This report proposes a modification method of the mathematical optimization technique by the Standard Powell method which is proposed by A. Nagai and M. Kadoya, that is nessary to identfy the runoff model involving parameters which have quantitative relationships of a different order. The outline is as follows.
(1) The modified Standard Powell method is introduced, in which each model parameter is identified to a seached value as sum of an intial value and a value multipled for a constant small term of model parameter. It is considered that a siginificant digit for model parameter is setted to the small term. So, a set of model parameter is searched by a step wise till converging to optimum values.
(2) Three type models are applied to the identification of runoff model. The first type one is the long and short terms runoff model (LST-II), which is developed by Kadoya and Nagai (1988) for analyzing both flood and long term runoff successively. The second type one is the modified LST-H model, in which the effects of distribution of permiable area and impermiable one in the basin is expressed by apllying the structure of the upper zone of first tank LST-II. The third type one is a lumped type model combined the LST-II model with kinematic wave one, named as the lumped KiWS model.
(3) These three type runoff models are adapted to the small exprimental basin at land reclamation area in Masuda City and Hii river basin as large scale one. For the repesentative floods, the model are identified by using the modified SP method. It is shown that the proposed method is improved with respect to evaluate the set of model parameter in the any significant digit designated for each parameter. And it is recognized that the reproducibility of the peak discharge and the observed hydrograph is better for the modified LST-II model than the LST-II model and the lumped KiWS model.