TOPMODEL is a storage-type semi-distributed-parameter model with advantages existing in each of the distributed-parameter and lumped-parameter models. It can consider the physical characteristics in the rainfall-runoff process in mountainous basins to some extent and has lately attracted considerable attention in the research field of rainfall-runoff analysis. In the TOPMODEL, water balance in groundwater storage is calculated by a simple lumpedparameter model. A basin area is horizontally divided into small square grids and phenomena in surface soil layers, such as surface runoff and groundwater recharge, are expressed by distributed- parameter models on each square grid. Time-varying runoff-contributing area during flood events can also be calculated in the distributed-parameter models by judging the generation of surface runoff from the soil moisture deficit in each square grid. In this paper, the characteristics of TOPMODEL parameters and the ability of TOPMODEL to estimate the observed discharge were discussed using a parameter optimization and a sensitivity analysis of parameters. Firstly, TOPMODEL parameters were optimally searched by SCE-UA method that is relatively new optimization method, and together by Simple Genetic Algorithm in order to comparing with. The SCE-UA method is the optimal search method that combines the concept of group mixture and is similar to genetic algorithms. The relative error was used in the parameter optimizations. The sensitivity analysis was then calculated using the relative sensitivity. The results indicated that the parameter optimization was successfully executed for the TOPMODEL parameters that had a relatively large value of the relative sensitivity and that the SCE-UA method was effective for the optimization of the TOPMODEL parameters.
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