| Songhua river basin, which is rich in water and soil resources, is one of the major grain-producing regions in China. This basin is located in cold temperate continental monsoon climatic zone. The precipitation concentrates in summer, holding70percents of the annual rainfall. The floods in this basin have the features of high peak, large volume and long lasting time, which make the flood control very important for the area safety. With the rapid development of national economy, the increasing population and improvement of the people’s living standard, higher requirement are proposed for flood control safety. With the economy development, the underling surface of the basin and the river condition has changed, and a large number of water conservancy project have been built, which both have important influence on the flood characteristic. The upstream part to Fengman reservoir in Second Songhua river catchment was selected in this paper as the study area to analyze the impacts of such human activities as the changes of land use/land cover and the construcion of water conservancy project on flood process and simulate the historical flood process on current underlying surface. The study is organized as the followings:(1) Temporal variation of observed runoff in flood season and its influencing factorsAnnual distribution, inter-decadal and inter-annual variations of the flood process were analyzed in this section. Among those, inter-annual variation were analyzed from three aspects, i.e. long-term variety of flood season runoff and maximum peak, abrupt change and periodic variations of high flow and low flow. The effects of natural factor and interferences of human activities on runoff were also analyzed. Runoff responses to natural factors, i.e. climatic factor, El Nino and LaNina, solar spot, were discussed firstly. And the changes of land cover and the development of water conservancy projects over the past dozens of years were described, which could provide data support for the following study about the impact of human activities on flood process. (2) The distributed hydrological model (DHM) considering the spatial variability of land useIn this section, the distributed hydrological model EasyDHM was modified, and a space discrete method to divide the study area according to the land use variation was proposed. Equal elevation bands in original EasyDHM were further divided into several land use units according to their land use types. There were at most six land use units in each equal elevation band, i.e. cultivated land, forestland, grassland, paddy field, residential area and unused land units. The underlying surface features of each land use type were characterized by a set of land use related parameters. As those land use related parameters have definite physical meaning, the parameter values could be directly adopted by only adjusting area value of corresponding land use unit if the land use patterns changed. The modified EasyDHM was employed in this study to analyze the impacts of land use changes on runoff generation and the runoff contribution per unit area for each land-use type. The study area showed9.18%,0.64%,-1.63%,-5.03%,-3.08%, and24.59%variations in runoff with changes in the areas of cultivated land, forest land, grassland, water surface, residential area, and unused land, respectively, during the20-year study period. The magnitude of the runoff per unit area for each land-use type was in the order water surface> residential area> unused land> cultivated land> woodland> grassland. Thus, well-vegetated areas (i.e., grassland and woodland areas) were likely to generate less runoff than areas with less vegetation cover (i.e., unused land and cultivated land) under the same rainfall conditions.(3) Development of cascading reservoirs operation module into the DHMA module for describing the most important aspects of reservoir dynamics with scare data availability was developed. Large and medium-sized reservoirs, which can be located accurately, were used to mark off their reservoir divisions. Small reservoirs, which cannot be located exactly, were packed as a virtual reservoir according to sub-catchments. In each sub-catchment, smaller reservoirs were located at the upstream of larger reservoirs. The method assumed that small reservoirs in each sub-catchment have similar storages capacity and they are shunt connected. The operation of all reservoirs without observation data was control by operation rule. The operation rule curces and other reservoir parameters could be identified by using optimization algorithms. The distributed hydrological model with reservoirs operation module was applied to analyze the influence of reservoirs on flood processes and simulate the historical flood occurrence on current underlying surface. The simulation results were improved after considering reservoirs’influences. Reservoirs played prominent parts by clipping peak, alternating peak and storing flood water in flood-control of rivers. Small floods were more sensitive to change of storage capacity than heavy floods. Historical floods before1990were simulated on current underlying surface, and the simulated flood peak and volume were less than that on historical underlying surface. In addition, occurrence of the flood peak delayed.(4) An optimization technology with parameter sensitivity guiding search directionMotivated by different sensitivity of hydrological model parameters, a modified dynamically dimensioned search (MDDS) algorithm with parameter sensitivity guiding search direction was proposed. The MDDS algorithm was developed in the context of solving the problem of high-dimensional parameter optimization. The sensitivity analysis preceded and was separated from DDS in the MDDS algorithm. In the modified MDDS algorithms, parameter mutation probability showed positive correlation with parameter’s sensitivity. The sensitive parameters that produced more obvious responses in the output model were given greater opportunities to mutate; however, the MDDS algorithm also allowed the insensitive parameters to participate in mutation. The MDDS strategy combined a preference for sensitive parameters with a use of random perturbations, as this yielded a higher probability of finding an improved solution. This evolution strategy enabled the high search efficiency of MDDS algorithm under a low amount of calculation. Test results showed that the MDDS algorithm is better suited to a distributed hydrological model that includes many parameters. When the computational budget is limited, the superiority of the MDDS consists in the greater average best function value and the greater stability of the algorithm; and the MDDS algorithm is more computationally efficient and robust than SCE and DDS identifying only the15most sensitive parameters in the context of distributed hydrological model calibration.(5) Application system for historical flood simulation on current underlying surfaceThe system was developed on the basis of the Microsoft Visual Studio.NET environment. C Sharp language, which was characterized by a friendly graphical interface, was used as system integration language. And Fortran language, which was characterized by the powerful ability of scientific computing, was used as development language of hydrological model component. The dynamic-link library (DLL) in which function interface was supported was coded in Fortran language, and was then called by C#language to realize the operation of system. Mixed programming technique, MapWindow platform and SQL Server were used in this system. So this system was a comprehensive simulation system based on a distributed hydrological model considering human activity disturbance, GIS platform, computer technology and database management system. This system integrates hydrologic analysis, water and rainfall information and engineering conditions query and analysis, historical flood simulation on current underlying surface, and simulation result display functions.(6) Study on historical flood simulation on current underlying surface The application system integrated the distributed hydrological model considering influence of human activities and high dimension parameter identification technology were used to simulate the historical flood on current underlying surface and summarize the influence law of human activities on floods. According to the historical flood simulation result in the outlet section of the basin, the simulation flood peak and volume show a decreasing trend, if they reoccurred on current underlying surface today. The simulation peak time are delayed for floods before1980. |
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