『Abstract
We investigate the chemical weathering processes and fluxes in
a small experimental watershed (SEW) through a modelling approach.
The study site is the Mule Hole SEW developed on a gneissic basement
located in the climatic gradient of the Western Ghats, South India.
The model couples a lumped hydrological model simulating the water
budget at the watershed scale to the WITCH model estimating the
dissolution/precipitation rates of minerals using laboratory kinetic
laws. Forcing functions and parameters of the simulation are defined
by the field data. The coupled model is calibrated with stream
and groundwater compositions through the testing of a large range
of smectite solubility and abundance in the soil horizons. We
found that, despite the low abundance of smectite in the dominant
soil type of the watershed (4 vol.％), their net dissolution provides
75％ of the export of dissolved silica, while primary silicate
mineral dissolution releases only 15％ of this flux. Overall, smectites
(modelled as montmorillonites) are not stable under the present
day climatic conditions. Furthermore, the dissolution of trace
carbonates in the saprolitic horizon provides 50％ of the calcium
export at the watershed scale. Modelling results show the contrasted
behavior of the two main soil types of the watershed: red soils
(88％ of the surface) are provider of calcium, while black soils
(smectite-rich and characterized by a lower drainage) consumes
calcium through overall carbonate precipitation. Our model results
stress the key role played by minor/accessory minerals and by
the thermodynamic properties of smectite minerals, and by the
drainage of the weathering profiles on the weathering budget of
a tropical watershed.
Keywords: Chemical weathering; Primary minerals; Smectites; Modelling;
Tropical; Watershed』
1. Introduction
2. Field settings
3. Model design, forcing parameters. and validation data
3.1. Hydrological modelling
3.1.1. Hydrological data
3.1.2. The hydrological model
3.1.3. Results of the hydrological model
3.2. Geochemical modelling
3.2.1. Defining the mineralogy of each reservoir
3.2.2. Atmospheric input chemistry (wet atmospheric deposit
and throughfalls)
3.2.3. Brief WITCH model description
3.3. Validation data
4. Model calibration
5. Model validation: comparison between simulated and measured
streamwater and groundwater compositions, and watershed weathering
fluxes (reference run)
5.1. Atmospheric inputs and streamwater chemical composition
5.2. Simulated and observed groundwater chemical composition
5.3. Simulated and observed weathering balance at the watershed
scale
6. Discussion
6.1. Weathering budget of the Mule Hole SEW
6.2. Current sensitivity of silica dissolved fluxes to smectite
volume percentages
6.3. Current sensitivity of Ca2+ dissolved fluxes
to carbonate volume percentages
6.4. Calcium origin in the carbonate nodules
6.5. Weathering system out of steady state
7. Conclusion
Acknowledgements
Appendix A. Measured major element chemical composition in
soils and saprolites sampled for this study; cation exchange capacity
(CEC); base saturation (S/T) and exchangeable cations
Appendix B. Total calcium carbonate content; bulk density,
grain size and organic carbon for soil and saprolites sampled
for this study
Appendix C. Measured mean chemical composition for various
solutions sampled in the Mule Hole watershed during the modelling
period
References