HydroCAL: an Integrated Surface-Subsurface Cellular Automata Hydrological Model

The Integrated Surface and Subsurface Hydrologic Modeling (ISSHM) approach, based on the coupling of physical-based surface and subsurface routing processes, evolved significantly in the last decades, also thanks to the continuously increasing capabilities offered by high-performance computing (HPC). The Extended Cellular Automata (XCA) paradigm perfectly fits the needs of HPC infrastructures, due to its inherent aptitude for parallel computing and other specific features like asynchronism that allow not only parallelization but also the reduction of the computational cost.We present HydroCAL, a new ISSHM based on the Extended Cellular Automata paradigm linking a two-dimensional weighted XCA surface routing model with a three-dimensional XCA subsurface model. The model was implemented in the parallel software library Open Computing Abstraction Layer (OpenCAL), which allows users to exploit several parallelization strategies, hardware architectures and XCA features.Preliminarily, the subsurface model was tested in several thousand synthetical test cases to assess the effects produced by an asynchronous functionality based on a fixed threshold rule on the hydraulic head difference. The results show the high efficiency of the asynchronous XCA model in terms of elapsed time, preserving the accuracy of the results.Then, the coupled surface and subsurface HydroCAL modules were tested with high-resolution (101 m) simulations in a small headwater Mediterranean catchment characterized by high hydrogeological heterogeneity. The model parameters were calibrated and validated using different events, characterized by several discharge peaks, during two years.The results show that the model can accurately catch the hydrological response, reproducing multi-peak events with correct peak times and discharge values, simulating adequately also the recession phases. At the same time, the XCA model implementation permits highly detailed coupled simulations with computational times adequate to operational (even real-time) purposes.Further study will regard the application of different asynchronism rules on both the surface and subsurface modules and the addition of other modules concerning subsurface-groundwater and land surface-atmosphere interaction.