Abstract

Hydrological and geochemical processes are intricately coupled at the watershed scale. Despite recent advances, modeling the complex hydro-thermo-geochemical interactions at the watershed scale has been challenging. Many efforts have been put forward to solve the well-known puzzles such as the “double paradox ” raised by Kirchner either mechanistically or through simplified numerical modeling. However, a major gap remains in explicitly modeling and integrating these processes  at the watershed scale.

This work presents an integrated approach to understanding and quantifying the hydrologic controls on water chemistry at the watershed scale. A fully coupled finite volume hydro-thermo-geochemical model, PIHM-RT (Penn State Integrated Hydrologic Model --Reactive Transport) has been developed based on the land surface hydrologic model, Flux-PIHM. Flux-PIHM is capable of simulating the terrestrial water cycle and the surface energy balance (SEB) to reproduce the spatially distributed observations of water, temperature, and saturation . Adding the reactive transport module enables explicit modeling of the evolving water chemistry, which is controlled by hydrologic processes and geochemical reactions. The reactions include mineral dissolution, precipitation and ion exchange. PIHM-RT utilizes an a priori database EQ3\EQ6 that is widely used for geochemical thermodynamics and kinetics. The RT module utilizes an operator splitting scheme described in Zysset et al. (1994), to solve for the advection- dispersion-reaction equation (ADR). The advection dispersion equation was solved using the Euler forward method and the reaction process was solved implicitly. In addition, because the reaction and transport processes differ significantly between the unsaturated and saturated zones, we implemented a volume explicit mass conservation law to account for the variable depth of groundwater and the mixing process involved at the boundary between the saturated and unsaturated zone. The use of an unstructured mesh in PIHM-RT provides an optimal representation of heterogeneity in both hydrological and geochemical parameters. In sum, PIHM-RT presents a powerful tool to test hypotheses against key hydrologic and geochemical observations at the watershed scale (HUC-12 sized or less). We will demonstrate the use of PIHM-RT with constraints from field data collected at the Shale Hills watershed (0.08 km²). PIHM-RT is expected to be a general tool for understanding coupled hydro-geochemical processes in the Critical Zone.

Citation

Bao, C., Li, L., Shi, Y., Qiao, C., Sullivan, P.L., Brantley, S.L., Duffy, C. (2013): Understanding the Hydrological Controls on the Water Chemistry at the Watershed Scale Using an Integrated Hydro-Thermo-Geochemical Model PIHM-RT. Abstract H51A-1174 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..

This Paper/Book acknowledges NSF CZO grant support.