By investigating the historical meteorological data, disaster situation data and disaster bearing body information during 1951-2020, this paper establishes a risk survey database to accurately evaluate the hazard of rainstorm disaster in Changsha County of Hunan Province. Comprehensively considering the impact of multiple meteorological disaster causing factors of rainstorm disaster and inducing environment such as geology and drainage network of watershed, this paper carries out the analysis of basic characteristics of rainstorm and risk assessment of disaster, and forms a risk assessment model. Geological disasters in Changsha County mainly occur in the northeast of Changsha County. The urban waterlogging disaster mainly affects the county seat and the surrounding areas with high urbanization. Mountain torrents mainly affect the central and northern areas of Changsha County. The risk of rainstorm disaster in Changsha County tends to weaken from the southeast and northeast to the middle. Generally speaking, the risk of rainstorm in the whole county is not high, only Kaihui Town, Gaoqiao Town and Huanghua town are high-risk, and rainstorm events are easy to cause secondary disasters in these three towns. There are two relatively high value centers in the county, which are located in Huanghua town and Jiangbei town in the southeast and Kaihui town and Gaoqiao Town in the north. The comprehensive risk level of rainstorm in Ansha Town, Guoyuan Town, Chunhua Town and urban area in the middle is low.

During the landfall of Typhoon Billis in July 2006, Wujiang River, a tributary of the Beijiang River basin, was hit by an unusual rainstorm in history, resulting in the largest flood ever recorded in the basin. Based on precipitation record and Doppler weather radar data, it is found that the "train effect" induced by a convective storm, which originated in southern Hunan and crossed the divide, is the main reason for the extreme precipitation in Wujiang River basin. The study of the convective storm activity by using GIS technology shows that the complex topography of Wujiang River basin changed the direction of the convective storm several times, which greatly prolongs the duration of the storm in Lechang city, resulting in local rainstorm and flash flood. The study also found that the Huangcen Reservoir in Qitianling Mountain area was affected by typhoon, which probably enhanced local convections. The study reveals the influence of mesoscale and miso-scale topography on typhoon rainstorm, which contribute to the prevention of typhoon-induced disasters.

In view of the differences between national, continental and global scale precipitation data sets, there are many studies, but the reliability of data sets on river basin scale is rarely considered. To this end, the maximum/minimum difference percentage method and TFPM-MK trend analysis method were used to study and evaluate the differences between 12 widely used precipitation data sets in precipitation estimation and change trends in 6 292 basins around the world. The results show that there are large differences between precipitation data sets. There are 2 469 basins with an annual average precipitation difference of more than 500 mm/a, covering an area of 4 419.6×10⁴km², mainly distributed in Greenland, Africa, Oceania and West Asia. The existing data products cannot reach an agreement on the precipitation trend of all basins, and it is difficult to provide a reasonable scientific basis for the formulation and implementation of effective policies to ensure water conservancy and power generation, domestic water supply, flood control and early warning in the basin.

As a widely used function in hydrologic frequency and flood computation, the Gamma function has many calculation methods, and the scope of application and computation accuracy also vary. In order to explore a high-precision Gamma function computation method suitable for hydrological computation, different Gamma function asymptotic expansion formulas such as Stirling series and its derived approximation formula are used for computation and analysis. Comparing the accuracy and application range of various methods, the results show that the piecewise polynomial method has the smallest truncation error and the highest accuracy; The second is the Ramanujan asymptotic expansion and the Stirling first tetranomial. The recommendatory high-precision Gamma function computation method can effectively improve the computation accuracy of hydrological results, which can provide fast and accurate results for the planning and design of various hydro projects to determine project scale and management decisions.

To analyze the potential influence of the construction and operation of the cascade junction on the hydrologic regime of four major Chinese carps spawning sites in the downstream of the Xinjiang River, 21 parameters were selected to establish the hydrological index system of fish spawning sites. The one-dimensional hydrodynamic model was adopted to analyze the hydrological regime of Daxidu spawning site before and after the construction of the cascade junction. The range of variability approach was used to analyze the hydrological changes of spawning grounds during the breeding period before and after the construction of cascade junction. The results show that the flow during the breeding period decreases after the operation of the cascading junction, which made the variation range of water flow limited in a small range. Furthermore, the number of high flow pulse decreased, which reduced the stimulation signal to spawning of four large fishes. This study quantified the impact of the construction of cascade junction in the downstream of the Xinjiang River on the spawning grounds during the breeding period, and provided scientific guidance for the design of the local hub operation scheme.

