MOHID Land is a physically-based, spatially distributed, continuous, variable time step model for the water and property cycles. It integrates four compartments or mediums (atmosphere, porous media, soil surface, and river network). Water moves through the mediums based on mass and momentum conservation equations. The atmosphere is not explicitly simulated but provides data necessary for imposing surface boundary conditions to the model (precipitation, solar radiation, wind, etc.) that may be space and time variant. The model is based on finite-volumes organized into a structured grid, rectangular in the horizontal plane, and Cartesian type in the vertical plane. The surface land is described by a 2D horizontal grid and the porous media is a 3D domain which includes the same horizontal grid as the surface complemented with a vertical grid with variable layer thickness. The river network is a 1D domain defined from the digital terrain model (DTM), with reaches linking surface cell centers. Fluxes are computed over the faces of the finite volumes and state variables are computed at the center to assure conservation of transported properties. The model uses an explicit algorithm with a variable time step, that is maximum during the dry season when fluxes are reduced and minimum when fluxes increase (e.g. during rain events).
The model will be implemented in case studies of Portugal, Spain, and Italy for simulating soil water dynamics and nutrient transport at the plot and catchment scales.