HydroCAL: An Integrated Surface-Subsurface Cellular Automata Hydrological Model to simulate streamflow and river network dynamics

One of the main constraints to the operational use of Integrated Surface and Subsurface Hydrologic Modelling (ISSHM) is the computational cost of such a complex approach. The growth of High-Performance Computing (HPC) has continuously pushed forward this limit, targeting the objective of "hyper-resolution" modelling. The Extended Cellular Automata (XCA) paradigm allows easy parallelization of numerical code and can be used in different HPC systems. Moreover, XCA have other unique features, like asynchronism, that can further break down the elapsed time.

HydroCAL is an integrated surface-subsurface Cellular Automata Layer hydrological model built by coupling a diffusive-like 2D water surface routing module and a 3D subsurface routing module based on the variably saturated Richards' equation. The model was implemented by adopting the parallel scientific software library Open Computing Abstraction Layer (OpenCAL), which allows researchers to exploit different parallelization techniques, hardware architectures and XCA-specific features.

Here we extend the HydroCAL model's capabilities, including groundwater and evapotranspiration modules that significantly contribute to the baseflow generation and river reach activation/deactivation dynamics, allowing continuous simulations beyond the storm-event scale. The enhanced model is used at ultra-high resolution (10⁰ m) in a small steep-orography headwater Mediterranean catchment characterized by high hydrogeological heterogeneity. A multivariate calibration and validation approach is adopted over long-term simulations, using the observed active stream network dynamics and the recorded streamflow at the catchment outlet.

The results show that HydroCAL can adequately reproduce the hydrological response, simulating several multipeak events and reproducing the recession phases. Moreover, groundwater behaviour contributes to the simulation of the complex river network activation and deactivation dynamics. Overall, the HydroCAL model implementation upon the XCA paradigm allows highly detailed coupled simulations for long periods with reasonable computational times.