Knowledge of water movement allows its proper management and planning over a watershed. The aim of this work was to model the trend of spatio – temporal change of runoff in a hilly watershed. The area of study is placed in the experimental watershed of the Belisario Creek, in the Southwest of the Buenos Aires province, Argentina. Runoff was determined using the Curve Number method (CN), within the Geographic Information System Idrisi Taiga ®. Land Change Modeler allowed us to determine the trend of change for runoff, over the last forty five years (1966 – 2011). Increase of runoff volume for the present situation was mainly detected in the Depredated grassland, mainly due to soil compaction and degradation, caused by overgrazing. No changes in land use and runoff were detected in the upper watershed. It is recommended to implement conservation measures over the watershed, in order to mitigate its degradation and possible effects of future damages to people and watershed system as well.