Stream‐Discharge Surges Generated by Groundwater Flow

Abstract

16 Catchments respond to rainfall by storing and releasing water according to their internal 17 dynamics. Groundwater had long been treated as the slow reservoir in this process, but 18 isotopic measurements showed how responsive it can be. Here, we investigate the mechanics 19 of groundwater’s contribution to floods. To do so, we monitored over three years the shape 20 of the water table in, and the runoff out of, a small tropical catchment. We find that 21 groundwater and runoff respond within minutes of a rainfall event. Using an asymptotic 22 theory inspired by recent laboratory experiments, we suggest that the peak water discharge 23 at the catchment’s outlet increases like the rainfall rate to the power of 3/2. This formula 24 consistently predicts the stream’s response to the 137 isolated rainfall events recorded during 25 our field survey. In addition, its prefactor yields an estimate of the average groundwater 26 storage. 27 Plain Language Summary Rainwater infiltrates into the ground, accumulates in porous 28 rocks, and eventually flows towards a neighboring stream. Although this underground travel 29 often takes millennia, groundwater can contribute quickly to floods. To understand how an 30 underground flow can be so responsive, we have recorded the motion of the groundwater 31 surface in a small tropical catchment during three years. We find that groundwater swells 32 within minutes of a rain event, and that this deformation directly pushes more water into the 33 stream. The resulting stream-discharge peak strengthens faster than the rainfall intensity: a 34 three-fold increase of the latter causes a five-fold increase of the stream discharge. Including 35 this mechanism into flood-forecasting models should allow us to better predict the impact 36 of extreme precipitations. Finally, we introduce a method to measure how much water 37 an aquifer stores during a rainfall event, before releasing it—a central parameter for the 38 management of water resources. 39