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Specified head and/or specified flux are typically employed as boundary conditions in numerical simulations involving unsaturated flow. Some modelers incorrectly refer to a specified flux condition as a second‐type boundary. In the mathematical literature a second‐type boundary is defined as a boundary along which the gradient of the dependent variable is specified. This note identifies physical scenarios in which a second‐type boundary (specified gradient) may be the most appropriate boundary condition, discusses a procedure for implementing a specified gradient boundary condition in a flow code, and presents an application of the procedure in the simulation of two practical problems.
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James T. McCord (1991) studied this question.
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