This study aims to evaluate nutrient flux to the coast from the inland areas of Shiretoko in order to understand the key factors controlling the ecological systems of the coast. As an external force, rainfall is considered one of the most significant components controlling nutrient supply to coastal systems in this area. Therefore, to estimate nutrient supply in the future, the bias correction was applied by using Meteorological Research Institute Global Climate Model, which shows good agreement with Automated Meteorological Data Acquisition System data. A synthetic generation technique is used to produce hourly rainfall data, which is necessary for evaluating nutrient supply in Shiretoko. The robustness of the duplicated hourly rainfall intensity was investigated, which reveals that its standard deviation controls nutrient flux when nonlinearity becomes stronger for the evaluation of nutrient supply from a river basin.