Watershed Reactive Transport

Abstract

A watershed (also drainage basin, river basin, or catchment) is defined as "… the area that topographically appears to contribute all the water that passes through a specified cross section of a stream (the outlet)" (Dingman 2015). In this chapter, I choose to use the term "watershed" as it is a broadly used one; it should be understood more as small watersheds or catchments. Watersheds are the fundamental units that support river networks, the blood vessels at Earth's surface ultimately draining into the ocean. Watersheds are complex hydro-biogeochemical reactors. They receive water, mass, and energy, transport them to distinct compartments, and transform them into various forms (Fig.1). Hydrological processes partition precipitation to the atmosphere, to the ground surface, and to the subsurface, eventually entering streams. Similarly, plants translate sunlight, water, CO 2 , and nutrients into organic matters (leaves, stems, roots) that fall and deposit in soil. As water routes through soils, it interacts with roots, microbes, and reactive gases (i.e., CO 2 and O 2), releases solutes, and ultimately transporting them out of watersheds. The water flow (discharge) and solute concentrations measured at stream outlets therefore reflect convoluted signature of ecohydrological and biogeochemical coupling. The process interactions and feedbacks are dictated not only by hydroclimatic forcing but also the architecture of watersheds, in particular the above-ground characteristics such as land cover and surface topography, as well as the below-ground structure including soil depth, soil type, geology, and root architecture. The external forcing and internal idiosyncrasies dictate the magnitude, timing, and spatial distribution of water flow and chemistry (Chorover et al. 2011; Brooks et al. 2015), giving rise to non-linear emergent behaviors that are unique of watershed reactive transport processes. Understanding feedbacks and non-linearity requires process-based models that integrate multiple interacting processes. Such integration however does not come easily with traditional boundaries of hydrology versus biogeochemistry relevant disciplines. As will be discussed later in this chapter, relevant model development has been advancing along two separate lines: hydrology models that solve for water storage and fluxes at the watershed scale and beyond (Fatichi et al. 2016), and reactive transport models (RTMs) that center on aqueous and solid concentration changes arising from transport and multi-component biogeochemical reactions typically in "closed" groundwater systems without much interactions with "open" watersheds directly receiving precipitation and sun light (Steefel et al. 2015; Li et al. 2017b). This comes along with a history of hydrologists often trained as physicists studying fluid mechanics, and biogeochemists typically grow up as geologists, chemists, or environmental engineers. There are however considerable needs to reach beyond disciplinary boundaries and integrate the two lines to develop watershed reacti...

Toward catchment hydro‐biogeochemical theories

“…Solute transport and reaction modules have been developed as add‐ons to hydrological models (e.g., (Arnold & Soil, 1994; Santhi et al, 2001)), but they often do not explicitly represent the multi‐component reaction thermodynamics and the kinetic theory that lie at the core of geochemistry and biogeochemistry. Only recently have RTMs been brought to the catchment scale (Bao, Li, Shi, & Duffy, 2017; Li et al, 2017; Li, 2019; Yeh et al, 2006; Zhi et al, 2020).…”

Streamflow Generation From Catchments of Contrasting Lithologies: The Role of Soil Properties, Topography, and Catchment Size

“…Flux-PIHM is a physically based, spatially distributed model (Shi et al, 2013) that couples a land surface scheme adapted from the Noah Land Surface Model with the PIHM (Qu & Duffy, 2007). In addition, there is a family of PIHM-related codes with different simulation capabilities (Duffy et al, 2014), including landscape evolution (Zhang et al, 2016), ecosystem biogeochemistry (Shi et al, 2018), and catchment-scale reactive transport (Bao et al, 2017;Li, 2019;Zhi et al, 2019). The code discretizes the land surface into triangular elements and rivers into rectangular segments that are projected vertically down to the bedrock to generate 10.1029/2018WR023736…”

Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

“…Existing studies however have primarily focused on the role of soil CO 2 and organic acids (Drever, 1994;Lawrence et al, 2014;Gaillardet et al, 2019;Hauser et al, 2020). Systematic studies on coupled effects of hydrological flow paths and soil CO 2 distribution are missing, owing to the limitation in data that detail rooting effects on flow partitioning and complex hydrologicalbiogeochemical interactions (Li et al, 2020). Here we ask the following questions.…”