System-dependent boundary condition for water flow from subsurface source

Infiltration rate of water from a subsurface cavity is affected by many factors, including the pressure in the cavity, its size and geometry, and the hydraulic properties of the surrounding soil. When a predetermined discharge of a subsurface source (e.g., a subsurface emitter) is larger than the soil infiltration capacity, the pressure head in the source outlet increases and becomes positive. The built up pressure may significantly reduce the source discharge rate. The main objective of this work was to develop a boundary condition that describes this process while considering the source characteristics, and to implement this boundary condition into the transient numerical model Hydrus-2D. This new, system-dependent boundary condition allows calculation of the source discharge while considering source properties, inlet pressure, and effects of the soil hydraulic properties. The updated numerical model was verified against existing analytical solutions for simplified steady-state conditions and validated against transient experimental data. Good agreement was found between transient cavity pressures measured in laboratory experiments and those calculated using the updated numerical model. The modified program allows using any analytical model that describes the soil hydraulic properties and source characteristics, simulating both short and long duration infiltration events, as well as considering various geometrical shapes of subsurface sources.