openKARST: A novel open-source flow simulator for karst systems

Abstract: We introduce the open-source Python-based code openKARST for flow in karst conduit networks. Flow and transport in complex karst systems remain a challenging area of hydrogeological research due to the heterogeneous nature of conduit networks. Flow regimes in these systems are highly dynamic, with transitions from free-surface to fully pressurized and laminar to turbulent flow conditions with Reynolds numbers often exceeding one million. These transitions can occur simultaneously within a network, depending on conduit roughness properties and diameter distributions. openKARST solves the transient dynamic wave equation using an iterative scheme and is optimized with an efficient vectorized structure. Transitions from free-surface to pressurized flows in smooth and rough circular conduits are realized via a Preissmann slot approach in combination with an implementation of the Darcy-Weisbach and Manning equations to compute friction losses. To mitigate numerical fluctuations commonly encountered in the Colebrook-White equation, the dynamic switching from laminar to turbulent flows is modeled with a continuous Churchill formulation for the friction factor computation. openKARST supports common boundary conditions encountered in karst systems, as well as functionalities for network import, export and visualization. The code is verified via comparison against several analytical solutions and validated against a laboratory experiment. Finally, we demonstrate the application of the openKARST by simulating a synthetic recharge event in one of the largest explored karst networks, the Ox Bel Ha system in Mexico.