Groundwater is the world's largest accessible store of fresh water and supplies 36% of the world's drinking water and ~42% of the water used for irrigation 1 . Groundwater is the only reliable source of fresh water in many semi-arid and arid regions where surface waters are seasonally or perennially absent 9 . The long-term viability of groundwater resources as well as the ecosystems and livelihoods that they sustain, depends on replenishment of groundwater by recharge. Over the past 50 years, groundwater depletion has been estimated and observed in several aquifers throughout the tropics and sub-tropics [10][11][12][13] . Such depletion not only threatens ecosystem function and the livelihoods of groundwater-dependent communities in some of the world's poorest regions but is also estimated to contribute to sea-level rise 12,13 . A conceptual understanding of the relationship between rainfall and recharge is fundamental to the development of robust estimates and projections of not only groundwater recharge and depletion but of all components of the terrestrial water balance under changing climates and increasing freshwater demand.