Abstract
Abstract. Surface flow and subsurface flow constitute a naturally linked hydrologic continuum that has not traditionally been simulated in an integrated fashion. Recognizing the interactions between these systems has encouraged the development of integrated hydrologic models (IHMs) capable of treating surface and subsurface systems as a single integrated resource. IHMs are dynamically evolving with improvements in technology, and the extent of their current capabilities are often only known to the developers and not general users. This article provides an overview of the core functionality, capability, applications, and ongoing development of one open-source IHM, ParFlow. ParFlow is a parallel, integrated, hydrologic model that simulates surface and subsurface flows. ParFlow solves the Richards equation for three-dimensional variably saturated groundwater flow and the two-dimensional kinematic wave approximation of the shallow water equations for overland flow. The model employs a conservative centered finite-difference scheme and a conservative finite-volume method for subsurface flow and transport, respectively. ParFlow uses multigrid-preconditioned Krylov and Newton–Krylov methods to solve the linear and nonlinear systems within each time step of the flow simulations. The code has demonstrated very efficient parallel solution capabilities. ParFlow has been coupled to geochemical reaction, land surface (e.g., the Common Land Model), and atmospheric models to study the interactions among the subsurface, land surface, and atmosphere systems across different spatial scales. This overview focuses on the current capabilities of the code, the core simulation engine, and the primary couplings of the subsurface model to other codes, taking a high-level perspective.
Highlights
Surface water and subsurface water are connected components of a hydrologic continuum (Kumar et al, 2009)
The recognition that flow systems are a single integrated resource has stimulated the development of integrated hydrologic models (IHMs), which include codes like ParFlow (Ashby and Falgout, 1996; Kollet and Maxwell, 2006), HydroGeoSphere (Therrien and Sudicky, 1996), PIHM (Kumar, 2009), and CATHY (Camporese et al, 2010)
Most IHMs adopt a similar physically based approach to describe watershed dynamics, whereby the governing equations of three-dimensional variably saturated subsurface flow are coupled to shallow water equations for surface runoff
Summary
Surface water and subsurface (unsaturated and saturated zones) water are connected components of a hydrologic continuum (Kumar et al, 2009). ParFlow is well documented and has been applied to surface and subsurface flow problems, including simulating the dynamic nature of groundwater and surface–subsurface interconnectivity in large domains (e.g., over 600 km2) (Kollet and Maxwell, 2008a; Ferguson and Maxwell, 2012; Condon et al, 2013; Condon and Maxwell, 2014), small catchments (e.g., approximately 30 km2) (Ashby et al, 1994; Kollet and Maxwell, 2006; Engdahl et al, 2016), complex terrain with highly heterogenous subsurface permeability such as the Rocky Mountain National Park, Colorado, United States (Engdahl and Maxwell, 2015; Kollet et al, 2017), large watersheds (Abu-El-Sha’r and Rihani, 2007; Kollet et al, 2010), continental-scale flows (Condon et al, 2015; Maxwell et al, 2015), and even subsurface–surface and atmospheric coupling (Maxwell et al, 2011; Williams and Maxwell, 2011; Williams et al, 2013; Gasper et al, 2014; Shrestha et al, 2015). A terrain-following mesh formulation has been implemented (Maxwell, 2013) that allows ParFlow to handle problems with fine space discretization near the ground surface that comes with variable vertical discretization flexibility, which offer modelers the advantage to increase the resolution of the shallow soil layers (these are discussed in detail below)
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