A major flash flood took place in the Chacarilla catchment in January 2000 as indicated by offlapping overbank mudflow deposits. The peak flood has been determined at 450 (+/-50) m(3)/s and it is possible to trace the origin of this flood to a quantifiable storm event in the Altiplano. Downstream, the surface water flood infiltrated the alluvial fan to recharge groundwater aquifers. A monitoring well in the fan toe showed a water level rise due to infiltration of 0.45 m that can be translated into a recharge event of 25 million m(3). The storm giving rise to this recharge has a return period of 4 years and so mean annual recharge is equivalent to similar to200 l/s. The catchment response is shown to be nonlinear with positive feedback, so that rarer events provide proportionately greater recharge. Long-term declines in groundwater level may be partly due to climatic fluctuations and the causes of such fluctuations are discussed.