The determination of feasible design flood is the most important to control flood damage in river management.
Concentration time and storage constant in the Clark unit hydrograph method mainly affects magnitude of peak
flood and shape of hydrograph. Model parameters should be calibrated using observed discharge but due to
deficiency of observed data the parameters have been adopted by empirical formula. This study is to suggest
concentration time and storage constant based on the observed rainfall-runoff data at GongDo stage station in the
Ansung river basin. To do this, five criteria have been suggested to compute root mean square error(RMSE) and
residual of oserved value and computed one. Once concentration time and storage constant have been determined
from three rainfall-runoff event selected at the station, the five criteria based on observed hydrograph and computed
hydrograph by the Clark model have been computed to determine the value of concentration time and storage
constant. A criteria has been proposed to determine concentration time and storage constant based on the results of
the observed hydrograph and the Clark model. It has also been shown that an exponent value of concentration
time-cumulative area curve should be determined based on the shape of watershed.