Surface runoff and soil erosion of the Upper Lagawe River Sub-watershed, Ifugao Province, Philippines were quantified using Geographic Information Systems (GIS)-based numerical simulation. PCRaster software was used as a framework for the model. Manning’s and Rose, and Freebairn’s Equations were utilized to compute surface runoff velocity and sediment mass. Analysis and comparison of the measured and predicted streamflow yielded a mean Nash and Sutcliffe Efficiency Index of 0.70, a mean percent bias of - 8.38%, and a mean root mean square error (RMSE) of 0.16. Based on the model, super typhoon Lando (international name: Koppu) generated an estimated surface runoff volume of 186000 m3 and an estimated sediment mass of 36587 kg for the entire sub-watershed. The mass of eroded sediments equated to 18.6 kg/ ha of erosion rate just for the said super typhoon. Stable ecosystems, such as the Upper Lagawe River Sub-watershed, where forest vegetation is intact and farmers practice soil conservation techniques are still vulnerable to the effects of extreme rainstorm events, can generate voluminous surface runoff and eroded sediments.