<p>This study evaluated the efficiency of reduction facilities installed in agricultural drainage channels in Gyeongsangnam-do, South Korea. The stormwater management model was employed to evaluate the efficiency of the reduction facilities, and to enhance the simulation capability of the rainfall-runoff model, topographic information of the target region was obtained through field surveys. Unmanned aerial vehicles were used to analyze land-use patterns and drainage-flow structures and to acquire topographic information of areas with dense networks of agricultural drainage channels. Precise spatial information was applied to the model by overlapping the current land-use characteristics and digital topographic maps. For the simulation, we considered 10 scenarios for the installation of the reduction facilities in the channels (SR1–SR10), with different biological oxygen demands, total nitrogen concentrations, and total installation costs. Among all the scenarios concerning the reduction efficiency, the simultaneous application of SR4, SR8, and SR10 yielded the best results. SR7 was the most suitable scenario when prioritizing installation costs, with the total cost being USD 775,144. When considering both reduction efficiency and installation costs, SR3 and SR7 were the most suitable scenarios. Our study presents an effective method for selecting the location of pollutant reduction facilities in agricultural drainage channels to reduce the nonpoint source pollution load in these channels. Notably, our study can serve as a foundation for policymakers and planners to mitigate environmental pollution caused by agricultural activities.</p>