacity before and after water replenishment were quantitatively evaluated by scenario simulation analysis. The results show that the inflow runoff sequence of Guanting Reservoir can be divided into three stages: natural stage, human weak interference stage and human strong interference stage. During the three ecological replenishment periods from 2019 to 2020, the flow of the main section of the Beijing section of Yongding River is much larger than the ecological flow of human strong interference stage. In the autumn of 2021, the loss of permeability coefficient and leakage of ecological water supplement was the lowest, indicating that the leakage of Yongding River was significantly reduced through three consecutive years of large-scale ecological water supplement. Taking the ecological replenishment in autumn of 2021 as an example, the fitting functions of channel storage and replenishment flow, submerged area and replenishment flow, submerged area and channel storage, permeability coefficient and replenishment flow were established, which can provide some guidance for the development of ecological replenishment in Yongding River. The results of this study can effectively guide the optimization and formulation of ecological water supplement scheme in the Beijing section of the Yongding River, and provide reference for the comprehensive management of the Yongding River Basin and the exploration of ecological restoration of long-term river courses in North China.

As an essential influencing factor of permissible pollution bearing capacity, hydrological condition was changed significantly due to hydropower cascade development. According to the water functional division and water quality control target in Panzhihua reach of Jinsha River, the variation of dynamic permissible pollution bearing capacity (COD, NH3-N) with hydropower cascade development were calculated depending on 2-D hydrodynamic and water quality models. Furthermore, response relationship between permissible pollution bearing capacity and hydrological factors was established to evaluate the effect of hydropower cascade. The results show that the dynamic permissible pollution bearing capacity of COD and NH3-N decreased by 5.2%-27.8% and 3.8%-41.2%, respectively, and its decreased range was found to be more evident in dry season. The variation trend of permissible pollution bearing capacity and near-bank discharge was similar for different conditions. Simple coefficients between the variation of near-bank discharge and permissible pollution bearing capacity were calculated. However, a significant correlation was found with permissible pollution bearing capacity of NH3-N only.

Based on the understanding of the concept of happiness river, taking 6 rivers with moderate river scale in Qiqihar Areas as an example, the TOPSIS evaluation model is constructed by combining the proportional scale AHP and AEM to carry out the evaluation on the happiness river, whose goal was divided into 5 guiding layers and 20 indicator layers. The evaluation results show that the overall happiness of the rivers in Qiqihar Areas is grade Ⅲ, which the happiness of Yalu River is grade Ⅳ, the happiness of Alen River, Nuomin River, Yin River and Runjin River is grade Ⅲ, and the happiness of Chaoer River is grade Ⅱ. In addition, the common problems of poor evaluation grade in the results were analyzed and the treatment suggestions were put forward.

The nonlinear equation was established for the incipient motion of sediment under constant flow, including Reynolds number, Hiltz number and sediment erosion intensity. Based on cusp catastrophe theory, this experimental process focused on the main factors affecting incipient motion of sediment and bottom riverbed scouring, and considerd different flow intensity conditions. The Reynolds number of sand particles from other experiments was used in the nonlinear equation of this study to verify the influence of Reynolds number. The theoretical value calculated by this formula has a strong consistency with the test value, after comparing the results and data. Therefore, this nonlinear equation can basically meet the requirements for predicting the scouring intensity of fine sediment.

To determine the appropriate ecological flow for small watershed assessment, taking Qingshui River in Pearl River Basin as an example, the monthly ecological flow was calculated by using six hydrological methods such as Tennant method, Texas method and frequency curve method. Furthermore, combined with hydrological rhythm and ecological flow guarantee rate, the advantages and disadvantages of the six methods were compared and analyzed in the dry and flood periods, and the most suitable ecological flow calculation method for small watershed was obtained to determine the ecological flow. The results show that the ecological flow of Hongqi Reservoir section is between 0.12-1.52 m³/s, and the ecological flow of Puzhehei section is between 0.30-4.95 m³/s. The fitting coefficients of hydrological rhythm are 0.993 and 0.998, respectively. The ecological flow guarantee rate is at a good level, and the ecological flow selection result is reasonable. The results can provide a reference for similar national small watersheds to select appropriate calculation method to determine the ecological flow.

In order to make up for the shortcomings of the single Gini coefficient method in the evaluation of spatial equilibrium of water resources, it needs to systematically evaluate the matching relationship between water resources and social development. Selecting arable land area, population size, industrial and agricultural output value and gross product in Xinjiang as the indicators, which affects the carrying capacity of water resources, combination of correlation number method and Lorenz curves was used to quantitatively analyze the equilibrium between regional and sub-regional water resources and indicators. Five element subtractive set of potentials was adopted to identify the development of equilibrium of sub-regional water resources. The results show that the overall water balance in Xinjiang has been rising and then falling during 2011-2018, and is uneven from year to year. The analysis of the indicators shows that the water balance fluctuates between uneven and uneven. The analysis of the sub-regional water balance and trends shows that most areas are in an uneven and inverse state, and exacerbates the unevenness of water resources and identifies the targets for water balance regulation, which points out the direction for reasonable improvement of the balance of water resources.