HydroCAL: A novel integrated surface-subsurface hydrological model based on the Cellular Automata paradigm

Integrated Surface-Subsurface Hydrological Models (ISSHMs) are advanced tools for reproducing hydrological processes that are difficult to manage due to their complexity and computational cost. This paper introduces HydroCAL (Hydrological Cellular Automata Layer), an event -based spatially distributed ISSHM entirely based on the Cellular Automata (CA) paradigm through the OpenCAL software library. The primary benefit of the CA paradigm is reducing the computational burden through parallelisation and other specific optimisation features, like asynchronous functionality. HydroCAL was tested on literature benchmarks and, for the first time, considering real events in a headwater catchment, correctly reproducing streamflow (NSE = 0.86; KGE = 0.88) and providing a fully spatially distributed output of several other hydrological variables. The combined effect of parallelisation and a novel version of the asynchronous functionality accounting for spatially distributed soil hydraulic properties significantly reduces the elapsed time in real -world applications. The results suggest that HydroCAL is suitable for simulating the hydrological cycle over large watersheds while keeping a hyper -resolution detail.