Impact of Vegetation and Decennial Rainfall Fluctuations on the Weathering Fluxes Exported from a Dry Tropical Forest (Mule Hole)

The small experimental watershed of Mule Hole has been monitored for hydrology and chemistry since the last decade at hourly frequency for stream and monthly frequency for groundwater. It is covered by a dry tropical forest which generates intense evapotranspiration, limiting both runoff and groundwater recharge. Stream and groundwater fluxes are then disconnected, which provides a unique opportunity for distinguishing surficial from deep biogeochemical processes occurring in the watershed. Here, monthly monitoring of groundwater levels and chemistry were combined with hydrological modeling for studying how vegetation mediates the water stock in the vadose zone and the groundwater flux, and how this impacts the output flux of sodium by groundwater at seasonal and decadal scales. It is found that evapotranspiration intensity controls the pore water saturation with Na-plagioclase, which determines the depth of chemical weathering (soil vs saprolite). Moreover, evapotranspiration intensity regulates the water residence time in the vadose zone, from about a year downslope to 20 years upslope and the local groundwater discharge. These delays induce long term fluctuations in groundwater output fluxes and therefore in silicate weathering fluxes.