Scientific assessment of the hydrological regime is an important part of the assessment of the health of the river ecological environment. The IHA indicators were used to quantitatively evaluate the change of hydrological indicators of Gaobazhou Station during the construction period and operation period of water conservancy projects in the Qingjiang River Basin. On the basis of analyzing the change law of five different IHA indicators, the overall hydrological situation change of Qingjiang River was analyzed by selecting three different RVA methods, which include the Nemero index method, the RVA method based on the European distance method and the RVA method considering the comprehensive weight. It shows that the results of the analysis of the hydrological situation in the Qingjiang River Basin by the three methods during the construction period and the comprehensive utilization period are moderate and highly changed, respectively, and the hydrological situation change in the Qingjiang River Basin has an increasing trend. The evaluation results of the RVA method considering comprehensive weights are similar to the previous two methods and are reasonable, which can better reflect the changes in the river hydrological situation after the construction of the water conservancy project in the Qingjiang River Basin.

In order to better understand the impact of vegetation restoration on hydrological processes, the SWATMODFLOW model and dynamic HRU was coupled, and LU-SWAT-MODFLOW model was developed to accurately simulate the dynamic change of vegetation and groundwater in the lower reaches of the Bayin River Basin. The results revealed that the LAI, ET and groundwater level simulated by LU-SWAT-MODFLOW model are better; After vegetation restoration in the study area, the monthly evapotranspiration of different sub-basins increased by 0-1.5 mm, and the annual evapotranspiration increased by 0-6 mm; The average monthly groundwater recharge increased about 1.27 mm, and the average annual groundwater recharge increased 14.02 mm. The exchange of the surface water and groundwater in the study area have obvious changes in the areas where the vegetation restoration type is low-coverage grassland and bare land converted to forestland.

The expansion of cities and towns has caused a large amount of waters to be compressed, occupied or even disappeared, and the risk of floods in the region has increased sharply. This study is based on Hengyang, a typical city in midstream of Xiangjiang River. Based on expansion speed index and intensity index of Xiangjiang midstream from 2001 to 2016, the linkage relationship between the two is quantitatively analyzed, and the response of water system change under the background of urban expansion is analyzed. The results show that the urban development in midstream of Xiangjiang River can be divided into three stages. The urban expansion is characterized by a high speed and then a low intensity. During the study period, the total number of river systems showed a declining trend, and the channel morphology showed a shrinking state. The change of the structure of river systems was more severe than the change of the number. The basin with few river systems and single structure had a sharp decline. Urban expansion has a close relationship with the change of river system, and has a greater impact on the change of the number of river systems. And it is highly related to the basin with few water systems and single structure. It can be seen that human activities such as urban construction and expansion should minimize the adverse impact on such watersheds.

In order to study the effect of arrangement of non-submerged rigid vegetation on surface flow hydraulic characteristics, this paper established a hydraulic model based on the actual situation. The average vegetation was evenly arranged as the control group, the vegetation was arranged in a strip pattern and the distance between the strips increased in multiples of 1.333, 1.667, 2.000, 2.333, 2.667, while the distance within the strips decreased by 0.833, 0.667, 0.500, 0.333, 0.167 as the experimental group. The numerical simulation based on Fluent verified the effects of changes in the arrangement of vegetation with the 5% coverage (210 plants) on the water flow and hydraulic characteristics. The results show that the vegetation coverage was the same and the vegetation arrangement tended to be uniform. Under the influence of water depth, the flow rate decreased, and the Reynolds stress in the lower half of the water depth increased. The surface flow rate on the non-submerged vegetation decreased, and the turbulent flow increased. The water energy is consumed to prevent water and reduce the erosion of water surface. This study provides reference for flood control and disaster reduction.

In order to accurately judge the start and end time of snowmelt period, the upstream of the Danba basin, the main source of snowmelt runoff in Daduhe River, was taken as an example. Based on hydrometeorological data from 2009 to 2020, set pair analysis method, systematic clustering method and K-means clustering method were used to calculate the snowmelt period in the dry season (November to the next May), and the rationality of the staging results was evaluated. The division scheme of final snowmelt period in the basin was determined. The results show that the main snowmelt period is from January 11 to May 10, the preceding period is from November 1 to January 10, and the following period is from May 11 to May 31, which is the beginning period of rainfall runoff (there is still some snowmelt runoff). The research results can provide reference for the classification of snowmelt periods in high latitude areas.