To investigate the hydrologic and landscape controls on shale weathering and their influence on first-order stream solute fluxes, we coupled a numerical chemical weathering model, WITCH, with a physically-based land surface hydrologic model, Flux-PIHM (Penn State Integrated Hydrologic Model). Using this approach, we were able to simulate vertical water and chemical fluxes across two representative landscape features (planar or swale hillslopes). We extrapolate the results from the soil profiles to estimate the chemical loads to the stream over a 32 year period (1980-2012) at the Susquehanna Shale Hills Critical Zone Observatory (SSH-CZ0). Observed soil water (10 cm increments) and stream water chemistry collected between 2006-2011 were used to validate the modelling results. The simulation predicted the saturation state of soil water with respect to illite, smectite and kaolinite. Saturation was driven by seasonal drying, with supersaturated conditions persistent around the rooting zone (25-70 cm) during the summer months. As expected, the modelled weathering rates of illite, chlorite and k-feldspar were elevated near the soil surface where fluids were dilute, especially at the onset of fall when solute fluxes (eg. Mg, Na and Si) reached their maximum. Results from Flux-PIHM suggest a more extensive hydrologic connection between the swale hillslopes and the stream as compared with the planar hillslopes. WITCH then documented the role of this hydrologic connection on the stream water chemistry, with swales contributing bigger solute fluxes.
Pamela L. Sullivan, Yves Godderis, Yuning Shi, Jacques Schott, Christopher J. Duffy, Susan L. Brantley (2013): Using WITCH to quantify landscape and hydrologic controls on solute fluxes in the Critical Zone (Susquehanna Shale Hills Observatory, PA) . Mineralogical Magazine, 77(5) 2284.
This Paper/Book acknowledges NSF CZO grant support.