Accurate observation of hillslope groundwater storage and instantaneous
recharge remains difficult due to limited monitoring and the complexity
of mountainous landscapes. We introduce a novel storage-discharge method
to estimate hillslope recharge and the recharge ratio---the fraction of
precipitation that recharges groundwater. The method, which relies on
streamflow data, is corroborated by independent measurements of water
storage dynamics inside the Rivendell experimental hillslope at the Eel
River Critical Zone Observatory, California USA. We find that
along-hillslope patterns in bedrock weathering and plant-driven storage
dynamics govern the seasonal evolution of recharge ratios. Thinner
weathering profiles and smaller root-zone storage deficits near-channel
are replenished before larger ridge-top deficits. Consequently,
precipitation progressively activates groundwater from channel to
divide, with an attendant increase in recharge ratios throughout the wet
season. Our novel approach and process observations offer valuable
insights into controls on groundwater recharge, enhancing our
understanding of a critical flux in the hydrologic cycle.