The hydrological response of a medium scale mountainous watershed (Mexico) is analysed over half a century. The hydrograph separation highlights an increasing surface runoff contribution since the early 1970's. This increase is attributed to land use changes while the meteorological forcing (rains) remains statistically stable over the same period. As a consequence, the intensity of annual extreme floods has tripled up over the period of survey, increasing flood risks in the region. The paper ends with a climatic projection over the 21st century. The decrease of precipitation and the increase of temperature should accentuate the trend engaged since the 1970's by reducing groundwater resources and increasing surface-runoff and associated risks.