Rapid climate change by global warming effects has frequently affected intake water qualities in water treatment process. This study evaluated a coagulant reduction method using multiple water resource blending. In normal conditions defined by turbidity 0-25NTU, TOC 0-8mg/L, and NO3-N 1-8mg/L, PASC dose of 15mg/L was determined based on the jar test and the pilot test. While insufficient water quantity or degradation of water quality was measured, ground water was blended a front or back of sandfilter to satisfy water quality and quantity. In emergency condition defined by turbidity 25-400NTU, TOC over 8mg/L, NO3-N over 8mg/L, pH 6.5-8.0, and TDS over 100 mg/L, the coagulant dose was determined 17.4mg/L at turbidity 100NTU, 23.2mg/L at 130NTU and 29mg/L at 150NTU. From over 400NTU, coagulant dose did not degrade water quality enough for sandfiltration. By that point, ground water was blended in the blending tank in order to reduce water contaminant levels. All results were satisfied with quality purpose of drinking water production.