Parallelization of the WASH123D Code—Phase III: 1-Dimensional Channel, 2-Dimensional Overland, and 3-Dimensional Subsurface Flows

Watershed models are used to simulate and predict major hydrological processes, such as surface or subsurface flows, which may occur on different spatial domains and temporal scales. A key feature of watershed models is the ability to model interactions among different processes and domains. Such interactions can be strongly or weakly coupled depending on the relevant time scales for each process. WASH123D is a first-principles, physics-based model for simulating a coupled system of channel flow, overland flow, and subsurface flow. A parallel version, pWASH, has been used for the calibration, validation, and evaluation of proposed alternatives of the Biscayne Bay Coastal Wetlands project. The goal is to rehydrate wetlands using the best alternative based on the simulation results. In the pWASH code, channel flow is modeled as a one-dimensional (1-D) channel network, overland flow as a 2-D process, and subsurface flow as a 3-D process. Different algorithms are implemented to account for the interactions between these different domains. The pWASH code is designed to tackle large watershed problems on parallel high performance computers. A software-engineering approach was used to efficiently parallelize the complex coupling algorithms. The resulting software toolkit encapsulates the parallel data structures and message passing required for multi- domain/process interactions. In this paper, the authors briefly describe the numerical methods in WASH123D, the parallelization of pWASH, and the scalability of coupled flow problems running on parallel high performance computers.