Jangchan reservoir is located in Okcheon county, Chungbuk province, of which watershed area is $29.4;km^2$ from outside, and $5.1;km^2$ from inside watershed, effective storage capacity is $392{ imes}10^4;m^3$, paddy area to be irrigated is 474 ha. To determine inflows from Keumcheon weir located in outside watershed on an optimum level, a repeated procedure which is composed of simulation of inflows to Keumcheon weir, setting of range of water taking at Keumcheon weir, simulation of inflows to Jangchan reservoir, estimation of paddy water from Jangchan reservoir, and simulation of water storages in Jangchan reservoir was selected. Parameters of DAWAST model for simulating inflows to Jangchan reservoir were determined to UMAX of 315 mm, LMAX of 21 mm, FC of 130 mm, CP of 0.018, and CE of 0.007 with absolute sum of errors in reservoir water storages minimized using unconstrained Simplex method because of no inflows data. Inflows to Keumcheon weir were simulated to $2,132{ imes}10^4;m^3$ on an annual average. Optimal range of water taking at Keumcheon weir to transfer to Jangchan reservoir were $0.81{sim}50;mm/km^2/d$, which were summed up to $1,397{ imes}10^4;m^3$ in 66% of total on an annual average. Inflows to Jangchan reservoir were simulated to $1,739{ imes}10^4;m^3$ on an annual average of which were 80 % from Keumcheon weir of outside watershed. Requirements to paddy water from Jangchan reservoir were estimated to $543{ imes}10^4;m^3$ on an annual average.