This study modified the generalized watershed loading functions (GWLF) model, and applied the modified model to simulate the water flow of the catchment area of the Lan-Yang River, by entering the current status of land use, hydrological data, and meteorological data into the model. This modified model took soil resistance into consideration when calculating evapotranspiration and added the parameters of moisture content in the unsaturated zone and hydraulic conductivity to calculate percolation. This model also considered the percolation that directly penetrated into the saturated zone. The modifications enabled the GWLF model to simulate the nature of daily stream flows. The results indicated that the modified GWLF model could improve the calculation of percolation and evapotranspiration in the unsaturated zone for the dry season. Because percolation enhanced groundwater flow, we adjusted the underestimations for low-rainfall periods to simulate the daily stream flow. The results showed that the modified GWLF model was appropriate for simulating not only monthly, but also daily stream flows.