As the frequency of drought increases due to climate change, water scarcity in agriculture would be a main issue. However, it seems difficult to solvethe water scarcity by securing alternative water sources. The aim of this study is to analyze optimal water supply capacity of agricultural reservoir foremergency operation connecting reservoirs and dams. First, we simulated the water storage of agricultural reservoir playing the role emergency watersupplier to other water facility such as dams and other reservoirs. In particular, the results of simulation of water storage through K-HAS model wascalibrated using the optimization process based on ratio correction factors of outflow and inflow. Finally, the optimal amount of water supply securingwater supply reliability in emergency interconnection operation was analyzed. The results of this study showed that Janchi reservoir could supply 12.8thousand m3/day maintaining 90 % water supply reliability. The result of this study could suggest the standard for connecting water facilities asemergency water supply.