This study was performed to ascertain the possibility of securing inflows to reservoir with low ratio of watershed to paddy field areas by outside diversion weir. The case of Maengdong reservoir and Samryong diversion weir was selected. Most of inflows to Maengdong reservoir with watershed area of $7.06;km^2$ and total storage capacity of $1,269{ imes}10^4;m^3$ are filled with intake water from outside Samryong diversion weir. Only using water storage data in Maengdong reservoir from 1991 to 2009, the range of water intake in Samryong diversion weir to Maengdong reservoir was optimized to 0.135~30 mm/d, from which water intake to Maengdong reservoir was $1,672.9{ imes}10^4;m^3$ (70.1 %) and downstream outflow to Weonnam reservoir was $714.4{ imes}10^4;m^3$ (29.9 %). The parameters of DAWAST model for reservoir inflow were determined to UMAX of 313.8 mm, LMAX 20.3 mm, FC 136.8 mm, CP 0.018, and CE 0.007. Inflows to Maengdong reservoir were $427.1{ imes}10^4;m^3$ (20.3 %) from inside watershed, and $1,672.9{ imes}10^4;m^3$ (79.7 %) from outside. Paddy irrigation water requirements were estimated to $1,549{ imes}10^4;m^3$ on annual average. Operation rule curve was drawn by using daily inflow and irrigation requirement data. By securing the amount of inflow to Maengdong reservoir to about 80 % from outside Samryong diversion weir, water supply capacity for irrigation of $1,549{ imes}10^4;m^3/yr$ was analyzed to be enough. Additional water supplies for instream flow were analyzed to $1,412;m^3/d$ in normal reservoir operation, $36,000;m^3/d$ in withdrawal limit operation by operation rule curve from October to March of non irrigation period.