A development of numerical model on the estimation of the long-term run-off for the design of riverheads works.

Although long-term runoff analysis is important as much as flood analysis in the design of water works, the technological level of the former is relatively lower than that of the latter. In this respect, the precise estimation model for the volume of successive runoff should be developed as soon as possible. Up to now, in Korea, Gajiyama's formula has been widely used in long-term runoff analysis, which has many problems in applying in real situation. On the other hand, in flood analysis, unit hydrograph method has been exclusively used. Therefore, this study aims at trying to apply unit hydrograph method in long-term runoff analysis for the betterment of its estimation. Four test catchment areas were selected; Maesan area in Namhan river as a representative area of Han river system, Cheongju area in Musim river as one of Geum river system, Hwasun area in Hwasun river as one of Yongsan river system, and Supyung area in Geum river as one of Nakdong river system. In the analysis of unit hydrograph, separation of effective rainfall was carried out firstly. Considering that effective rainfall and moisture condidtion of catchment area are inside and outside of a phenomenon respectively and the latter is not considered in the analysis, initial base flow (qb) was selected as an index of moisture condition. AT the same time, basic equation (Eq.7) was established, in which qb can take a role as a parameter in relating between cumulative rainfall (P) and cumulative loss of rainfall (Ld). Based on the above equation, computer program for estimation model of qb was separately developed according to the range of qb, developed model was applied to measured hydrographs and hyetographs for total 10 years in 4 test areas and effective rainfall was estimated. Estimation precision of model was checked as shown in Tab-6 and Fig.8. In the next stage, based on the estimated effective rainfall (R) and runoff (Qd), a runoff distribution ratio was calculated for each test area by computerised least square method and used in making unit hydrographs in each test area.