Tools and Algorithms to Link Horizontal Hydrologic and Vertical Hydrodynamic Models and Provide a Stochastic Modeling Framework

In this article, we present an algorithm to automatically link an overland flow model to a water quality and hydrodynamic model spatially and temporally. We introduce the spatial and temporal linking algorithms, the tools developed to link and run these models automatically,  and illustrate how these tools were implemented in the Watershed Modeling System (WMS). We demonstrate these algorithms by linking the Gridded Surface Subsurface Hydrological Analysis (GSSHA) model for overland flow with the CE-QUAL-W2 model for water quality and reservoir hydrodynamics. These models have different spatial and temporal domains; GSSHA uses a two-dimensional horizontal grid while CE-QUAL-W2 uses a two-dimensional vertical grid. We briefly discuss an algorithm implemented to facilitate and analyze stochastic simulations using the linked models. The ability to automatically link models allows modelers to use the most appropriate model for specific flow regimes to answer questions on a watershed scale. The ability to use stochastic simulations can help address and understand how parameter variability translates to uncertainty in results and provides tools to determine how to reduce, or at least manage, this uncertainty.