It is important to accurately evaluate storm runoff loading for preserving the water quality of stagnant water bodies. In this study, we analyzed the effects of hydrological factors on the estimation of storm runoff loading using hourly observed data (rainfall, river flow and water quality) at 2 points located in the center of the Yahagi River basin, and applied simple regression models to estimate the loading. Firstly, we observed close relationships among rainfall, river flow, water quality and loading. Secondly, by evaluating the differences between estimated and observed loading rates during the rainy season, we proposed conditions for upgrading the accuracy of loading estimation. Thirdly, we analyzed the factors that affect storm runoff loading from hydrological aspects, and identified relationships between the cumulative loading residual and factors such as preceding dry weather period. Finally, some conditions for the estimation of storm runoff loading are outlined, which include improving the application of the regression models to estimate the loading.