Abstract

River hydrographs generally exhibit intense flood events during which the discharge increases quickly during rainfall, and decreases slowly afterwards. In this seminar, we show that the dynamics of groundwater in an unconfined aquifer can account for these features.

A laboratory aquifer (homogeneous and bidimensional), made of a tank filled with glass beads, is submitted to artificial rainfall. Rainwater infiltrates into the porous material to join a groundwater reservoir. Groundwater then flows slowly to exit this laboratory aquifer through one side of the tank. This simplified experimental setup generates a realistic flood signal, in the absence of surface runoff.

A theory based on Darcy’s law and the shallow water approximation reveals two asymptotic regimes. At the beginning of a rain event, the water discharge increases linearly with time, with a slope proportional to the rainfall rate at the power of $3/2$. Long after the rain has stopped, it decreases as the inverse time squared, as predicted by Polubarinova-Kochina (1962). These predictions compare well against our experimental data.

Field observation in the catchment of the Quiock Creek, Guadeloupe reveals a similar behaviour. The water table and the river discharge evolve simultaneously during rainfall, and exhibit the same asymptotic behaviours. Like in our laboratory experiment, this aquifer reacts non-linearly to forcing by rainfall. This observation suggests that these results could be extended to a broader class of groundwater flows.