| The Yangzte River basin (also called the Changjiang River basin) is the largest river basin in China, which has complicated geomorphological features and diverse climatic patterns, and frequent natrual disaters caused by flood and drought.Understanding the hydrological response to climate change and human activities, assessment of the changes in water resources and the flood forecast are the key research topics for integrated watershed management with new anthropo-concept of health and harmonious river. Building on the geomorphology-based hydrological model -- GBHM, a large-scale distributed hydrological model has been established in the upper Yangtze River. Applications of the model for water resources assessment, drought severity evaluation and flood forecast have been addressed in this research.Using meteorological data from 154 weather stations together with river discharge from 26 hydrological gauges, it analyzed the spatio-temporal variation of temperature, precipitation and runoff during the last half century. The TFPW-MK statistical test has been applied to detect the significance of trends. Both runoff and precipitation showed a significant increasing trend in summer along the middle and lower reaches in the 1990s. However, a decreasing trend for runoff and precipitation was found in spring and autumn, particularly in the upper reaches.Then, a large scale distributed hydrological model has been established in the upper Yangtze River with area of 1 million km2. In this model, the study basin is divided into a discrete grid system of 10-km size, and each grid is represented by a number of geometrically-symmetrical hillslopes. The basin is divided into a number of sub-basins with the proper size and the river network is identified up to the main stream of the minimum sub-basin. The hydrological components, including runoff generation from the hillslopes and the flow routing in the river network are modelled using physically-based approaches. Taking the meteorological inputs, the model simulated river discharge during 1961~2000 at different locations in the river network, and temporal changes and spatial distributions of soil moisture and evapotranspiration, which provide an inside investigation into water resources in the study basin. Results showed that the ratio of seasonal runoff to annual one has a significant increasing trend in summer in the eastern Sichuan basin and the Three Gorges region in the 1990s, but a decreasing trend in autumn. This implies an increasing flood risk in summer and water shortage in autumn. Based on the hydrological simulation, a new drought index, GBHM-PDSI, was proposed. It was found that the new drought index has obvious advantages on describing the temporal change of drought severity and the spatial variation of dryness.In order to reduce the uncertainties of real time flood forecast in the Three Gorges region, the radar rainfall data has been used together with the distributed hydrological model. Results showed that in addition to the rain gauge network the weather radar can provide better spatial distribution of rainfall. By means of physically-based distributed hydrological model combining with radar rainfall data, it can capture adequately the spatial variation of rainstorm, and provide better flood forecast at real time.For meeting with the requirements of integrated operation of the Three Gorges reservoir, a new space-nested hydrological model with different spatial scales (present used 10km and 1km grid sizes) has been built. Inflow simulation from the upper streams of the reservior, inflow simulation from the inter-basin of the Three Gorges and the flood routing in the reservoir were coupled into one model system. The preliminary results showed that this model could sucessfuly simulate the dynamic change in reservoir water level and provide a useful tool for the reservoir operation.Using this model as the core, a Digital Hydrology Simulation System for the Yangtze River basin has been proposed as the tool for the integrated river management in future.
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